linux-kernel - Re: Perf and ftrace [was Re: PyTimechart]

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Date:	Wed, 12 May 2010 20:49:02 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Pierre Tardy <tardyp@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Tom Zanussi <tzanussi@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org, arjan@...radead.org,
	ziga.mahkovec@...il.com, davem <davem@...emloft.net>
Subject: Re: Perf and ftrace [was Re: PyTimechart]

On Wed, 2010-05-12 at 14:37 -0400, Mathieu Desnoyers wrote:
> * Peter Zijlstra (peterz@...radead.org) wrote:
> > On Wed, 2010-05-12 at 14:04 -0400, Mathieu Desnoyers wrote:
> > > Can't we keep multiple references to each page ? (shared page) so it's still in
> > > the buffer, also accessed by mmap(), and in addition accessed by splice.
> > 
> > I'm not sure, the problem seems to be that a splice-consumer might want
> > to inject the page into a whole different address-space, over-writing
> > page->mapping/->index etc.
> 
> OK, I see. In LTTng, I dropped the mmap() support when I integrated splice(). In
> both case, I can share the pages between the "output" (mmap or splice) and the
> ring buffer because my ring buffer does not care about
> page->mapping/->index/etc, so I never have to swap them.
> 
> However, doing mmap() and splice() at the same time on the same pages seems
> problematic for the reason you point out here (and not very useful anyway).
> But I think restrictions could be done more transparently than what you propose,
> e.g.:
> 
> 1) create buffer -> return fd
>    (perform pfn alignment for the architecture worse-case, e.g. support mmap()
>     on sparc)
> 
> 2a) mmap(fd)
>     return -EBUSY if any of the pages has non-NULL mapping.
> 3a) munmap(fd)
> 
> 2b) splice(fd)
>     return -EBUSY if any of the pages has non-NULL mapping.
> 
> 2c) read(fd)
>     Could probably be done concurrently with splice() or mmap().
> 
> This way we would ensure that only mmap or splice is used on the buffer at a
> given time without crippling the API.
> 
> Thoughts ?

Right, so the problem is that we now use mmap() to size the buffer. I
guess we could go adding a size attribute to perf_event_attr, but I
think its makes more sense to separate the actual event and the output
buffer objects.

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