linux-kernel - Re: [PATCH] vfs: dcache: cond_resched in shrink_dentry

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Message-Id: <20130410164455.a3cbcbdf86bc72455c22f420@linux-foundation.org>
Date:	Wed, 10 Apr 2013 16:44:55 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Greg Thelen <gthelen@...gle.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Dave Chinner <david@...morbit.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] vfs: dcache: cond_resched in shrink_dentry_list

On Tue, 09 Apr 2013 17:37:20 -0700 Greg Thelen <gthelen@...gle.com> wrote:

> > Call cond_resched() in shrink_dcache_parent() to maintain
> > interactivity.
> >
> > Before this patch:
> >
> > void shrink_dcache_parent(struct dentry * parent)
> > {
> > 	while ((found = select_parent(parent, &dispose)) != 0)
> > 		shrink_dentry_list(&dispose);
> > }
> >
> > select_parent() populates the dispose list with dentries which
> > shrink_dentry_list() then deletes.  select_parent() carefully uses
> > need_resched() to avoid doing too much work at once.  But neither
> > shrink_dcache_parent() nor its called functions call cond_resched().
> > So once need_resched() is set select_parent() will return single
> > dentry dispose list which is then deleted by shrink_dentry_list().
> > This is inefficient when there are a lot of dentry to process.  This
> > can cause softlockup and hurts interactivity on non preemptable
> > kernels.
> >
> > This change adds cond_resched() in shrink_dcache_parent().  The
> > benefit of this is that need_resched() is quickly cleared so that
> > future calls to select_parent() are able to efficiently return a big
> > batch of dentry.
> >
> > These additional cond_resched() do not seem to impact performance, at
> > least for the workload below.
> >
> > Here is a program which can cause soft lockup on a if other system
> > activity sets need_resched().

I was unable to guess what word was missing from "on a if other" ;)

> Should this change go through Al's or Andrew's branch?

I'll fight him for it.


Softlockups are fairly serious, so I'll put a cc:stable in there.  Or
were the changes which triggered this problem added after 3.9?

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