linux-kernel - Re: [PATCH] perf_events: improve DS/BTS/PEBS buffer allocation

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Date:	Mon, 13 Sep 2010 17:08:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	eranian@...gle.com
Cc:	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, markus.t.metzger@...el.com
Subject: Re: [PATCH] perf_events: improve DS/BTS/PEBS buffer allocation

On Mon, 2010-09-13 at 16:55 +0200, Stephane Eranian wrote:
> The DS, BTS, and PEBS memory regions were allocated using kzalloc(), i.e.,
> requesting contiguous physical memory. There is no such restriction on
> DS, PEBS and BTS buffers. Using kzalloc() could lead to error in case
> no contiguous physical memory is available. BTS is requesting 64KB,
> thus it can cause issues. PEBS is currently only requesting one page.
> Both PEBS and BTS are static buffers allocated for each CPU at the
> first user. When the last user exists, the buffers are released.
> 
> All buffers are only accessed on the CPU they are attached to.
> kzalloc() does not take into account NUMA, thus all allocations
> are taking place on the NUMA node where the perf_event_open() is
> made.

I guess that should have been a alloc_pages_node() indeed.

> This patch switches allocation to vmalloc_node() to use non-contiguous
> physical memory and to allocate on the NUMA node corresponding to each
> CPU. We switched DS and PEBS although they do not cause problems today,
> to, at least, make the allocation on the correct NUMA node. In the future,
> the PEBS buffer size may increase. DS may also grow bigger than a page.
> This patch eliminates the memory allocation imbalance.

I'm not really a fan of vmalloc, have you actually observed allocation
failures for these 64k (order-4) allocations?


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