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

Mathieu Desnoyers
2010-09-13T19:31:26Z

* Stephane Eranian (eranian@google.com) wrote:
> On Mon, Sep 13, 2010 at 7:35 PM, Peter Zijlstra <peterz@infradead.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?

Well, a thought is striking me: it sounds like you are re-doing YAORB (short
for Yet Another Ring Buffer, which I start to expect will become a frequently
used acronym). Have you looked at my "generic ring buffer library" ? It's at:

git://git.kernel.org/pub/scm/linux/kernel/git/compudj/linux-2.6-ringbuffer.git
current branch: tip-current-ringbuffer-0.248

documentation is under Documentation/ringbuffer/.

I think it can save you a lot of trouble. E.g., it does stitch pages togeter by
software.

Thanks,

Mathieu

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.