Message-ID: <CAA85sZs8Li7+8BQWj0e+Qrxes1VF6K_Ukqrqgs1E3hHmaXqsbQ@mail.gmail.com>
Date:   Mon, 14 Dec 2020 23:56:31 +0100
From:   Ian Kumlien <ian.kumlien@...il.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Kai-Heng Feng <kai.heng.feng@...onical.com>,
        linux-pci <linux-pci@...r.kernel.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        "Saheed O. Bolarinwa" <refactormyself@...il.com>,
        Puranjay Mohan <puranjay12@...il.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Tony Nguyen <anthony.l.nguyen@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Heiner Kallweit <hkallweit1@...il.com>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] PCI/ASPM: Use the path max in L1 ASPM latency check

On Mon, Dec 14, 2020 at 8:19 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
>
> On Mon, Dec 14, 2020 at 04:47:32PM +0100, Ian Kumlien wrote:
> > On Mon, Dec 14, 2020 at 3:02 PM Bjorn Helgaas <helgaas@...nel.org> wrote:
> > > On Mon, Dec 14, 2020 at 10:14:18AM +0100, Ian Kumlien wrote:
> > > > On Mon, Dec 14, 2020 at 6:44 AM Bjorn Helgaas <helgaas@...nel.org> wrote:
> > > > >
> > > > > [+cc Jesse, Tony, David, Jakub, Heiner, lists in case there's an ASPM
> > > > > issue with I211 or Realtek NICs.  Beginning of thread:
> > > > > https://lore.kernel.org/r/20201024205548.1837770-1-ian.kumlien@gmail.com
> > > > >
> > > > > Short story: Ian has:
> > > > >
> > > > >   Root Port --- Switch --- I211 NIC
> > > > >                        \-- multifunction Realtek NIC, etc
> > > > >
> > > > > and the I211 performance is poor with ASPM L1 enabled on both links
> > > > > in the path to it.  The patch here disables ASPM on the upstream link
> > > > > and fixes the performance, but AFAICT the devices in that path give us
> > > > > no reason to disable L1.  If I understand the spec correctly, the
> > > > > Realtek device should not be relevant to the I211 path.]
> > > > >
> > > > > On Sun, Dec 13, 2020 at 10:39:53PM +0100, Ian Kumlien wrote:
> > > > > > On Sun, Dec 13, 2020 at 12:47 AM Bjorn Helgaas <helgaas@...nel.org> wrote:
> > > > > > > On Sat, Oct 24, 2020 at 10:55:46PM +0200, Ian Kumlien wrote:
> > > > > > > > Make pcie_aspm_check_latency comply with the PCIe spec, specifically:
> > > > > > > > "5.4.1.2.2. Exit from the L1 State"
> > > > > > > >
> > > > > > > > Which makes it clear that each switch is required to
> > > > > > > > initiate a transition within 1μs from receiving it,
> > > > > > > > accumulating this latency and then we have to wait for the
> > > > > > > > slowest link along the path before entering L0 state from
> > > > > > > > L1.
> > > > > > > > ...
> > > > > > >
> > > > > > > > On my specific system:
> > > > > > > > 03:00.0 Ethernet controller: Intel Corporation I211 Gigabit Network Connection (rev 03)
> > > > > > > > 04:00.0 Unassigned class [ff00]: Realtek Semiconductor Co., Ltd. Device 816e (rev 1a)
> > > > > > > >
> > > > > > > >             Exit latency       Acceptable latency
> > > > > > > > Tree:       L1       L0s       L1       L0s
> > > > > > > > ----------  -------  -----     -------  ------
> > > > > > > > 00:01.2     <32 us   -
> > > > > > > > | 01:00.0   <32 us   -
> > > > > > > > |- 02:03.0  <32 us   -
> > > > > > > > | \03:00.0  <16 us   <2us      <64 us   <512ns
> > > > > > > > |
> > > > > > > > \- 02:04.0  <32 us   -
> > > > > > > >   \04:00.0  <64 us   unlimited <64 us   <512ns
> > > > > > > >
> > > > > > > > 04:00.0's latency is the same as the maximum it allows so as
> > > > > > > > we walk the path the first switchs startup latency will pass
