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

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Date:	Mon, 13 Sep 2010 20:49:56 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, paulus@...ba.org,
	davem@...emloft.net, perfmon2-devel@...ts.sf.net,
	eranian@...il.com, robert.richter@....com,
	"markus.t.metzger" <markus.t.metzger@...el.com>
Subject: Re: [PATCH] perf_events: improve DS/BTS/PEBS buffer allocation

On Mon, Sep 13, 2010 at 8:42 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, 2010-09-13 at 20:40 +0200, Stephane Eranian wrote:
>> Ok, so can we play the same trick you're playing with the sampling
>> buffer, i.e., you use alloc_pages_node() for one page at a time, and
>> then you stitch them on demand via SW?
>
> Not for BTS, it wants a linear range, getting the vmalloc vs NMI thing
> sorted would be best I think.
>
What is annoying in this is that you run into the problem even though
you may not be using BTS nor PEBS.

What mitigates the problem, I think, is the NMI watchdog. It is the first
user of perf_events. As such, the BTS and PEBS buffers get allocated
during kernel initialization thereby increasing the chances of finding
contiguous chunks of memory. What would partly help would be to use of
kmalloc_node() to at least balance allocations amongst the various NUMA
nodes. That would be until the vmalloc() vs. NMI is sorted out.
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