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

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Message-ID: <20161230093654.GC13301@dhcp22.suse.cz>
Date:   Fri, 30 Dec 2016 10:36:55 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Mel Gorman <mgorman@...e.de>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Rik van Riel <riel@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/7] vm, vmscan: enahance vmscan tracepoints

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@...e.de>

Thanks!
-- 
Michal Hocko
SUSE Labs