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

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Message-ID: <20120206153408.GA31237@aftab>
Date:	Mon, 6 Feb 2012 16:34:08 +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,
	Borislav Petkov <bp@...64.org>,
	Andreas Herrmann <andreas.herrmann3@....com>
Subject: Re: [BUG] perf: perf sched warning possibly due to clock granularity
 on AMD

+ Andreas.

On Mon, Feb 06, 2012 at 03:26:43PM +0100, Peter Zijlstra wrote:
> On Mon, 2012-02-06 at 14:25 +0100, Stephane Eranian wrote:
> > In summary, two issues:
> >   - Why is sched_clock_stable not set or even tested on recent AMD systems?

AFAICT, sched_clock_stable is set on Intel under the following conditions:

        /*
         * c->x86_power is 8000_0007 edx. Bit 8 is TSC runs at constant rate
         * with P/T states and does not stop in deep C-states.
         *
         * It is also reliable across cores and sockets. (but not across
         * cabinets - we turn it off in that case explicitly.)
         */
        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;
        }

and yes, we can do CONSTANT_TSC and NONSTOP_TSC on the now older F10h
already, so Bulldozer can do that too, implicitly.

I don't see why sched_clock_stable shouldn't be moved to generic x86
code and set based on the two CPUID cap flags above.

HTH.

-- 
Regards/Gruss,
Boris.

Advanced Micro Devices GmbH
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