linux-kernel - Re: [PATCH rfc 0/5] mm: introduce shrinker sysfs interface

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Date:   Tue, 19 Apr 2022 09:33:48 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Roman Gushchin <roman.gushchin@...ux.dev>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Dave Chinner <dchinner@...hat.com>,
        linux-kernel@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Yang Shi <shy828301@...il.com>
Subject: Re: [PATCH rfc 0/5] mm: introduce shrinker sysfs interface

On Mon, Apr 18, 2022 at 10:27:34AM -0700, Roman Gushchin wrote:
> On Mon, Apr 18, 2022 at 12:27:36PM +0300, Mike Rapoport wrote:
> > On Fri, Apr 15, 2022 at 05:27:51PM -0700, Roman Gushchin wrote:
> > > There are 50+ different shrinkers in the kernel, many with their own bells and
> > > whistles. Under the memory pressure the kernel applies some pressure on each of
> > > them in the order of which they were created/registered in the system. Some
> > > of them can contain only few objects, some can be quite large. Some can be
> > > effective at reclaiming memory, some not.
> > > 
> > > The only existing debugging mechanism is a couple of tracepoints in
> > > do_shrink_slab(): mm_shrink_slab_start and mm_shrink_slab_end. They aren't
> > > covering everything though: shrinkers which report 0 objects will never show up,
> > > there is no support for memcg-aware shrinkers. Shrinkers are identified by their
> > > scan function, which is not always enough (e.g. hard to guess which super
> > > block's shrinker it is having only "super_cache_scan"). They are a passive
> > > mechanism: there is no way to call into counting and scanning of an individual
> > > shrinker and profile it.
> > > 
> > > To provide a better visibility and debug options for memory shrinkers
> > > this patchset introduces a /sys/kernel/shrinker interface, to some extent
> > > similar to /sys/kernel/slab.
> > 
> > Wouldn't debugfs better fit the purpose of shrinker debugging?
> 
> I think sysfs fits better, but not a very strong opinion.
> 
> Even though the interface is likely not very useful for the general
> public, big cloud instances might wanna enable it to gather statistics
> (and it's certainly what we gonna do at Facebook) and to provide
> additional data when something is off.  They might not have debugfs
> mounted. And it's really similar to /sys/kernel/slab.

And there is also similar /proc/vmallocinfo so why not /proc/shrinker? ;-)

I suspect slab ended up in sysfs because nobody suggested to use debugfs
back then. I've been able to track the transition from /proc/slabinfo to
/proc/slubinfo to /sys/kernel/slab, but could not find why Christoph chose
sysfs in the end.

> Are there any reasons why debugfs is preferable?

debugfs is more flexible because it's not stable kernel ABI so if there
will be need/desire to change the layout and content of the files with
debugfs it can be done more easily.

Is this a real problem for Facebook to mount debugfs? ;-)
 
> Thanks!

-- 
Sincerely yours,
Mike.