linux-kernel - Re: [RFC PATCH 1/2] perf_events: add support for per-cpu per-cgroup monitoring (v3)

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Message-ID: <4C89EF37.5080803@kernel.org>
Date:	Fri, 10 Sep 2010 10:41:27 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Stephane Eranian <eranian@...gle.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, paulus@...ba.org,
	davem@...emloft.net, fweisbec@...il.com,
	perfmon2-devel@...ts.sf.net, eranian@...il.com,
	robert.richter@....com, acme@...hat.com
Subject: Re: [RFC PATCH 1/2] perf_events: add support for per-cpu per-cgroup
 monitoring (v3)

Hello,

On 09/10/2010 10:16 AM, Peter Zijlstra wrote:
> On Thu, 2010-09-09 at 23:41 +0200, Stephane Eranian wrote:
>>> alloc_percpu() is zalloc_percpu() in fact, memory is already cleared.
>>>
>> I remember thinking about this and trying to trace to the code down
>> to figure this out. But it is rather complicated. If alloc_percpu() always
>> clears the memory, then I think that calling is zalloc_percpu()
>> would be more helpful.... 

Maybe but at this point it might be a bit too late.  The allocator has
been that way since the beginning.

> pcpu_populate_chunk() in mm/percpu-vm.c does indeed do that memset, the
> one in mm/percpu-km.c does not.
> 
> It is not obviously clear to me the -km allocator does indeed result in
> zero filled memory.

Nice catch.  Fortunately, the -km allocator isn't currently being used
in upstrea although it was enabled for linux-next a couple of days
ago.  I'll fix it up.

Thank you.

-- 
tejun
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