Date:   Mon, 16 Oct 2017 06:25:53 +0000
From:   Yuval Mintz <yuvalm@...lanox.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>,
        "davem@...emloft.net" <davem@...emloft.net>
CC:     "huangdaode@...ilicon.com" <huangdaode@...ilicon.com>,
        "xuwei5@...ilicon.com" <xuwei5@...ilicon.com>,
        "liguozhu@...ilicon.com" <liguozhu@...ilicon.com>,
        "Yisen.Zhuang@...wei.com" <Yisen.Zhuang@...wei.com>,
        "gabriele.paoloni@...wei.com" <gabriele.paoloni@...wei.com>,
        "john.garry@...wei.com" <john.garry@...wei.com>,
        "linuxarm@...wei.com" <linuxarm@...wei.com>,
        "salil.mehta@...wei.com" <salil.mehta@...wei.com>,
        "lipeng321@...wei.com" <lipeng321@...wei.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH net-next 0/2] Add mqprio hardware offload support in hns3
 driver

> Hi, Yuval
> 
> On 2017/10/15 13:14, Yuval Mintz wrote:
> >> Hi, Yuval
> >>
> >> On 2017/10/13 4:21, Yuval Mintz wrote:
> >>>> This patchset adds a new hardware offload type in mqprio before
> adding
> >>>> mqprio hardware offload support in hns3 driver.
> >>>
> >>> I think one of the biggest issues in tying this to DCB configuration is the
> >>> non-immediate [and possibly non persistent] configuration.
> >>>
> >>> Scenario #1:
> >>> User is configuring mqprio offloaded with 3 TCs while device is in willing
> >> mode.
> >>> Would you expect the driver to immediately respond with a success or
> >> instead
> >>> delay the return until the DCBx negotiation is complete and the
> operational
> >>> num of TCs is actually 3?
> >>
> >> Well, when user requsts the mqprio offloaded by a hardware shared by
> DCB,
> >> I expect
> >> the user is not using the dcb tool.
> >> If user is still using dcb tool, then result is undefined.
> >>
> >> The scenario you mention maybe can be enforced by setting willing to
> zero
> >> when user
> >> is requesting the mqprio offload, and restore the willing bit when
> unloaded
> >> the mqprio
> >> offload.
> >
> > Sounds a bit harsh but would probably work.
> >
> >> But I think the real issue is that dcb and mqprio shares the tc system in the
> >> stack,
> >> the problem may be better to be fixed in the stack rather than in the
> driver,
> >> as you
> >> suggested in the DCB patchset. What do you think?
> >
> > What did you have in mind?
> 
> I was thinking maybe the tc system can provide a notification to mqprio and
> dcb.
> mqprio and dcb register a callback to the tc system, when there is some
> change of
> tc configuration, the tc system call the callback from mqprio and dcb.
> 
> >
> >>
> >>>
> >>> Scenario #2:
> >>> Assume user explicitly offloaded mqprio with 3 TCs, but now DCB
> >> configuration
> >>> has changed on the peer side and 4 TCs is the new negotiated
> operational
> >> value.
> >>> Your current driver logic would change the number of TCs underneath
> the
> >> user
> >>> configuration [and it would actually probably work due to mqprio being a
> >> crappy
> >>> qdisc]. But was that the user actual intention?
> >>> [I think the likely answer in this scenario is 'yes' since the alternative is no
> >> better.
> >>> But I still thought it was worth mentioning]
> >>
> >> You are right, the problem also have something to do with mqprio and dcb
> >> sharing
> >> the tc in the stack.
> >>
> >> Druing testing, when user explicitly offloaded mqprio with 3 TCs, all
> >> queue has a default pfifo mqprio attached, after DCB changes the tc num
> to
> >> 4,
> >> using tc qdisc shows some queue does not have a default pfifo mqprio
> >> attached.
> >
> > Really? Then what did it show?
> > [I assume it has some pfifo attached, and it's an mqprio dump kind of an
> issue]
> 
> When queue size of the ndev is 16 and tc num is 3, we set the real queue size
> to
> 15 ( 5 * 3 = 15), mqprio only attach pfifo to the first 15 queue, when tc num
> change
> to 4 by DCB, we set the real queue size to 16 (4 * 4 = 16).
> So tc qdisc shows the last queue has no qdisc attached.

So there is a qdisc attached - mqprio_attach() attches to all transmission
queues [num_tx_queues] and not only the active ones.
But the flow for mqprio might be lacking the additional qdisc_hash_add()
for the additional queue's qdisc.

> 
> >
> >>
> >> Maybe we can add a callback to notify mqprio the configuration has
> changed.
> >>
> >
> > Which would do what?
> > You already have the notifications available for monitoring using dcbnl logic
> if the
> > configuration change [for user]; So user can re-configure whatever it
> wants.
> 
> Yes, if user is only using dcb tool.
> 
> > But other than dropping all the qdisc configurations and going back to the
> default
> > qdiscs, what default action would mqprio be able to do when configuration
> changes
> > that actually makes sense?
> 
> As explained above, after dcb changing the configuration, some queue may
> have no qdisc
> attached, so I was thinking maybe we can add pfifo to it if there is no qdsic
> attached
> to it.
> 
> Thanks,
> Yunsheng Lin
> 
> >
> >> Thanks
> >> Yunsheng Lin
> >>
> >>>
> >>> Cheers,
> >>> Yuval
> >>>
> >>>>
> >>>> Yunsheng Lin (2):
> >>>>   mqprio: Add a new hardware offload type in mqprio
> >>>>   net: hns3: Add mqprio hardware offload support in hns3 driver
> >>>>
> >>>>  drivers/net/ethernet/hisilicon/hns3/hnae3.h        |  1 +
> >>>>  .../net/ethernet/hisilicon/hns3/hns3pf/hclge_dcb.c | 23
> +++++++++++
> >>>>  .../net/ethernet/hisilicon/hns3/hns3pf/hns3_enet.c | 46
> >> ++++++++++++++-
> >>>> -------
> >>>>  include/uapi/linux/pkt_sched.h                     |  1 +
> >>>>  4 files changed, 55 insertions(+), 16 deletions(-)
> >>>>
> >>>> --
> >>>> 1.9.1
> >>>
> >>>
> >>>
> >

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