[PATCH 1/2] mm/vmscan: add sync_shrinkers function v2

[Daniel Vetter]

On Thu, Aug 26, 2021 at 03:27:30PM +0200, Daniel Vetter wrote:
> On Fri, Aug 20, 2021 at 02:05:27PM +0200, Christian König wrote:
> > From: Christian König <ckoenig.leichtzumerken at gmail.com>
> > 
> > While unplugging a device the TTM shrinker implementation
> > needs a barrier to make sure that all concurrent shrink
> > operations are done and no other CPU is referring to a
> > device specific pool any more.
> > 
> > Taking and releasing the shrinker semaphore on the write
> > side after unmapping and freeing all pages from the device
> > pool should make sure that no shrinker is running in
> > paralell.
> > 
> > This allows us to avoid the contented mutex in the TTM pool
> > implementation for every alloc/free operation.
> > 
> > v2: rework the commit message to make clear why we need this
> > 
> > Signed-off-by: Christian König <christian.koenig at amd.com>
> > Acked-by: Huang Rui <ray.huang at amd.com>
> > Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> > ---
> >  include/linux/shrinker.h |  1 +
> >  mm/vmscan.c              | 10 ++++++++++
> >  2 files changed, 11 insertions(+)
> > 
> > diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> > index 9814fff58a69..1de17f53cdbc 100644
> > --- a/include/linux/shrinker.h
> > +++ b/include/linux/shrinker.h
> > @@ -93,4 +93,5 @@ extern void register_shrinker_prepared(struct shrinker *shrinker);
> >  extern int register_shrinker(struct shrinker *shrinker);
> >  extern void unregister_shrinker(struct shrinker *shrinker);
> >  extern void free_prealloced_shrinker(struct shrinker *shrinker);
> > +extern void sync_shrinkers(void);
> >  #endif
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 4620df62f0ff..fde1aabcfa7f 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -638,6 +638,16 @@ void unregister_shrinker(struct shrinker *shrinker)
> >  }
> >  EXPORT_SYMBOL(unregister_shrinker);
> >  
> > +/**
> > + * sync_shrinker - Wait for all running shrinkers to complete.
> 
> I think it would be good to add a bit more text here maybe:
> 
> "This is equivalent to calling unregister_shrink() and
> register_shrinker(), but atomically and with less overhead. This is useful
> to guarantee that all shrinker invocations have seen an update, before
> freeing memory, similar to rcu."
> 
> Also a bit a bikeshed, but if we look at the equivalent in irq land it's
> called synchronize_irq() and synchronize_hardirq(). I think it'd be good
> to bikeshed that for more conceptual consistency.

Oh also synchronize_*rcu* also spells them all out, so even more reasons
to do the same.
-Daniel

> > + */
> > +void sync_shrinkers(void)
> > +{
> > +	down_write(&shrinker_rwsem);
> > +	up_write(&shrinker_rwsem);
> > +}
> > +EXPORT_SYMBOL(sync_shrinkers);
> > +
> >  #define SHRINK_BATCH 128
> >  
> >  static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
> > -- 
> > 2.25.1
> > 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch