linux-kernel - RE: [2.6.25-rc5-mm1][regression] ia64: hackbench doesn't finish >12 hour

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Date:	Thu, 20 Mar 2008 00:23:59 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	"Yu, Fenghua" <fenghua.yu@...el.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"Luck, Tony" <tony.luck@...el.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-ia64@...r.kernel.org,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: RE: [2.6.25-rc5-mm1][regression] ia64: hackbench doesn't finish
	>12 hour

On Tue, 2008-03-18 at 17:14 -0700, Yu, Fenghua wrote:
> >this paramter mean use all physical memory and about 1GB swap space.
> >Could you expand swap space?
> 
> We can reproduce the soft lockup issue now and root cause the issue as
> well.
> 
> Since the ptc.g patch uses semaphore ptcg_sem to serialize multiple
> ptc.g instructions in ia64_global_tlb_purge(). This requires the code
> path should be safe to sleep in down(). But the code path can not sleep
> during swap because it holds some spin locks (e.g. anon_vma_lock). Going
> to sleep finally causes soft lockup.
> 
> Actually we though of this issue before releasing the ptcg patch and
> wrote some non-sleeping versions of ptcg patches. But since we couldn't
> see the sleeping issue during our testing, we didn't release a
> non-sleeping ptcg patch. If replacing the ptcg patch in -mm1 tree with
> one of our non-sleeping ptcg patches, the issue goes away.
> 
> Tony and I are working on releasing a final ptcg patch to solve the
> issue.

Which makes me wonder, why did you ever use a semaphore here? Looking at
the code its a straight forward mutex. And when you would have used a
mutex lockdep would have warned about this.

There is hardly ever a good reason to use semaphores in new code, we're
trying very hard to get rid of them.

Hmm, then again, does ia64 have lockdep?

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