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

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Date:	Mon, 13 Sep 2010 20:40:16 +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 7:35 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, 2010-09-13 at 17:55 +0200, Stephane Eranian wrote:
>>
>> Ok, so you're saying there is no allocator that will give non-contiguous
>> physical memory WITHOUT requiring a page fault to populate the pte.
>>
>> On the other hand, with vmalloc_node() the pte are populated when
>> you first touch the memory. That happens as part of memset() right after
>> the allocation and thus outside of NMI interrupt handler.
>>
>> Does this sound right?
>
> Nope, in particular read: http://lkml.org/lkml/2010/7/14/465
>
> The issue is that the vmalloc space can be mapped in different
> processes, and that memset() will only ensure its mapped in the current
> process, but the next one might need that fault to populate.
>
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?
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