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Date: Mon, 06 Feb 2012 17:37:26 +0100 From: Peter Zijlstra <peterz@...radead.org> To: Borislav Petkov <bp@...64.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> Subject: Re: [BUG] perf: perf sched warning possibly due to clock granularity on AMD On Mon, 2012-02-06 at 16:34 +0100, Borislav Petkov wrote: > + 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. stable means more than both those flags together.. - CONSTANT_TSC means freq independent - NONSTOP_TSC means it doesn't get stopped in any C state Together they make TSC completely C-state independent. sched_clock_stable actually means that TSC is both _AND_ is consistent across all CPUs in the entire system. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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