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Subject: Re: [PATCH] perf_events: improve DS/BTS/PEBS buffer allocation
From: Peter Zijlstra <peterz@infradead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Stephane Eranian <eranian@google.com>,
        Frederic Weisbecker <fweisbec@gmail.com>, linux-kernel@vger.kernel.org,
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Date: Mon, 13 Sep 2010 21:35:21 +0200
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On Mon, 2010-09-13 at 21:34 +0200, Peter Zijlstra wrote:
> On Mon, 2010-09-13 at 15:31 -0400, Mathieu Desnoyers 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?
> > 
> > Well, a thought is striking me: it sounds like you are re-doing YAORB (short
> > for Yet Another Ring Buffer, 
> 
> He's not.. the hardware needs a large (virtually) contiguous region to
> poke data into, we need to read it out from NMI context.

Which just made me realize, we really need vmalloc_sync_all() for this,
the hardware will want to walk the pagetables as well, it cannot lazy
fault the pages in.