Re: [RFC] Tracer Ring Buffer splice() vs page cache [was: Re: Perfand ftrace [was Re: PyTimechart]]

From: Mathieu Desnoyers
Date: Tue May 18 2010 - 11:16:34 EST

* Peter Zijlstra (peterz@xxxxxxxxxxxxx) wrote:
> On Mon, 2010-05-17 at 18:42 -0400, Mathieu Desnoyers wrote:
> > I'll continue to look into this. One of the things I noticed that that we could
> > possibly use the "steal()" operation to steal the pages back from the page cache
> > to repopulate the ring buffer rather than continuously allocating new pages. If
> > steal() fails for some reasons, then we can fall back on page allocation. I'm
> > not sure it is safe to assume anything about pages being in the page cache
> > though.
> Also, suppose it was still in the page-cache and still dirty, a steal()
> would then punch a hole in the file.

page_cache_pipe_buf_steal starts by doing a wait_on_page_writeback(page); and
then does a try_to_release_page(page, GFP_KERNEL). Only if that succeeds is the
action of stealing succeeding.

> > Maybe the safest route is to just allocate new pages for now.
> Yes, that seems to be the only sane approach.

Yes, a good start anyway.



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