> > > > > > > > the acceptable latency limit for the link, and as a
> > > > > > > > side-effect it fixes my issues with 03:00.0.
> > > > > > > >
> > > > > > > > Without this patch, 03:00.0 misbehaves and only gives me ~40
> > > > > > > > mbit/s over links with 6 or more hops. With this patch I'm
> > > > > > > > back to a maximum of ~933 mbit/s.
> > > > > > >
> > > > > > > There are two paths here that share a Link:
> > > > > > >
> > > > > > >   00:01.2 --- 01:00.0 -- 02:03.0 --- 03:00.0 I211 NIC
> > > > > > >   00:01.2 --- 01:00.0 -- 02:04.0 --- 04:00.x multifunction Realtek
> > > > > > >
> > > > > > > 1) The path to the I211 NIC includes four Ports and two Links (the
> > > > > > >    connection between 01:00.0 and 02:03.0 is internal Switch routing,
> > > > > > >    not a Link).
> > > > > >
> > > > > > >    The Ports advertise L1 exit latencies of <32us, <32us, <32us,
> > > > > > >    <16us.  If both Links are in L1 and 03:00.0 initiates L1 exit at T,
> > > > > > >    01:00.0 initiates L1 exit at T + 1.  A TLP from 03:00.0 may see up
> > > > > > >    to 1 + 32 = 33us of L1 exit latency.
> > > > > > >
> > > > > > >    The NIC can tolerate up to 64us of L1 exit latency, so it is safe
> > > > > > >    to enable L1 for both Links.
> > > > > > >
> > > > > > > 2) The path to the Realtek device is similar except that the Realtek
> > > > > > >    L1 exit latency is <64us.  If both Links are in L1 and 04:00.x
> > > > > > >    initiates L1 exit at T, 01:00.0 again initiates L1 exit at T + 1,
> > > > > > >    but a TLP from 04:00.x may see up to 1 + 64 = 65us of L1 exit
> > > > > > >    latency.
> > > > > > >
> > > > > > >    The Realtek device can only tolerate 64us of latency, so it is not
> > > > > > >    safe to enable L1 for both Links.  It should be safe to enable L1
> > > > > > >    on the shared link because the exit latency for that link would be
> > > > > > >    <32us.
> > > > > >
> > > > > > 04:00.0:
> > > > > > DevCap: MaxPayload 128 bytes, PhantFunc 0, Latency L0s <512ns, L1 <64us
> > > > > > LnkCap: Port #0, Speed 5GT/s, Width x1, ASPM L0s L1, Exit Latency L0s
> > > > > > unlimited, L1 <64us
> > > > > >
> > > > > > So maximum latency for the entire link has to be <64 us
> > > > > > For the device to leave L1 ASPM takes <64us
> > > > > >
> > > > > > So the device itself is the slowest entry along the link, which
> > > > > > means that nothing else along that path can have ASPM enabled
> > > > >
> > > > > Yes.  That's what I said above: "it is not safe to enable L1 for both
> > > > > Links."  Unless I'm missing something, we agree on that.
> > > > >
> > > > > I also said that it should be safe to enable L1 on the shared Link
> > > > > (from 00:01.2 to 01:00.0) because if the downstream Link is always in
> > > > > L0, the exit latency of the shared Link should be <32us, and 04:00.x
> > > > > can tolerate 64us.
> > > >
> > > > Exit latency of shared link would be max of link, ie 64 + L1-hops, not 32
> > >
> > > I don't think this is true.  The path from 00:01.2 to 04:00.x includes
> > > two Links, and they are independent.  The exit latency for each Link
> > > depends only on the Port at each end:
> >
> > The full path is what is important, because that is the actual latency
> > (which the current linux code doesn't do)
>
> I think you're saying we need to include the 04:00.x exit latency of
> 64us even though L1 is not enabled for 04:00.x.  I disagree; the L1
> exit latency of Ports where L1 is disabled is irrelevant.

I will redo the without patch and look again, I know that I have to
wait a while for it to happen.

