Re: [PATCH] mm: do not call add_nr_deferred() with zero deferred

[Roman Gushchin <roman.gushchin@linux.dev>]

On Fri, Apr 22, 2022 at 11:19:05AM +1000, Dave Chinner wrote:
> On Tue, Apr 19, 2022 at 09:42:30AM -0700, Roman Gushchin wrote:
> > On Tue, Apr 19, 2022 at 02:56:06PM +0200, David Hildenbrand wrote:
> > > On 16.04.22 02:41, Roman Gushchin wrote:
> > > > add_nr_deferred() is often called with next_deferred equal to 0.
> > > > For instance, it's happening under low memory pressure for any
> > > > shrinkers with a low number of cached objects. A corresponding trace
> > > > looks like:
> > > >   <...>-619914 [005] .... 467456.345160: mm_shrink_slab_end: \
> > > >   super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1	     \
> > > >   unused scan count 0 new scan count 0 total_scan 0	     \
> > > >   last shrinker return val 0
> > > > 
> > > >   <...>-619914 [005] .... 467456.345371: mm_shrink_slab_end: \
> > > >   super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1	     \
> > > >   unused scan count 0 new scan count 0 total_scan 0	     \
> > > >   last shrinker return val 0
> > > > 
> > > >   <...>-619914 [005] .... 467456.345380: mm_shrink_slab_end: \
> > > >   super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1 unused \
> > > >   scan count 0 new scan count 0 total_scan 0	             \
> > > >   last shrinker return val 0
> > > > 
> > > > This lead to unnecessary checks and atomic operations, which can be
> > > > avoided by checking next_deferred for not being zero before calling
> > > > add_nr_deferred(). In this case the mm_shrink_slab_end trace point
> > > > will get a potentially slightly outdated "new scan count" value, but
> > > > it's totally fine.
> > > 
> > > Sufficient improvement to justify added complexity for anybody reading
> > > that code?
> > 
> > I don't have any numbers and really doubt the difference is significant,
> 
> Never been able to measure it myself.
> 
> HwoeverI'd much prefer the tracepoint output stays accurate - I've had to
> post-process and/or graph the shrinker progress as reported by the
> start/end tracpoints to find problems in the algorithms in the past.
> That's why there is the additional complexity in the code to make
> sure the coutners are accurate in the first place.

Sure, no problems.

Andrew, can you, please, drop this patch?

Thanks!