linux-kernel - Re: [git pull] scheduler updates

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Date:	Mon, 17 Nov 2008 15:04:03 -0800
From:	Venki Pallipadi <venkatesh.pallipadi@...el.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>
Subject: Re: [git pull] scheduler updates

On Mon, Nov 17, 2008 at 02:50:18PM -0800, Ingo Molnar wrote:
> 
> * Venki Pallipadi <venkatesh.pallipadi@...el.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


Thanks,
Venki
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