Witch patch 3 i get:
dec 14 13:44:40 localhost kernel: pci 0000:04:00.0: ASPM latency
exceeded, disabling: L1:0000:01:00.0-0000:00:01.2

And it should only check links that has L1 aspm enabled, as per the
original code.

> > >   Link 1 (depends on 00:01.2 and 01:00.0): max(32, 32) = 32us
> > >   Link 2 (depends on 02:04.0 and 04:00.x): max(32, 64) = 64us
> > >
> > > If L1 is enabled for Link 1 and disabled for Link 2, Link 2 will
> > > remain in L0 so it has no L1 exit latency, and the exit latency of
> > > the entire path should be 32us.
> >
> > My patch disables this so yes.
> >
> > > > > > > > The original code path did:
> > > > > > > > 04:00:0-02:04.0 max latency 64    -> ok
> > > > > > > > 02:04.0-01:00.0 max latency 32 +1 -> ok
> > > > > > > > 01:00.0-00:01.2 max latency 32 +2 -> ok
> > > > > > > >
> > > > > > > > And thus didn't see any L1 ASPM latency issues.
> > > > > > > >
> > > > > > > > The new code does:
> > > > > > > > 04:00:0-02:04.0 max latency 64    -> ok
> > > > > > > > 02:04.0-01:00.0 max latency 64 +1 -> latency exceeded
> > > > > > > > 01:00.0-00:01.2 max latency 64 +2 -> latency exceeded
> > > > > > >
> > > > > > > [Nit: I don't think we should add 1 for the 02:04.0 -- 01:00.0 piece
> > > > > > > because that's internal Switch routing, not a Link.  But even without
> > > > > > > that extra microsecond, this path does exceed the acceptable latency
> > > > > > > since 1 + 64 = 65us, and 04:00.0 can only tolerate 64us.]
> > > > > >
> > > > > > It does report L1 ASPM on both ends, so the links will be counted as
> > > > > > such in the code.
> > > > >
> > > > > This is a bit of a tangent and we shouldn't get too wrapped up in it.
> > > > > This is a confusing aspect of PCIe.  We're talking about this path:
> > > > >
> > > > >   00:01.2 --- [01:00.0 -- 02:04.0] --- 04:00.x multifunction Realtek
> > > > >
> > > > > This path only contains two Links.  The first one is
> > > > > 00:01.2 --- 01:00.0, and the second one is 02:04.0 --- 04:00.x.
> > > > >
> > > > > 01:00.0 is a Switch Upstream Port and 02:04.0 is a Switch Downstream
> > > > > Port.  The connection between them is not a Link; it is some internal
> > > > > wiring of the Switch that is completely opaque to software.
> > > > >
> > > > > The ASPM information and knobs in 01:00.0 apply to the Link on its
> > > > > upstream side, and the ASPM info and knobs in 02:04.0 apply to the
> > > > > Link on its downstream side.
> > > > >
> > > > > The example in sec 5.4.1.2.2 contains three Links.  The L1 exit latency
> > > > > for the Link is the max of the exit latencies at each end:
> > > > >
> > > > >   Link 1: max(32, 8) = 32us
> > > > >   Link 2: max(8, 32) = 32us
> > > > >   Link 3: max(32, 8) = 32us
> > > > >
> > > > > The total delay for a TLP starting at the downstream end of Link 3
> > > > > is 32 + 2 = 32us.
> > > > >
> > > > > In the path to your 04:00.x Realtek device:
> > > > >
> > > > >   Link 1 (from 00:01.2 to 01:00.0): max(32, 32) = 32us
> > > > >   Link 2 (from 02:04.0 to 04:00.x): max(32, 64) = 64us
> > > > >
> > > > > If L1 were enabled on both Links, the exit latency would be 64 + 1 =
> > > > > 65us.
> > > >
> > > > So one line to be removed from the changelog, i assume... And yes, the
> > > > code handles that - first disable is 01:00.0 <-> 00:01.2
> > > >
> > > > > > I also assume that it can power down individual ports... and enter
> > > > > > rest state if no links are up.
> > > > >
> > > > > I don't think this is quite true -- a Link can't enter L1 unless the
> > > > > Ports on both ends have L1 enabled, so I don't think it makes sense to
> > > > > talk about an individual Port being in L1.
> > > > >
> > > > > > > > It correctly identifies the issue.
> > > > > > > >
> > > > > > > > For reference, pcie information:
> > > > > > > > https://bugzilla.kernel.org/show_bug.cgi?id=209725
> > > > > > >
> > > > > > > The "lspci without my patches" [1] shows L1 enabled for the shared
> > > > > > > Link from 00:01.2 --- 01:00.0 and for the Link to 03:00.0 (I211), but
> > > > > > > not for the Link to 04:00.x (Realtek).
> > > > > > >
> > > > > > > Per my analysis above, that looks like it *should* be a safe
> > > > > > > configuration.  03:00.0 can tolerate 64us, actual is <33us.  04:00.0
> > > > > > > can tolerate 64us, actual should be <32us since only the shared Link
> > > > > > > is in L1.
> > > > > >
> > > > > > See above.
> > > > >
> > > > > As I said above, if we enabled L1 only on the shared Link from 00:01.2
> > > > > to 01:00.0, the exit latency should be acceptable.  In that case, a
> > > > > TLP from 04:00.x would see only 32us of latency:
> > > > >
> > > > >   Link 1 (from 00:01.2 to 01:00.0): max(32, 32) = 32us
> > > > >
> > > > > and 04:00.x can tolerate 64us.
> > > >
> > > > But, again, you're completely ignoring the full link, ie 04:00.x would
> > > > also have to power on.
> > >
> > > I think you're using "the full link" to refer to the entire path from
> > > 00:01.2 to 04:00.x.  In PCIe, a "Link" directly connects two Ports.
> > > It doesn't refer to the entire path.
> > >
> > > No, if L1 is disabled on 02:04.0 and 04:00.x (as Linux apparently does
> > > by default), the Link between them never enters L1, so there is no
> > > power-on for this Link.
> >
> > It doesn't do it by default, my patch does
>
> I'm relying on [1], your "lspci without my patches" attachment named
> "lspci-5.9-mainline.txt", which shows:
>
>   02:04.0 LnkCtl: ASPM Disabled
>   04:00.0 LnkCtl: ASPM Disabled
>
> so I assumed that was what Linux did by default.

