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

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Date:	Tue, 18 Nov 2008 00:13:05 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Venki Pallipadi <venkatesh.pallipadi@...el.com>
Cc:	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


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

> 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

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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