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

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Date:	Fri, 10 Sep 2010 10:59:41 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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)

On Fri, Sep 10, 2010 at 10:41 AM, Tejun Heo <tj@...nel.org> wrote:
>
> 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.
>
Then add a comment in the header or at the definition of the function
to make this more explicit.


> > 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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