Interesting, they are disabled.

> > > > > > > However, the commit log at [2] shows L1 *enabled* for both
> > > > > > > the shared Link from 00:01.2 --- 01:00.0 and the 02:04.0
> > > > > > > --- 04:00.x Link, and that would definitely be a problem.
> > > > > > >
> > > > > > > Can you explain the differences between [1] and [2]?
> > > > > >
> > > > > > I don't understand which sections you're referring to.
> > > > >
> > > > > [1] is the "lspci without my patches" attachment of bugzilla #209725,
> > > > > which is supposed to show the problem this patch solves.  We're
> > > > > talking about the path to 04:00.x, and [1] show this:
> > > > >
> > > > >   01:00.2 L1+               # <-- my typo here, should be 00:01.2
> > > > >   01:00.0 L1+
> > > > >   02:04.0 L1-
> > > > >   04:00.0 L1-
> > > > >
> > > > > AFAICT, that should be a legal configuration as far as 04:00.0 is
> > > > > concerned, so it's not a reason for this patch.
> > > >
> > > > Actually, no, maximum path latency 64us
> > > >
> > > > 04:00.0 wakeup latency == 64us
> > > >
> > > > Again, as stated, it can't be behind any sleeping L1 links
> > >
> > > It would be pointless for a device to advertise L1 support if it could
> > > never be used.  04:00.0 advertises that it can tolerate L1 latency of
> > > 64us and that it can exit L1 in 64us or less.  So it *can* be behind a
> > > Link in L1 as long as nothing else in the path adds more latency.
> >
> > Yes, as long as nothing along the entire path adds latency - and I
> > didn't make the component
> > I can only say what it states, and we have to handle it.
> >
> > > > > [2] is a previous posting of this same patch, and its commit log
> > > > > includes information about the same path to 04:00.x, but the "LnkCtl
> > > > > Before" column shows:
> > > > >
> > > > >   01:00.2 L1+               # <-- my typo here, should be 00:01.2
> > > > >   01:00.0 L1+
> > > > >   02:04.0 L1+
> > > > >   04:00.0 L1+
> > > > >
> > > > > I don't know why [1] shows L1 disabled on the downstream Link, while
> > > > > [2] shows L1 *enabled* on the same Link.
> > > >
> > > > From the data they look switched.
> > > >
> > > > > > > > Kai-Heng Feng has a machine that will not boot with ASPM without
> > > > > > > > this patch, information is documented here:
> > > > > > > > https://bugzilla.kernel.org/show_bug.cgi?id=209671
> > > > > > >
> > > > > > > I started working through this info, too, but there's not
> > > > > > > enough information to tell what difference this patch
> > > > > > > makes.  The attachments compare:
> > > > > > >
> > > > > > >   1) CONFIG_PCIEASPM_DEFAULT=y without the patch [3] and
> > > > > > >   2) CONFIG_PCIEASPM_POWERSAVE=y *with* the patch [4]
> > > > > > >
> > > > > > > Obviously CONFIG_PCIEASPM_POWERSAVE=y will configure
> > > > > > > things differently than CONFIG_PCIEASPM_DEFAULT=y, so we
> > > > > > > can't tell what changes are due to the config change and
> > > > > > > what are due to the patch.
> > > > > > >
> > > > > > > The lspci *with* the patch ([4]) shows L0s and L1 enabled
> > > > > > > at almost every possible place.  Here are the Links, how
> > > > > > > they're configured, and my analysis of the exit latencies
> > > > > > > vs acceptable latencies:
> > > > > > >
> > > > > > >   00:01.1 --- 01:00.0      L1+ (                  L1 <64us vs unl)
> > > > > > >   00:01.2 --- 02:00.0      L1+ (                  L1 <64us vs 64us)
> > > > > > >   00:01.3 --- 03:00.0      L1+ (                  L1 <64us vs 64us)
> > > > > > >   00:01.4 --- 04:00.0      L1+ (                  L1 <64us vs unl)
> > > > > > >   00:08.1 --- 05:00.x L0s+ L1+ (L0s <64ns vs 4us, L1  <1us vs unl)
> > > > > > >   00:08.2 --- 06:00.0 L0s+ L1+ (L0s <64ns vs 4us, L1  <1us vs unl)
> > > > > > >
> > > > > > > So I can't tell what change prevents the freeze.  I would
> > > > > > > expect the patch would cause us to *disable* L0s or L1
> > > > > > > somewhere.
> > > > > > >
> > > > > > > The only place [4] shows ASPM disabled is for 05:00.1.
> > > > > > > The spec says we should program the same value in all
> > > > > > > functions of a multi-function device.  This is a non-ARI
