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SubjectRe: [GIT PULL rcu/next] fixes and breakup of memory-barrier-decrease patch

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

> On Sat, May 21, 2011 at 04:28:44PM +0200, Ingo Molnar wrote:
> >
> > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> >
> > > Hello, Ingo,
> > >
> > > This pull requests covers some RCU bug fixes and one patch rework.
> > >
> > > The first group breaks up the infamous now-reverted (but ultimately
> > > vindicated) "Decrease memory-barrier usage based on semi-formal proof"
> > > commit into five commits. These five commits immediately follow the
> > > revert, and the diff across all six of these commits is empty, so that
> > > the effect of the five commits is to revert the revert.
> >
> > But ... the regression that was observed with that commit needs to be fixed
> > first, or not? In what way was the barrier commit vindicated?
>
> From what I can see, the hang was fixed by Frederic's patch at
> https://lkml.org/lkml/2011/5/19/753. I was interpreting that as vindication,
> perhaps ill-advisedly.

I mean, without Frederic's patch we are getting very long hangs due to the
barrier patch, right?

Even if the barrier patch is not to blame - somehow it still managed to produce
these hangs - and we do not understand it yet.

> Yinghai said that he was still seeing a delay, adn that he was seeing it even
> with the "Decrease memory-barrier usage based on semi-formal proof" reverted:
> https://lkml.org/lkml/2011/5/20/427. This hang seems to happen when he uses
> gcc 4.5.0, but not when using gcc 4.5.1, assuming I understood his sequence
> of emails. So I was interpreting that as meaning that the delay was unlikely
> to be caused by that commit, probably by one of the later commits.
>
> I clearly need to figure out what is causing this delay. I asked Yinghai to
> apply c7a378603 (Remove waitqueue usage for cpu, node, and boost kthreads)
> from Peter Zijlstra because the long delays that Yinghai is seeing (93
> seconds for memory_dev_init() rather than 3 or 4 seconds) might be due to my
> less-efficient method of awakening the RCU kthreads, so that Peter's
> approache might help.
>
> If that doesn't speed things up for Yinghai, then I will work out some
> tracing to help localize the slowdown that he is seeing.
>
> Of course, if you would rather that I get to the bottom of this before
> pulling, fair enough!

We should fix the delay regression i suspect - do we have to revert more stuff
perhaps?

Would it be possible to figure out what caused that other delay for Yinghai?

Thanks,

Ingo


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