Re: [PATCH v2 8/8] rcu/kfree: Fix kfree_rcu_shrink_count() return value

[Joel Fernandes <joel@joelfernandes.org>]

On Mon, Jun 27, 2022 at 02:43:59PM -0700, Paul E. McKenney wrote:
> On Mon, Jun 27, 2022 at 09:18:13PM +0000, Joel Fernandes wrote:
> > On Mon, Jun 27, 2022 at 01:59:07PM -0700, Paul E. McKenney wrote:
> > > On Mon, Jun 27, 2022 at 08:56:43PM +0200, Uladzislau Rezki wrote:
> > > > > As per the comments in include/linux/shrinker.h, .count_objects callback
> > > > > should return the number of freeable items, but if there are no objects
> > > > > to free, SHRINK_EMPTY should be returned. The only time 0 is returned
> > > > > should be when we are unable to determine the number of objects, or the
> > > > > cache should be skipped for another reason.
> > > > > 
> > > > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > > > > ---
> > > > >  kernel/rcu/tree.c | 2 +-
> > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > > index 711679d10cbb..935788e8d2d7 100644
> > > > > --- a/kernel/rcu/tree.c
> > > > > +++ b/kernel/rcu/tree.c
> > > > > @@ -3722,7 +3722,7 @@ kfree_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
> > > > >  		atomic_set(&krcp->backoff_page_cache_fill, 1);
> > > > >  	}
> > > > >  
> > > > > -	return count;
> > > > > +	return count == 0 ? SHRINK_EMPTY : count;
> > > > >  }
> > > > >  
> > > > >  static unsigned long
> > > > > -- 
> > > > > 2.37.0.rc0.104.g0611611a94-goog
> > > > > 
> > > > Looks good to me!
> > > > 
> > > > Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > > 
> > > Now that you mention it, this does look independent of the rest of
> > > the series.  I have pulled it in with Uladzislau's Reviewed-by.
> > 
> > Thanks Paul and Vlad!
> > 
> > Paul, apologies for being quiet. I have been working on the series and the
> > review comments carefully. I appreciate your help with this work.
> 
> Not a problem.  After all, this stuff is changing some of the trickier
> parts of RCU.  We must therefore assume that some significant time and
> effort will be required to get it right.

To your point about trickier parts of RCU, the v2 series though I tested it
before submitting is now giving me strange results with rcuscale. Sometimes
laziness does not seem to be in effect (as pointed out by rcuscale), other
times I am seeing stalls.

So I have to carefully look through all of this again. I am not sure why I
was not seeing these issues with the exact same code before (frustrated).

thanks,

 - Joel