> > > > > > > device, so "only capabilities enabled in all functions are
> > > > > > > enabled for the component as a whole."  That would mean
> > > > > > > that L0s and L1 are effectively disabled for 05:00.x even
> > > > > > > though 05:00.0 claims they're enabled.  But the latencies
> > > > > > > say ASPM L0s and L1 should be safe to be enabled.  This
> > > > > > > looks like another bug that's probably unrelated.
> > > > > >
> > > > > > I don't think it's unrelated, i suspect it's how PCIe works with
> > > > > > multiple links...  a device can cause some kind of head of queue
> > > > > > stalling - i don't know how but it really looks like it.
> > > > >
> > > > > The text in quotes above is straight out of the spec (PCIe r5.0, sec
> > > > > 7.5.3.7).  Either the device works that way or it's not compliant.
> > > > >
> > > > > The OS configures ASPM based on the requirements and capabilities
> > > > > advertised by the device.  If a device has any head of queue stalling
> > > > > or similar issues, those must be comprehended in the numbers
> > > > > advertised by the device.  It's not up to the OS to speculate about
> > > > > issues like that.
> > > > >
> > > > > > > The patch might be correct; I haven't actually analyzed
> > > > > > > the code.  But the commit log doesn't make sense to me
> > > > > > > yet.
> > > > > >
> > > > > > I personally don't think that all this PCI information is required,
> > > > > > the linux kernel is currently doing it wrong according to the spec.
> > > > >
> > > > > We're trying to establish exactly *what* Linux is doing wrong.  So far
> > > > > we don't have a good explanation of that.
> > > >
> > > > Yes we do, linux counts hops + max for "link" while what should be done is
> > > > counting hops + max for path
> > >
> > > I think you're saying we need to include L1 exit latency even for
> > > Links where L1 is disabled.  I don't think we should include those.
> >
> > Nope, the code does not do that, it only adds the l1 latency on L1
> > enabled hops
> >
> > > > > Based on [1], in the path to 03:00.0, both Links have L1 enabled, with
> > > > > an exit latency of <33us, and 03:00.0 can tolerate 64us.  That should
> > > > > work fine.
> > > > >
> > > > > Also based on [1], in the path to 04:00.x, the upstream Link has L1
> > > > > enabled and the downstream Link has L1 disabled, for an exit latency
> > > > > of <32us, and 04:00.0 can tolerate 64us.  That should also work fine.
> > > >
> > > > Again, ignoring the exit latency for 04:00.0
> > > >
> > > > > (Alternately, disabling L1 on the upstream Link and enabling it on the
> > > > > downstream Link should have an exit latency of <64us and 04:00.0 can
> > > > > tolerate 64us, so that should work fine, too.)
> > > >
> > > > Then nothing else can have L1 aspm enabled
> > >
> > > Yes, as I said, we should be able to enable L1 on either of the Links
> > > in the path to 04:00.x, but not both.
> >
> > The code works backwards and disables the first hop that exceeds the
> > latency requirements -
> > we could argue that it should try to be smarter about it and try to
> > disable a minimum amount of links
> > while still retaining the minimum latency but... It is what it is and
> > it works when patched.
> >
> > > The original problem here is not with the Realtek device at 04:00.x
> > > but with the I211 NIC at 03:00.0.  So we also need to figure out what
> > > the connection is.  Does the same I211 performance problem occur if
> > > you remove the Realtek device from the system?
> >
> > It's mounted on the motherboard, so no I can't remove it.
>
> If you're interested, you could probably unload the Realtek drivers,
> remove the devices, and set the PCI_EXP_LNKCTL_LD (Link Disable) bit
> in 02:04.0, e.g.,
>
>   # RT=/sys/devices/pci0000:00/0000:00:01.2/0000:01:00.0/0000:02:04.0
>   # echo 1 > $RT/0000:04:00.0/remove
>   # echo 1 > $RT/0000:04:00.1/remove
>   # echo 1 > $RT/0000:04:00.2/remove
>   # echo 1 > $RT/0000:04:00.4/remove
>   # echo 1 > $RT/0000:04:00.7/remove
>   # setpci -s02:04.0 CAP_EXP+0x10.w=0x0010
>
> That should take 04:00.x out of the picture.

