Re: [Patch] mm tracepoints update - use case.

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 23 Apr 2009 09:48:04 +0900 (JST) KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

> > On Wed, 2009-04-22 at 08:07 -0400, Larry Woodman wrote:
> > > On Wed, 2009-04-22 at 11:57 +0200, Ingo Molnar wrote:
> > > > * KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:
> > 
> > > > > In past thread, Andrew pointed out bare page tracer isn't useful. 
> > > > 
> > > > (do you have a link to that mail?)

http://lkml.indiana.edu/hypermail/linux/kernel/0903.0/02674.html

And Larry's example use case here tends to reinforce what I said then.  Look:

: In addition I could see that the priority was decremented to zero and
: that 12342 pages had been reclaimed rather than just enough to satisfy
: the page allocation request.
: 
: -----------------------------------------------------------------------------
: # tracer: nop
: #
: #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
: #              | |       |          |         |
: <mem>-10723 [005]  6976.285610: mm_directreclaim_reclaimzone: reclaimed=12342, priority=0

and

: -----------------------------------------------------------------------------
: # tracer: nop
: #
: #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
: #              | |       |          |         |
:            <mem>-10723 [005]   282.776271: mm_pagereclaim_shrinkzone: reclaimed=12342
:            <mem>-10723 [005]   282.781209: mm_pagereclaim_shrinkzone: reclaimed=3540
:            <mem>-10723 [005]   282.801194: mm_pagereclaim_shrinkzone: reclaimed=7528
: -----------------------------------------------------------------------------

This diagnosis was successful because the "reclaimed" number was weird.
By sheer happy coincidence, page-reclaim is already generating the
aggregated numbers for us, and the tracer just prints it out.

If some other problem is being worked on and if there _isn't_ some
convenient already-present aggregated result for the tracer to print,
the problem won't be solved.  Unless a vast number of trace events are
emitted and problem-specific userspace code is written to aggregate
them into something which the developer can use.