linux-kernel - Re: Why kmem_cache_free occupy CPU for more than 10 seconds?

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Message-Id: <1176369596.6893.105.camel@twins>
Date:	Thu, 12 Apr 2007 11:19:56 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Zhao Forrest <forrest.zhao@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: Why kmem_cache_free occupy CPU for more than 10 seconds?

On Thu, 2007-04-12 at 00:55 -0700, Andrew Morton wrote:
> On Thu, 12 Apr 2007 09:39:25 +0200 Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> 
> > On Wed, 2007-04-11 at 15:30 -0700, Andrew Morton wrote:
> > 
> > > There used to be a cond_resched() in invalidate_mapping_pages() which would
> > > have prevented this, but I rudely removed it to support
> > > /proc/sys/vm/drop_caches (which needs to call invalidate_inode_pages()
> > > under spinlock).
> > > 
> > > We could resurrect that cond_resched() by passing in some flag, I guess. 
> > > Or change the code to poke the softlockup detector.  The former would be
> > > better.
> > 
> > cond_resched() is conditional on __resched_legal(0)
> 
> What's __resched_legal()?

Hmm, I was looking at the RHEL-5 sources, it seems this function was
removed later (commmit 9414232fa0cc28e2f51b8c76d260f2748f7953fc).

> > which should take
> > care of being called under a spinlock.
> 
> We only increment preempt_count() in spin_lock() if CONFIG_PREEMPT.

Right, ok, that makes the whole thing fall apart indeed.

> > Index: linux-2.6-mm/mm/truncate.c
> > ===================================================================
> > --- linux-2.6-mm.orig/mm/truncate.c
> > +++ linux-2.6-mm/mm/truncate.c
> > @@ -292,6 +292,8 @@ unsigned long invalidate_mapping_pages(s
> >  			pgoff_t index;
> >  			int lock_failed;
> >  
> > +			cond_resched();
> > +
> >  			lock_failed = TestSetPageLocked(page);
> 
> Is deadlocky on the drop_caches path and if CONFIG_PREEMPT we'll get
> scheduling-in-spinlock warnings.

*nod*

> For the blkdev_close() path the change is unneeded if CONFIG_PREEMPT and
> will fix things if !CONFIG_PREEMPT.
> 
> We can presumably just remove the invalidate_mapping_pages() call from the
> kill_bdev() path (at least) - kill_bdev()'s truncate_inode_pages() will do
> the same thing.

Certainly looks that way, I'll see if I can spot a hole in that.

> It might be time to remove that unused-for-six-years destroy_dirty_buffers
> too.

ok.

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