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Date: Fri, 30 Dec 2016 10:20:45 +0000
From: Mel Gorman <mgorman@suse.de>
Subject: Re: [PATCH 0/7] vm, vmscan: enahance vmscan tracepoints
Message-ID: <20161230102045.y2fiszc2o2uqmny4@suse.de>
References: <20161228153032.10821-1-mhocko@kernel.org>
 <20161230091117.nkxn3bnhle3bofhw@suse.de>
 <20161230093654.GC13301@dhcp22.suse.cz>
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To: Michal Hocko <mhocko@kernel.org>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>, Johannes Weiner <hannes@cmpxchg.org>, Vlastimil Babka <vbabka@suse.cz>, Rik van Riel <riel@redhat.com>, LKML <linux-kernel@vger.kernel.org>

On Fri, Dec 30, 2016 at 10:36:55AM +0100, Michal Hocko wrote:
> 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.
>  

I didn't spot one. The path where I saw the most overhead is already
quite heavy.

-- 
Mel Gorman
SUSE Labs

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