linux-kernel - Re: [PATCH 2/7] mm: shrinker: Add a .to

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Message-ID: <20231204181553.tyxeq7zy54cuc3o7@moria.home.lan>
Date:   Mon, 4 Dec 2023 13:15:53 -0500
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Roman Gushchin <roman.gushchin@...ux.dev>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Linux-MM <linux-mm@...ck.org>, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH 2/7] mm: shrinker: Add a .to_text() method for shrinkers

On Mon, Dec 04, 2023 at 11:33:12AM +0100, Michal Hocko wrote:
> On Fri 01-12-23 16:25:06, Kent Overstreet wrote:
> > On Fri, Dec 01, 2023 at 11:04:23AM +0100, Michal Hocko wrote:
> > > On Thu 30-11-23 20:47:45, Kent Overstreet wrote:
> > > > On Thu, Nov 30, 2023 at 09:14:35AM +0100, Michal Hocko wrote:
> > > [...]
> > > > > All that being said, I am with you on the fact that the oom report in
> > > > > its current form could see improvements.
> > > > 
> > > > I'm glad we're finally in agreement on something!
> > > > 
> > > > If you want to share your own ideas on what could be improved and what
> > > > you find useful, maybe we could find some more common ground.
> > > 
> > > One thing that I would consider an improvement is to have a way to
> > > subscribe drivers with excessive memory consumption or those which are
> > > struggling to dump their state.
> > 
> > Remember the memory allocation profiling patchset? The one where you
> > kept complaining about "maintenancy overhead"?
> 
> Yes, I still maintain my opinion on that approach. I have never
> questioned usefulness of the information.
> 
> > We can plug that into the show_mem report too, and list the top 10
> > allocations by file and line number.
> > 
> > > Maybe your proposal can be extended that way but the crucial point is to
> > > not dump all sorts of random shrinkers' state and end up with unwieldy
> > > reports.  If, on the other hand, any particular shrinker struggles to
> > > reclaim memory and it is sitting on a lot of memory it could be able to
> > > flag itself to be involved in the dump.
> > 
> > Great, since as was mentioned in the original commit message it's not
> > "all sorts of random shrinkers", but top 10 by objects reported, what
> > I've got here should make you happy.
> 
> Can we do better and make that a shrinker decision rather than an
> arbitrary top N selection? The thing is that shrinkers might even not
> matter in many cases so their output would be just a balast. The number
> of objects is not universaly great choice. As Dave mentioned metdata
> might be pinning other objects.
> 
> That being said, if you want to give more debugability power to
> shrinkers then it makes more sense to allow them to opt-in for the oom
> report rather than control which of them to involve from the oom
> reporting code which doesn't have enough context on its own.

If you've got an idea for a refinement, please submit your own patch and
I'll look at incorporating it into the series.