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

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Message-ID: <20231129223655.shi74lnm2tolun77@moria.home.lan>
Date:   Wed, 29 Nov 2023 17:36:55 -0500
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Michal Hocko <mhocko@...e.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>
Subject: Re: [PATCH 2/7] mm: shrinker: Add a .to_text() method for shrinkers

On Wed, Nov 29, 2023 at 05:02:35PM +0100, Michal Hocko wrote:
> On Tue 28-11-23 12:48:53, Kent Overstreet wrote:
> > On Tue, Nov 28, 2023 at 11:01:16AM +0100, Michal Hocko wrote:
> > > On Wed 22-11-23 18:25:07, Kent Overstreet wrote:
> > > [...]
> > > > +void shrinkers_to_text(struct seq_buf *out)
> > > > +{
> > > > +	struct shrinker *shrinker;
> > > > +	struct shrinker_by_mem {
> > > > +		struct shrinker	*shrinker;
> > > > +		unsigned long	mem;
> > > > +	} shrinkers_by_mem[10];
> > > > +	int i, nr = 0;
> > > > +
> > > > +	if (!mutex_trylock(&shrinker_mutex)) {
> > > > +		seq_buf_puts(out, "(couldn't take shrinker lock)");
> > > > +		return;
> > > > +	}
> > > > +
> > > > +	list_for_each_entry(shrinker, &shrinker_list, list) {
> > > > +		struct shrink_control sc = { .gfp_mask = GFP_KERNEL, };
> > > 
> > > This seems to be global reclaim specific. What about memcg reclaim?
> > 
> > I have no fsckin idea how memcg reclaim works - and, for that matter,
> > the recent lockless shrinking work seems to have neglected to write even
> > an iterator macro, leaving _that_ a nasty mess so I'm not touching that
> > either.
> 
> OK, you could have made it more clearly that all of this is aiming at
> the global OOM handling. With an outlook on what should be done if this
> was ever required.
> 
> Another thing you want to look into is a NUMA constrained OOM (mbind,
> cpuset) where this output could be actively misleading.

Yeah.

It's not clear to me how (if at all) we want memcg to be represented in
this output; it's not something us filesystem developers think about a
lot. NUMA definitely should, though.