Messages in this thread | | | Date | Mon, 6 Feb 2012 13:19:44 -0800 | Subject | Re: [BUG] perf: perf sched warning possibly due to clock granularity on AMD | From | Venki Pallipadi <> |
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On Mon, Feb 6, 2012 at 12:37 PM, Borislav Petkov <bp@amd64.org> wrote: > 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. >
Looks like cabinet comment came from Ingo (commit 83ce4009) in reference to (We will turn this off in DMI quirks for multi-chassis systems)
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().
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