Re: [LSF/MM TOPIC] dying memory cgroups and slab reclaim issues

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Feb 20, 2019 at 06:27:07PM +1100, Dave Chinner wrote:
> freeable = 1
> 
> ratio	4.15	priority	4.16	4.18		new
> 1:100	  1	   12		0	batch		1
> 1.32	  1	    9		0	batch		1
> 1:12	  1	    6		0	batch		1
> 1:6	  1	    3		0	batch		1
> 1:1	  1	    1		1	batch		1

> @@ -479,7 +479,16 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>  
>  	total_scan = nr;
>  	if (shrinker->seeks) {
> -		delta = freeable >> priority;
> +		/*
> +		 * Use a small non-zero offset for delta so that if the scan
> +		 * priority is low we always accumulate some pressure on caches
> +		 * that have few freeable objects in them. This allows light
> +		 * memory pressure to turn over caches with few freeable objects
> +		 * slowly without the need for memory pressure priority to wind
> +		 * up to the point where (freeable >> priority) is non-zero.
> +		 */
> +		delta = ilog2(freeable);

The idea makes sense to me, but log2 fails us when freeable is
1. fls() should work, though.

> +		delta += freeable >> priority;
>  		delta *= 4;
>  		do_div(delta, shrinker->seeks);
>  	} else {