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    SubjectRe: [BUG] perf: perf sched warning possibly due to clock granularity on AMD
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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().

    Thanks,
    Venki
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