Re: [PATCH 12/28] shrinker: defer work only to kswapd

[Brian Foster]

On Fri, Nov 01, 2019 at 10:46:02AM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> 
> Right now deferred work is picked up by whatever GFP_KERNEL context
> reclaimer that wins the race to empty the node's deferred work
> counter. However, if there are lots of direct reclaimers, that
> work might be continually picked up by contexts taht can't do any
> work and so the opportunities to do the work are missed by contexts
> that could do them.
> 
> A further problem with the current code is that the deferred work
> can be picked up by a random direct reclaimer, resulting in that
> specific process having to do all the deferred reclaim work and
> hence can take extremely long latencies if the reclaim work blocks
> regularly. This is not good for direct reclaim fairness or for
> minimising long tail latency events.
> 
> To avoid these problems, simply limit deferred work to kswapd
> contexts. We know kswapd is a context that can always do reclaim
> work, and hence deferring work to kswapd allows the deferred work to
> be done in the background and not adversely affect any specific
> process context doing direct reclaim.
> 
> The advantage of this is that amount of work to be done in direct
> reclaim is now bound and predictable - it is entirely based on
> the cache's freeable objects and the reclaim priority. hence all
> direct reclaimers running at the same time should be doing
> relatively equal amounts of work, thereby reducing the incidence of
> long tail latencies due to uneven reclaim workloads.
> 
> Note that we use signed integers for everything except the freed
> count as the returns from the shrinker callouts cannot be guaranteed
> untainted. Indeed, the shrinkers can return scan counts larger that
> were fed in, so we need scan counts to underflow in a detectable
> manner to terminate loops. This is necessary to avoid a misbehaving
> shrinker from triggering endless scanning loops.
> 
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
>  include/linux/shrinker.h |   2 +-
>  mm/vmscan.c              | 100 ++++++++++++++++++++-------------------
>  2 files changed, 53 insertions(+), 49 deletions(-)
> 
> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> index 3405c39ab92c..30c10f42109f 100644
> --- a/include/linux/shrinker.h
> +++ b/include/linux/shrinker.h
> @@ -81,7 +81,7 @@ struct shrinker {
>  	int id;
>  #endif
>  	/* objs pending delete, per node */
> -	atomic_long_t *nr_deferred;
> +	atomic64_t *nr_deferred;
>  };
>  #define DEFAULT_SEEKS 2 /* A good number if you don't know better. */
>  
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 2d39ec37c04d..c0e2bf656e3f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -517,16 +517,16 @@ static int64_t shrink_scan_count(struct shrink_control *shrinkctl,
>  static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>  				    struct shrinker *shrinker, int priority)
>  {
> -	unsigned long freed = 0;
> -	long total_scan;
> +	uint64_t freed = 0;
>  	int64_t freeable_objects = 0;
>  	int64_t scan_count;
> -	long nr;
> -	long new_nr;
> +	int64_t scanned_objects = 0;
> +	int64_t next_deferred = 0;
> +	int64_t deferred_count = 0;
> +	int64_t new_nr;
>  	int nid = shrinkctl->nid;
>  	long batch_size = shrinker->batch ? shrinker->batch
>  					  : SHRINK_BATCH;
> -	long scanned = 0, next_deferred;
>  
>  	if (!(shrinker->flags & SHRINKER_NUMA_AWARE))
>  		nid = 0;
> @@ -537,47 +537,51 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>  		return scan_count;
>  
>  	/*
> -	 * copy the current shrinker scan count into a local variable
> -	 * and zero it so that other concurrent shrinker invocations
> -	 * don't also do this scanning work.
> -	 */
> -	nr = atomic_long_xchg(&shrinker->nr_deferred[nid], 0);
> -
> -	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 = scan_count;
> -		next_deferred = nr;
> -	} else
> -		next_deferred = total_scan;
> -
> -	/*
> -	 * We need to avoid excessive windup on filesystem shrinkers
> -	 * due to large numbers of GFP_NOFS allocations causing the
> -	 * shrinkers to return -1 all the time. This results in a large
> -	 * nr being built up so when a shrink that can do some work
> -	 * comes along it empties the entire cache due to nr >>>
> -	 * freeable. This is bad for sustaining a working set in
> -	 * memory.
> +	 * If kswapd, we take all the deferred work and do it here. We don't let
> +	 * direct reclaim do this, because then it means some poor sod is going
> +	 * to have to do somebody else's GFP_NOFS reclaim, and it hides the real
> +	 * amount of reclaim work from concurrent kswapd operations. Hence we do
> +	 * the work in the wrong place, at the wrong time, and it's largely
> +	 * unpredictable.
>  	 *
> -	 * Hence only allow the shrinker to scan the entire cache when
> -	 * a large delta change is calculated directly.
> +	 * By doing the deferred work only in kswapd, we can schedule the work
> +	 * according the the reclaim priority - low priority reclaim will do
> +	 * less deferred work, hence we'll do more of the deferred work the more
> +	 * desperate we become for free memory. This avoids the need for needing
> +	 * to specifically avoid deferred work windup as low amount os memory
> +	 * pressure won't excessive trim caches anymore.

