Message-ID: <20171213160242.GB29572@yu-chen.sh.intel.com>
Date:   Thu, 14 Dec 2017 00:02:42 +0800
From:   Yu Chen <yu.c.chen@...el.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Lukas Wunner <lukas@...ner.de>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Len Brown <len.brown@...el.com>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Rui Zhang <rui.zhang@...el.com>
Subject: Re: [PATCH 3/3][RFC/RFT] PM / sleep: Do not delay the
 synchronization of MTRR during resume

On Wed, Dec 13, 2017 at 01:31:50AM +0100, Rafael J. Wysocki wrote:
> On Tuesday, October 31, 2017 10:58:50 AM CET Yu Chen wrote:
> > From: Chen Yu <yu.c.chen@...el.com>
> > 
> > Sometimes it is too late for the APs to synchronize the MTRR
> > after all the APs have been brought up. In some cases the BIOS
> > might scribble the MTRR across suspend/resume, as a result we
> > might get insane MTRR after resumed back, thus every instruction
> > run on this AP would be extremely slow if it happended to be 'uncached'
> > in the MTRR, although after all the APs have been brought up, the
> > delayed invoking of set_mtrr_state() will adjust the MTRR on APs
> > thus brings everything back to normal. In practice it is necessary
> > to synchronize the MTRR as early as possible to get it fixed during
> > each AP's online. And this is what this patch does.
> > 
> > Moreover, since we have put the synchronization earlier, there is
> > a side effect that, during each AP's online, the other cpus already
> > online will be forced stopped to run mtrr_rendezvous_handler() and
> > reprogram the MTRR again and again. This symptom can be lessened
> > by checking if this cpu has already finished the synchronization
> > during the enable_nonboot_cpus() stage, if it is, we can safely
> > bypass the reprogramming action. (We can not bypass the
> > mtrr_rendezvous_handler(), because the other online cpus must be
> > stopped running the VMX transaction while one cpu is updating
> > its MTRR, which is described here:
> > Commit d0af9eed5aa9 ("x86, pat/mtrr: Rendezvous all the cpus for
> > MTRR/PAT init")
> > 
> > This patch does not impact the normal boot up and cpu hotplug.
> > 
> > On a platform with insane MTRR after resumed,
> > 1 .before this patch:
> >    [  619.810899] Enabling non-boot CPUs ...
> >    [  619.825537] smpboot: Booting Node 0 Processor 1 APIC 0x2
> >    [  621.723809] CPU1 is up
> >    [  621.840228] smpboot: Booting Node 0 Processor 3 APIC 0x3
> >    [  626.690900] CPU3 is up
> > 
> > So it took cpu1 621.723809 - 619.825537 = 1898.272 ms, and
> > cpu3 626.690900 - 621.840228 = 4850.672 ms.
> > 
> > 2. after this patch:
> >    [  106.931790] smpboot: Booting Node 0 Processor 1 APIC 0x2
> >    [  106.948360] CPU1 is up
> >    [  106.986534] smpboot: Booting Node 0 Processor 3 APIC 0x3
> >    [  106.990702] CPU3 is up
> > 
> > It took cpu1 106.948360 - 106.931790 = 16.57 ms, and
> > cpu3 106.990702 - 106.986534 = 4.16 ms.
> > 
> > For comparison, I also verify the suspend on a 88 cpus Xeon Broadwell
> > platform, and the result also shows that with this patch applied,
> > the overall APs online time has decreased a little bit, I think this
> > is because after we synchronize the MTRR earlier, the MTRRs also get
> > updated to the correct value earlier.
> > 
> > I've tested 5 times each, before/after the patch applied:
> > 
> > 1. before this patch:
> > [   64.549430] Enabling non-boot CPUs ...
> > [   66.253304] End enabling non-boot CPUs
> > 
> > overall cpu online: 1.703874 second
> > 
> > [   62.159063] Enabling non-boot CPUs ...
> > [   64.483443] End enabling non-boot CPUs
> > 
> > overall cpu online: 2.32438 second
> > 
> > [   58.351449] Enabling non-boot CPUs ...
> > [   60.796951] End enabling non-boot CPUs
> > 
> > overall cpu online: 2.445502 second
> > 
> > [   64.491317] Enabling non-boot CPUs ...
> > [   66.996208] End enabling non-boot CPUs
> > 
> > overall cpu online: 2.504891 second
> > 
> > [   60.593235] Enabling non-boot CPUs ...
> > [   63.397886] End enabling non-boot CPUs
> > 
> > overall cpu online: 2.804651 second
> > 
> > average: 2.3566596 second
> > 
> > 2. after this patch:
> > 
> > [   66.077368] Enabling non-boot CPUs ...
> > [   68.343374] End enabling non-boot CPUs
> > 
> > overall cpu online: 2.266006 second
> > 
> > [   64.594605] Enabling non-boot CPUs ...
> > [   66.092688] End enabling non-boot CPUs
> > 
> > overall cpu online: 1.498083 second
> > 
> > [   64.778546] Enabling non-boot CPUs ...
> > [   66.277577] End enabling non-boot CPUs
> > 
> > overall cpu online: 1.499031 second
> > 
> > [   63.773091] Enabling non-boot CPUs ...
> > [   65.601637] End enabling non-boot CPUs
> > 
> > overall cpu online: 1.828546 second
> > 
> > [   64.638855] Enabling non-boot CPUs ...
> > [   66.327098] End enabling non-boot CPUs
> > 
> > overall cpu online: 1.688243 second
> > 
> > average: 1.7559818 second
> > 
> > In one word, with the patch applied, the cpu online time during resume
> > has decreased by about 6 seconds on a bogus MTRR platform, and decreased
> > by about 600ms on a 88 cpus Xeon platform after resumed.
> > 
> > Cc: Len Brown <len.brown@...el.com>
> > Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > Cc: Rui Zhang <rui.zhang@...el.com>
> > Signed-off-by: Chen Yu <yu.c.chen@...el.com>
> 
> It will be better to combine this with patch [2/3] IMO, because that makes it
> clear why the changes in that patch are needed.
> 
> Also you can define the new flag in mtrr/main.c, set it in
> arch_enable_nonboot_cpus_begin() and clear it in
> arch_enable_nonboot_cpus_end().  It is better to put it into the arch-specific
> code as the flag itself is arch-specific.
> 
> Then, of course, you don't need patch [1/3] and all can be done in one patch.
> 
Ok, will rewrite the patch, thanks! 

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