linux-kernel - Re: [PATCH] mm: slub: fix ALLOC

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Message-ID: <alpine.DEB.2.02.1401211413440.1666@chino.kir.corp.google.com>
Date:	Tue, 21 Jan 2014 14:14:51 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Dave Hansen <dave@...1.net>
cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Christoph Lameter <cl@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>
Subject: Re: [PATCH] mm: slub: fix ALLOC_SLOWPATH stat

On Wed, 8 Jan 2014, David Rientjes wrote:

> > There used to be only one path out of __slab_alloc(), and
> > ALLOC_SLOWPATH got bumped in that exit path.  Now there are two,
> > and a bunch of gotos.  ALLOC_SLOWPATH can now get set more than once
> > during a single call to __slab_alloc() which is pretty bogus.
> > Here's the sequence:
> > 
> > 1. Enter __slab_alloc(), fall through all the way to the
> >    stat(s, ALLOC_SLOWPATH);
> > 2. hit 'if (!freelist)', and bump DEACTIVATE_BYPASS, jump to
> >    new_slab (goto #1)
> > 3. Hit 'if (c->partial)', bump CPU_PARTIAL_ALLOC, goto redo
> >    (goto #2)
> > 4. Fall through in the same path we did before all the way to
> >    stat(s, ALLOC_SLOWPATH)
> > 5. bump ALLOC_REFILL stat, then return
> > 
> > Doing this is obviously bogus.  It keeps us from being able to
> > accurately compare ALLOC_SLOWPATH vs. ALLOC_FASTPATH.  It also
> > means that the total number of allocs always exceeds the total
> > number of frees.
> > 
> > This patch moves stat(s, ALLOC_SLOWPATH) to be called from the
> > same place that __slab_alloc() is.  This makes it much less
> > likely that ALLOC_SLOWPATH will get botched again in the
> > spaghetti-code inside __slab_alloc().
> > 
> > Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> Acked-by: David Rientjes <rientjes@...gle.com>
> 

Pekka, are you going to pick this up for linux-next?  I think it would be 
nice to have for 3.14 for those of us who use the stats.
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