Re: [tip:sched/urgent] sched: Fix cross-cpu clock sync on remotewakeups

[Peter Zijlstra]

On Tue, 2011-05-31 at 14:56 +0200, Borislav Petkov wrote:
> Hey Peter,
> 
> On Tue, May 31, 2011 at 12:31:20PM +0000, tip-bot for Peter Zijlstra wrote:
> > Commit-ID:  f01114cb59d670e9b4f2c335930dd57db96e9360
> > Gitweb:     http://git.kernel.org/tip/f01114cb59d670e9b4f2c335930dd57db96e9360
> > Author:     Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> > AuthorDate: Tue, 31 May 2011 12:26:55 +0200
> > Committer:  Ingo Molnar <mingo@xxxxxxx>
> > CommitDate: Tue, 31 May 2011 14:19:56 +0200
> > 
> > sched: Fix cross-cpu clock sync on remote wakeups
> > 
> > Markus reported that commit 317f394160e ("sched: Move the second half
> > of ttwu() to the remote cpu") caused some accounting funnies on his AMD
> > Phenom II X4, such as weird 'top' results.
> > 
> > It turns out that this is due to non-synced TSC
> 
> this would mean that his machine doesn't pass the TSC sync check at boot
> but that's a F10h and they usu. have synchronized TSCs?
> 
> I'm confused.

Well, I don't have a modern AMD system to verify on, but the only
explanation is sched_clock weirdness (different code from the GTOD tsc
stuff). I could not reproduce on an Intel Westmere machine, but could on
a Core2.

The sched_clock_cpu stuff basically takes a GTOD timestamp every tick
and uses sched_clock() (tsc + cyc2ns) to provide delta increments, when
TSCs are synced every cpu should return the same value and the patch is
a nop.

If they aren't synced the per-cpu sched_clock_cpu() values can drift up
to about 2 jiffies (when the ticks drift about 1 and the slower of the
two has a stuck tsc while the faster of the two does progress at the
normal rate). In that case doing a clock update cross-cpu will ensure
time monotonicity between those two cpus.

So by this patch making a difference we prove that sched_clock_cpu()'s
aren't synced on the affected machines. That leaves two options, either
the TSC are screwy or there's a bug somewhere in the sched_clock* code.

I'd be more than happy if you could take a look at the relevant code
since all code can use more eyes :-)
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