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

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Message-ID: <alpine.DEB.2.02.1401081824170.15616@chino.kir.corp.google.com>
Date:	Wed, 8 Jan 2014 18:24:33 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Dave Hansen <dave@...1.net>
cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	cl@...ux-foundation.org, akpm@...ux-foundation.org,
	penberg@...nel.org
Subject: Re: [PATCH] mm: slub: fix ALLOC_SLOWPATH stat

On Mon, 6 Jan 2014, Dave Hansen 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>
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