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SubjectRe: [git pull] scheduler updates

* Venki Pallipadi <venkatesh.pallipadi@intel.com> wrote:

> On Mon, Nov 17, 2008 at 02:50:18PM -0800, Ingo Molnar wrote:
> >
> > * Venki Pallipadi <venkatesh.pallipadi@intel.com> wrote:
> >
> > > Patch being discussed on this thread (commit 0d12cdd) has a
> > > regression on one of the test systems here.
> > >
> > > With the patch, I see
> > >
> > > checking TSC synchronization [CPU#0 -> CPU#1]:
> > > Measured 28 cycles TSC warp between CPUs, turning off TSC clock.
> > > Marking TSC unstable due to check_tsc_sync_source failed
> > >
> > > Whereas, without the patch syncs pass fine on all CPUs
> > >
> > > checking TSC synchronization [CPU#0 -> CPU#1]: passed.
> > >
> > > Due to this, TSC is marke unstable, when it is not actually unstable.
> > > This is because syncs in check_tsc_wrap() goes away due to this commit.
> > >
> > > As per the discussion on this thread, correct way to fix this is to add
> > > explicit syncs as below?
> >
> > ah. Yes.
> >
> > Could you please check whether:
> >
> > > + rdtsc_barrier();
> > > start = get_cycles();
> > > + rdtsc_barrier();
> > > /*
> > > * The measurement runs for 20 msecs:
> > > */
> > > @@ -61,7 +63,9 @@ static __cpuinit void check_tsc_warp(voi
> > > */
> > > __raw_spin_lock(&sync_lock);
> > > prev = last_tsc;
> > > + rdtsc_barrier();
> > > now = get_cycles();
> > > + rdtsc_barrier();
> >
> > adding the barrier just _after_ the get_cycles() call (but not before
> > it) does the trick too? That should be enough in this case.
> >
>
> With barrier only after get_cycles, I do see syncs across first few
> CPUs passing. But later I see:
>
> checking TSC synchronization [CPU#0 -> CPU#13]: Measured 4 cycles
> TSC warp between CPUs, turning off TSC clock. Marking TSC unstable
> due to check_tsc_sync_source failed

yeah - has to be surrounded, to make sure our last_tsc observation
does not happen after the RDTSC.

I have applied your patch to tip/x86/urgent, thanks!

Ingo


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