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

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

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 will get a potentially slightly outdated "new scan count" value, but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,
however the added complexity is also small: one "if" statement.
Anyway, if you feel strongly against this change, I'm fine with dropping it.

> 
> > 
> > Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
> > ---
> >  mm/vmscan.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index d4a7d2bd276d..19d3d4fa1aad 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -808,7 +808,10 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
> >  	 * move the unused scan count back into the shrinker in a
> >  	 * manner that handles concurrent updates.
> >  	 */
> > -	new_nr = add_nr_deferred(next_deferred, shrinker, shrinkctl);
> > +	if (next_deferred)
> > +		new_nr = add_nr_deferred(next_deferred, shrinker, shrinkctl);
> > +	else
> > +		new_nr = nr;
> >  
> >  	trace_mm_shrink_slab_end(shrinker, shrinkctl->nid, freed, nr, new_nr, total_scan);
> >  	return freed;
> 
> And if we still want to do this optimization, why not put it into
> add_nr_deferred()?

Because of the semantics of add_nr_deferred(), which returns the deferred value.
It's not used for anything except tracing, so maybe it's a place for another
change.

Thanks!