Didn't actually change the behaviour, I'm suspecting an errata for AMD pcie...

So did this, with unpatched kernel:
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.00   sec  4.56 MBytes  38.2 Mbits/sec    0   67.9 KBytes
[  5]   1.00-2.00   sec  4.47 MBytes  37.5 Mbits/sec    0   96.2 KBytes
[  5]   2.00-3.00   sec  4.85 MBytes  40.7 Mbits/sec    0   50.9 KBytes
[  5]   3.00-4.00   sec  4.23 MBytes  35.4 Mbits/sec    0   70.7 KBytes
[  5]   4.00-5.00   sec  4.23 MBytes  35.4 Mbits/sec    0   48.1 KBytes
[  5]   5.00-6.00   sec  4.23 MBytes  35.4 Mbits/sec    0   45.2 KBytes
[  5]   6.00-7.00   sec  4.23 MBytes  35.4 Mbits/sec    0   36.8 KBytes
[  5]   7.00-8.00   sec  3.98 MBytes  33.4 Mbits/sec    0   36.8 KBytes
[  5]   8.00-9.00   sec  4.23 MBytes  35.4 Mbits/sec    0   36.8 KBytes
[  5]   9.00-10.00  sec  4.23 MBytes  35.4 Mbits/sec    0   48.1 KBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec  43.2 MBytes  36.2 Mbits/sec    0             sender
[  5]   0.00-10.00  sec  42.7 MBytes  35.8 Mbits/sec                  receiver

