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Subject: Re: [PATCH] perf_events: improve DS/BTS/PEBS buffer allocation
From: Peter Zijlstra <peterz@infradead.org>
To: Stephane Eranian <eranian@google.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        linux-kernel@vger.kernel.org, mingo@elte.hu, paulus@samba.org,
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Date: Mon, 13 Sep 2010 20:57:52 +0200
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On Mon, 2010-09-13 at 20:49 +0200, Stephane Eranian wrote:
> On Mon, Sep 13, 2010 at 8:42 PM, Peter Zijlstra <peterz@infradead.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.

Yes, one thing we could do is simply disable BTS when we fail that
alloc, instead of fail everything.

> 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.

Right, that would be a simple change to make.