linux-kernel - Re: [BUG] perf: perf sched warning possibly due to clock granularity on AMD

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Date:	Tue, 7 Feb 2012 10:50:00 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Borislav Petkov <bp@...64.org>
Cc:	Venki Pallipadi <venki@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephane Eranian <eranian@...gle.com>,
	linux-kernel@...r.kernel.org, acme@...hat.com,
	robert.richter@....com, eric.dumazet@...il.com,
	Andreas Herrmann <andreas.herrmann3@....com>
Subject: Re: [BUG] perf: perf sched warning possibly due to clock granularity
 on AMD


* Borislav Petkov <bp@...64.org> wrote:

> On Tue, Feb 07, 2012 at 09:32:53AM +0100, Ingo Molnar wrote:
> > > Yes. If these two flags are set, TSC should be consistent and 
> > > sched_clock_stable could be set and it will be reset if there 
> > > is a call to mark_tsc_unstable().
> > 
> > Most of the details swapped out from my brain meanwhile, but I 
> > have some vague memories of a DMI quirk for some high-end system 
> > that just did a sched_clock_stable = 0 or such.
> > 
> > So if the common case is that the TSC is entirely synchronized 
> > across CPUs, then we can default to that and rely on platform 
> > initialization code or DMI quirks setting the few large-NUMA 
> > systems to an unstable TSC.
> 
> There's also 14be1f7454ea96ee614467a49cf018a1a383b189 which removed
> the setting of sched_clock_stable to 1 due to UV systems not being
> TSC-synchronized across blades.
> 
> I guess, we could tentatively enable it on AMD provided nothing has
> marked the TSC as unstable already:
> 
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index f4773f4..ddee619 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -456,6 +456,8 @@ static void __cpuinit early_init_amd(struct cpuinfo_x86 *c)
>         if (c->x86_power & (1 << 8)) {
>                 set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
>                 set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
> +               if (!check_tsc_unstable())
> +                       sched_clock_stable = 1;
>         }

i'd go for that.

Thanks,

	Ingo
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