and:
echo 0 > /sys/devices/pci0000:00/0000:00:01.2/0000:01:00.0/link/l1_aspm

and:
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.00   sec   113 MBytes   951 Mbits/sec  153    772 KBytes
[  5]   1.00-2.00   sec   109 MBytes   912 Mbits/sec  276    550 KBytes
[  5]   2.00-3.00   sec   111 MBytes   933 Mbits/sec  123    625 KBytes
[  5]   3.00-4.00   sec   111 MBytes   933 Mbits/sec   31    687 KBytes
[  5]   4.00-5.00   sec   110 MBytes   923 Mbits/sec    0    679 KBytes
[  5]   5.00-6.00   sec   110 MBytes   923 Mbits/sec  136    577 KBytes
[  5]   6.00-7.00   sec   110 MBytes   923 Mbits/sec  214    645 KBytes
[  5]   7.00-8.00   sec   110 MBytes   923 Mbits/sec   32    628 KBytes
[  5]   8.00-9.00   sec   110 MBytes   923 Mbits/sec   81    537 KBytes
[  5]   9.00-10.00  sec   110 MBytes   923 Mbits/sec   10    577 KBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec  1.08 GBytes   927 Mbits/sec  1056             sender
[  5]   0.00-10.00  sec  1.07 GBytes   923 Mbits/sec                  receiver

But this only confirms that the fix i experience is a side effect.

The original code is still wrong :)

