Re: [PATCH] mm: cache largest vma

From: Ingo Molnar
Date: Mon Nov 04 2013 - 12:52:54 EST

* Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:

> On Mon, Nov 04, 2013 at 08:05:00AM +0100, Ingo Molnar wrote:
> >
> > * Davidlohr Bueso <davidlohr@xxxxxx> wrote:
> >
> > > Btw, do you suggest using a high level tool such as perf for getting
> > > this data or sprinkling get_cycles() in find_vma() -- I'd think that the
> > > first isn't fine grained enough, while the later will probably variate a
> > > lot from run to run but the ratio should be rather constant.
> >
> > LOL - I guess I should have read your mail before replying to it ;-)
> >
> > Yes, I think get_cycles() works better in this case - not due to
> > granularity (perf stat will report cycle granular just fine), but due
> > to the size of the critical path you'll be measuring. You really want
> > to extract the delta, because it's probably so much smaller than the
> > overhead of the workload itself.
> >
> > [ We still don't have good 'measure overhead from instruction X to
> > instruction Y' delta measurement infrastructure in perf yet,
> > although Frederic is working on such a trigger/delta facility AFAIK.
> > ]
> Yep, in fact Jiri took it over and he's still working on it. But yeah,
> once that get merged, we should be able to measure instructions or
> cycles inside any user or kernel function through kprobes/uprobes or
> function graph tracer.

So, what would be nice is to actually make use of it: one very nice
usecase I'd love to see is to have the capability within the 'perf top'
TUI annotated assembly output to mark specific instructions as 'start' and
'end' markers, and measure the overhead between them.

I.e. allow perf top / perf report to manage probes into interesting
functions - or create a similar TUI for 'perf probe' to allow easy live
marking/probing of various kernel functionality.


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