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

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Message-ID: <20120206203738.GB556@aftab>
Date:	Mon, 6 Feb 2012 21:37:38 +0100
From:	Borislav Petkov <bp@...64.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Stephane Eranian <eranian@...gle.com>,
	linux-kernel@...r.kernel.org, acme@...hat.com, mingo@...e.hu,
	robert.richter@....com, eric.dumazet@...il.com,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Venkatesh Pallipadi <venki@...gle.com>
Subject: Re: [BUG] perf: perf sched warning possibly due to clock granularity
 on AMD

On Mon, Feb 06, 2012 at 09:31:33PM +0100, Peter Zijlstra wrote:
> On Mon, 2012-02-06 at 21:27 +0100, Borislav Petkov wrote:
> > On Mon, Feb 06, 2012 at 05:54:19PM +0100, Peter Zijlstra wrote:
> > > On Mon, 2012-02-06 at 17:46 +0100, Borislav Petkov wrote:
> > > > > across all CPUs in the entire system.
> > > > 
> > > > Right, by the "entire system" you mean consistent across cores and
> > > > sockets but not necessarily across cabinets, as in the comment above,
> > > > correct?
> > > > 
> > > > If so, let me ask around if this holds true too.
> > > 
> > > Every CPU available to the kernel. So if you run a single system image
> > > across your cabinets, then yes those too.
> > 
> > Ok, but what about that sentence "(but not across cabinets - we turn
> > it off in that case explicitly.)" - I don't see any place where it is
> > turned off explicitly... Maybe a stale comment?
> 
> I suspect it might be the sched_clock_stable = 0 in mark_tsc_unstable(),
> but lets ask Venki, IIRC he wrote all that.

Yeah, I was looking at the code further and on Intel it does:

        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;
        }

while on AMD, in early_init_amd() we do:

        if (c->x86_power & (1 << 8)) {
                set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
                set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
        }

and having in mind that tsc_unstable is set on generic x86 paths,
nothing stops us to do the same on AMD too, and as a result, set
sched_clock_stable too.

But yeah, let's see what Venki has to say first.

Thanks.

-- 
Regards/Gruss,
Boris.

Advanced Micro Devices GmbH
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