Re: [PATCH] mm: vmscan: avoid a unnecessary reschedule in shrink_slab()

[Michal Hocko]

On Fri 16-10-20 12:48:23, Tianxianting wrote:
> Thanks, my understanding is,
> In shrink_slab(), do_shrink_slab() will do the real reclaim work, which will occupy current cpu and consume more cpu time, so we need to trigger a reschedule after reclaim.
> But if it jumps to 'out' label, that means we don't do the reclaim work at this time, it won't cause other thread getting starvation, so we don't need to call cond_resched() in this case.
> Is it right?

You are almost right. But consider situation when the lock is taken for
quite some time. do_shrink_slab cannot make any forward progress and
effectivelly busy loop. Unless the caller does cond_resched it might
cause soft lockups.

Anyway let me try to ask again. Why does would this be any problem that
deserves a fix?

> 
> -----Original Message-----
> From: Michal Hocko [mailto:mhocko@xxxxxxxx] 
> Sent: Friday, October 16, 2020 8:08 PM
> To: tianxianting (RD) <tian.xianting@xxxxxxx>
> Cc: akpm@xxxxxxxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH] mm: vmscan: avoid a unnecessary reschedule in shrink_slab()
> 
> On Fri 16-10-20 11:39:52, Xianting Tian wrote:
> > In shrink_slab(), it directly goes to 'out' label only when it can't 
> > get the lock of shrinker_rwsew. In this case, it doesn't do the real 
> > work of shrinking slab, so we don't need trigger a reschedule by 
> > cond_resched().
> 
> Your changelog doesn't explain why this is not needed or undesirable. Do you see any actual problem?
> 
> The point of this code is to provide a deterministic scheduling point regardless of the shrinker_rwsew.
> 
> > 
> > Signed-off-by: Xianting Tian <tian.xianting@xxxxxxx>
> > ---
> >  mm/vmscan.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/vmscan.c b/mm/vmscan.c index 466fc3144..676e97b28 
> > 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -687,8 +687,9 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
> >  	}
> >  
> >  	up_read(&shrinker_rwsem);
> > -out:
> > +
> >  	cond_resched();
> > +out:
> >  	return freed;
> >  }
> >  
> > --
> > 2.17.1
> > 
> 
> -- 
> Michal Hocko
> SUSE Labs

-- 
Michal Hocko
SUSE Labs