Re: [PATCH 0/7] vm, vmscan: enahance vmscan tracepoints

From: Michal Hocko
Date: Fri Dec 30 2016 - 04:37:03 EST

On Fri 30-12-16 09:11:17, Mel Gorman wrote:
> On Wed, Dec 28, 2016 at 04:30:25PM +0100, Michal Hocko wrote:
> > Hi,
> > while debugging [1] I've realized that there is some room for
> > improvements in the tracepoints set we offer currently. I had hard times
> > to make any conclusion from the existing ones. The resulting problem
> > turned out to be active list aging [2] and we are missing at least two
> > tracepoints to debug such a problem.
> >
> > Some existing tracepoints could export more information to see _why_ the
> > reclaim progress cannot be made not only _how much_ we could reclaim.
> > The later could be seen quite reasonably from the vmstat counters
> > already. It can be argued that we are showing too many implementation
> > details in those tracepoints but I consider them way too lowlevel
> > already to be usable by any kernel independent userspace. I would be
> > _really_ surprised if anything but debugging tools have used them.
> >
> > Any feedback is highly appreciated.
> >
> There is some minor overhead introduced in some paths regardless of
> whether the tracepoints are active or not but I suspect it's negligible
> in the context of the overhead of reclaim in general so;

I will work on improving some of them. E.g. I've dropped the change to
free_hot_cold_page_list because that is indeed a hot path but other than
that there shouldn't be any even medium hot path which should see any
overhead I can see. If you are aware of any, please let me know and I
will think about how to improve it.

> Acked-by: Mel Gorman <mgorman@xxxxxxx>

Michal Hocko