Re: [PATCH 11/28] mm: factor shrinker work calculations

[Brian Foster <bfoster@redhat.com>]

On Fri, Nov 01, 2019 at 10:46:01AM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> Start to clean up the shrinker code by factoring out the calculation
> that determines how much work to do. This separates the calculation
> from clamping and other adjustments that are done before the
> shrinker work is run. Document the scan batch size calculation
> better while we are there.
> 
> Also convert the calculation for the amount of work to be done to
> use 64 bit logic so we don't have to keep jumping through hoops to
> keep calculations within 32 bits on 32 bit systems.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---

I assume the kbuild warning thing will be fixed up...

>  mm/vmscan.c | 97 ++++++++++++++++++++++++++++++++++++++---------------
>  1 file changed, 70 insertions(+), 27 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a215d71d9d4b..2d39ec37c04d 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -459,13 +459,68 @@ EXPORT_SYMBOL(unregister_shrinker);
>  
>  #define SHRINK_BATCH 128
>  
> +/*
> + * Calculate the number of new objects to scan this time around. Return
> + * the work to be done. If there are freeable objects, return that number in
> + * @freeable_objects.
> + */
> +static int64_t shrink_scan_count(struct shrink_control *shrinkctl,
> +			    struct shrinker *shrinker, int priority,
> +			    int64_t *freeable_objects)
> +{
> +	int64_t delta;
> +	int64_t freeable;
> +
> +	freeable = shrinker->count_objects(shrinker, shrinkctl);
> +	if (freeable == 0 || freeable == SHRINK_EMPTY)
> +		return freeable;
> +
> +	if (shrinker->seeks) {
> +		/*
> +		 * shrinker->seeks is a measure of how much IO is required to
> +		 * reinstantiate the object in memory. The default value is 2
> +		 * which is typical for a cold inode requiring a directory read
> +		 * and an inode read to re-instantiate.
> +		 *
> +		 * The scan batch size is defined by the shrinker priority, but
> +		 * to be able to bias the reclaim we increase the default batch
> +		 * size by 4. Hence we end up with a scan batch multipler that
> +		 * scales like so:
> +		 *
> +		 * ->seeks	scan batch multiplier
> +		 *    1		      4.00x
> +		 *    2               2.00x
> +		 *    3               1.33x
> +		 *    4               1.00x
> +		 *    8               0.50x
> +		 *
> +		 * IOWs, the more seeks it takes to pull the item into cache,
> +		 * the smaller the reclaim scan batch. Hence we put more reclaim
> +		 * pressure on caches that are fast to repopulate and to keep a
> +		 * rough balance between caches that have different costs.
> +		 */
> +		delta = freeable >> (priority - 2);

Does anything prevent priority < 2 here?

> +		do_div(delta, shrinker->seeks);
> +	} else {
> +		/*
> +		 * These objects don't require any IO to create. Trim them
> +		 * aggressively under memory pressure to keep them from causing
> +		 * refetches in the IO caches.
> +		 */
> +		delta = freeable / 2;
> +	}
> +
> +	*freeable_objects = freeable;
> +	return delta > 0 ? delta : 0;
> +}
> +
>  static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>  				    struct shrinker *shrinker, int priority)
>  {
>  	unsigned long freed = 0;
> -	unsigned long long delta;
>  	long total_scan;
> -	long freeable;
> +	int64_t freeable_objects = 0;
> +	int64_t scan_count;
>  	long nr;
>  	long new_nr;
>  	int nid = shrinkctl->nid;
...
> @@ -487,25 +543,11 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>  	 */
>  	nr = atomic_long_xchg(&shrinker->nr_deferred[nid], 0);
>  
> -	total_scan = nr;
> -	if (shrinker->seeks) {
> -		delta = freeable >> priority;
> -		delta *= 4;
> -		do_div(delta, shrinker->seeks);
> -	} else {
> -		/*
> -		 * These objects don't require any IO to create. Trim
> -		 * them aggressively under memory pressure to keep
> -		 * them from causing refetches in the IO caches.
> -		 */
> -		delta = freeable / 2;
> -	}
> -
> -	total_scan += delta;
> +	total_scan = nr + scan_count;
>  	if (total_scan < 0) {
>  		pr_err("shrink_slab: %pS negative objects to delete nr=%ld\n",
>  		       shrinker->scan_objects, total_scan);
> -		total_scan = freeable;
> +		total_scan = scan_count;

Same question as before: why the change in assignment? freeable was the
->count_objects() return value, which is now stored in freeable_objects.

FWIW, the change seems to make sense in that it just factors out the
deferred count, but it's not clear if it's intentional...

Brian

>  		next_deferred = nr;
>  	} else
>  		next_deferred = total_scan;
> @@ -522,19 +564,20 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>  	 * Hence only allow the shrinker to scan the entire cache when
>  	 * a large delta change is calculated directly.
>  	 */
> -	if (delta < freeable / 4)
> -		total_scan = min(total_scan, freeable / 2);
> +	if (scan_count < freeable_objects / 4)
> +		total_scan = min_t(long, total_scan, freeable_objects / 2);
>  
>  	/*
>  	 * Avoid risking looping forever due to too large nr value:
>  	 * never try to free more than twice the estimate number of
>  	 * freeable entries.
>  	 */
> -	if (total_scan > freeable * 2)
> -		total_scan = freeable * 2;
> +	if (total_scan > freeable_objects * 2)
> +		total_scan = freeable_objects * 2;
>  
>  	trace_mm_shrink_slab_start(shrinker, shrinkctl, nr,
> -				   freeable, delta, total_scan, priority);
> +				   freeable_objects, scan_count,
> +				   total_scan, priority);
>  
>  	/*
>  	 * If the shrinker can't run (e.g. due to gfp_mask constraints), then
> @@ -559,7 +602,7 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>  	 * possible.
>  	 */
>  	while (total_scan >= batch_size ||
> -	       total_scan >= freeable) {
> +	       total_scan >= freeable_objects) {
>  		unsigned long ret;
>  		unsigned long nr_to_scan = min(batch_size, total_scan);
>  
> -- 
> 2.24.0.rc0
>