> > > 03:00.0 can tolerate 64us of latency, so even if L1 is enabled on both
> > > Links leading to it, the path exit latency would be <33us, which
> > > should be fine.
> >
> > Yes, it "should be" but due to broken ASPM latency calculations we
> > have some kind of
> > side effect that triggers a racecondition/sideeffect/bug that causes
> > it to misbehave.
> >
> > Since fixing the latency calculation fixes it, I'll leave the rest to
> > someone with a logic
> > analyzer and a die-hard-fetish for pcie links - I can't debug it.
> >
> > > > > > Also, since it's clearly doing the wrong thing, I'm worried that
> > > > > > dists will take a kernel enable aspm and there will be alot of
> > > > > > bugreports of non-booting systems or other weird issues... And the
> > > > > > culprit was known all along.
> > > > >
> > > > > There's clearly a problem on your system, but I don't know yet whether
> > > > > Linux is doing something wrong, a device in your system is designed
> > > > > incorrectly, or a device is designed correctly but the instance in
> > > > > your system is defective.
> > > >
> > > > According to the spec it is, there is a explanation of how to
> > > > calculate the exit latency
> > > > and when you implement that, which i did (before knowing the actual
> > > > spec) then it works...
> > > >
> > > > > > It's been five months...
> > > > >
> > > > > I apologize for the delay.  ASPM is a subtle area of PCIe, the Linux
> > > > > code is complicated, and we have a long history of issues with it.  I
> > > > > want to fix the problem, but I want to make sure we do it in a way
> > > > > that matches the spec so the fix applies to all systems.  I don't want
> > > > > a magic fix that fixes your system in a way I don't quite understand.
> > > >
> > > > > Obviously *you* understand this, so hopefully it's just a matter of
> > > > > pounding it through my thick skull :)
> > > >
> > > > I only understand what I've been forced to understand - and I do
> > > > leverage the existing code without
> > > > knowing what it does underneath, I only look at the links maximum
> > > > latency and make sure that I keep
> > > > the maximum latency along the path and not just link for link
> > > >
> > > > once you realise that the max allowed latency is buffer dependent -
> > > > then this becomes obviously correct,
> > > > and then the pcie spec showed it as being correct as well... so...
> > > >
> > > >
> > > > > > > [1] https://bugzilla.kernel.org/attachment.cgi?id=293047
> > > > > > > [2] https://lore.kernel.org/linux-pci/20201007132808.647589-1-ian.kumlien@gmail.com/
> > > > > > > [3] https://bugzilla.kernel.org/attachment.cgi?id=292955
> > > > > > > [4] https://bugzilla.kernel.org/attachment.cgi?id=292957
> > > > > > >
> > > > > > > > Signed-off-by: Ian Kumlien <ian.kumlien@...il.com>
> > > > > > > > Tested-by: Kai-Heng Feng <kai.heng.feng@...onical.com>
> > > > > > > > ---
> > > > > > > >  drivers/pci/pcie/aspm.c | 22 ++++++++++++++--------
> > > > > > > >  1 file changed, 14 insertions(+), 8 deletions(-)
> > > > > > > >
> > > > > > > > diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> > > > > > > > index 253c30cc1967..c03ead0f1013 100644
> > > > > > > > --- a/drivers/pci/pcie/aspm.c
> > > > > > > > +++ b/drivers/pci/pcie/aspm.c
> > > > > > > > @@ -434,7 +434,7 @@ static void pcie_get_aspm_reg(struct pci_dev *pdev,
> > > > > > > >
> > > > > > > >  static void pcie_aspm_check_latency(struct pci_dev *endpoint)
> > > > > > > >  {
> > > > > > > > -     u32 latency, l1_switch_latency = 0;
> > > > > > > > +     u32 latency, l1_max_latency = 0, l1_switch_latency = 0;
> > > > > > > >       struct aspm_latency *acceptable;
> > > > > > > >       struct pcie_link_state *link;
> > > > > > > >
> > > > > > > > @@ -456,10 +456,14 @@ static void pcie_aspm_check_latency(struct pci_dev *endpoint)
> > > > > > > >               if ((link->aspm_capable & ASPM_STATE_L0S_DW) &&
> > > > > > > >                   (link->latency_dw.l0s > acceptable->l0s))
> > > > > > > >                       link->aspm_capable &= ~ASPM_STATE_L0S_DW;
> > > > > > > > +
> > > > > > > >               /*
> > > > > > > >                * Check L1 latency.
> > > > > > > > -              * Every switch on the path to root complex need 1
> > > > > > > > -              * more microsecond for L1. Spec doesn't mention L0s.
> > > > > > > > +              *
> > > > > > > > +              * PCIe r5.0, sec 5.4.1.2.2 states:
> > > > > > > > +              * A Switch is required to initiate an L1 exit transition on its
> > > > > > > > +              * Upstream Port Link after no more than 1 μs from the beginning of an
> > > > > > > > +              * L1 exit transition on any of its Downstream Port Links.
> > > > > > > >                *
> > > > > > > >                * The exit latencies for L1 substates are not advertised
> > > > > > > >                * by a device.  Since the spec also doesn't mention a way
> > > > > > > > @@ -469,11 +473,13 @@ static void pcie_aspm_check_latency(struct pci_dev *endpoint)
> > > > > > > >                * L1 exit latencies advertised by a device include L1
> > > > > > > >                * substate latencies (and hence do not do any check).
> > > > > > > >                */
> > > > > > > > -             latency = max_t(u32, link->latency_up.l1, link->latency_dw.l1);
> > > > > > > > -             if ((link->aspm_capable & ASPM_STATE_L1) &&
> > > > > > > > -                 (latency + l1_switch_latency > acceptable->l1))
> > > > > > > > -                     link->aspm_capable &= ~ASPM_STATE_L1;
> > > > > > > > -             l1_switch_latency += 1000;
> > > > > > > > +             if (link->aspm_capable & ASPM_STATE_L1) {
> > > > > > > > +                     latency = max_t(u32, link->latency_up.l1, link->latency_dw.l1);
> > > > > > > > +                     l1_max_latency = max_t(u32, latency, l1_max_latency);
> > > > > > > > +                     if (l1_max_latency + l1_switch_latency > acceptable->l1)
> > > > > > > > +                             link->aspm_capable &= ~ASPM_STATE_L1;
> > > > > > > > +                     l1_switch_latency += 1000;
> > > > > > > > +             }
> > > > > > > >
> > > > > > > >               link = link->parent;
> > > > > > > >       }
> > > > > > > > --
> > > > > > > > 2.29.1
> > > > > > > >

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