That last sentence is hard to read. ;)

>  	 */
> -	if (scan_count < freeable_objects / 4)
> -		total_scan = min_t(long, total_scan, freeable_objects / 2);
> +	if (current_is_kswapd()) {
> +		int64_t	deferred_scan;
> +
> +		deferred_count = atomic64_xchg(&shrinker->nr_deferred[nid], 0);
> +
> +		/* we want to scan 5-10% of the deferred work here at minimum */
> +		deferred_scan = deferred_count;
> +		if (priority)
> +			do_div(deferred_scan, priority);
> +		scan_count += deferred_scan;
> +
> +		/*
> +		 * If there is more deferred work than the number of freeable
> +		 * items in the cache, limit the amount of work we will carry
> +		 * over to the next kswapd run on this cache. This prevents
> +		 * deferred work windup.
> +		 */
> +		deferred_count = min(deferred_count, freeable_objects * 2);
> +

Extra whitespace above.

> +	}
>  
>  	/*
>  	 * Avoid risking looping forever due to too large nr value:
>  	 * never try to free more than twice the estimate number of
>  	 * freeable entries.
>  	 */

The comment refers to a variable that no longer exists.

I also wonder if it's a little cleaner to move the deferred_count =
min(...); statement above down here and condense the two comments.

> -	if (total_scan > freeable_objects * 2)
> -		total_scan = freeable_objects * 2;
> +	scan_count = min(scan_count, freeable_objects * 2);
>  
> -	trace_mm_shrink_slab_start(shrinker, shrinkctl, nr,
> +	trace_mm_shrink_slab_start(shrinker, shrinkctl, deferred_count,
>  				   freeable_objects, scan_count,
> -				   total_scan, priority);
> +				   scan_count, priority);
>  
>  	/*
>  	 * If the shrinker can't run (e.g. due to gfp_mask constraints), then
> @@ -601,10 +605,10 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>  	 * scanning at high prio and therefore should try to reclaim as much as
>  	 * possible.
>  	 */
> -	while (total_scan >= batch_size ||
> -	       total_scan >= freeable_objects) {
> +	while (scan_count >= batch_size ||
> +	       scan_count >= freeable_objects) {
>  		unsigned long ret;
> -		unsigned long nr_to_scan = min(batch_size, total_scan);
> +		unsigned long nr_to_scan = min_t(long, batch_size, scan_count);
>  
>  		shrinkctl->nr_to_scan = nr_to_scan;
>  		shrinkctl->nr_scanned = nr_to_scan;
> @@ -614,29 +618,29 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>  		freed += ret;
>  
>  		count_vm_events(SLABS_SCANNED, shrinkctl->nr_scanned);
> -		total_scan -= shrinkctl->nr_scanned;
> -		scanned += shrinkctl->nr_scanned;
> +		scan_count -= shrinkctl->nr_scanned;
> +		scanned_objects += shrinkctl->nr_scanned;
>  
>  		cond_resched();
>  	}
> -
>  done:
> -	if (next_deferred >= scanned)
> -		next_deferred -= scanned;
> +	if (deferred_count)
> +		next_deferred = deferred_count - scanned_objects;
>  	else
> -		next_deferred = 0;
> +		next_deferred = scan_count;

Hmm.. so if there was no deferred count on this cycle, we set
next_deferred to whatever is left from scan_count and add that back into
the shrinker struct below. If there was a pending deferred count on this
cycle, we subtract what we scanned from that and add that value back.
But what happens to the remaining scan_count in the latter case? Is it
lost, or am I missing something?

For example, suppose we start this cycle with a large scan_count and
->scan_objects() returned SHRINK_STOP before doing much work. In that
scenario, it looks like whether ->nr_deferred is 0 or not is the only
thing that determines whether we defer the entire remaining scan_count
or just what is left from the previous ->nr_deferred. The existing code
appears to consistently factor in what is left from the current scan
with the previous deferred count. Hm?

>  	/*
>  	 * move the unused scan count back into the shrinker in a
>  	 * manner that handles concurrent updates. If we exhausted the
>  	 * scan, there is no need to do an update.
>  	 */
>  	if (next_deferred > 0)
> -		new_nr = atomic_long_add_return(next_deferred,
> +		new_nr = atomic64_add_return(next_deferred,
>  						&shrinker->nr_deferred[nid]);
>  	else
> -		new_nr = atomic_long_read(&shrinker->nr_deferred[nid]);
> +		new_nr = atomic64_read(&shrinker->nr_deferred[nid]);

It looks like we could kill new_nr and just reuse next_deferred here
too.

Brian

>  
> -	trace_mm_shrink_slab_end(shrinker, nid, freed, nr, new_nr, total_scan);
> +	trace_mm_shrink_slab_end(shrinker, nid, freed, deferred_count, new_nr,
> +					scan_count);
>  	return freed;
>  }
>  
> -- 
> 2.24.0.rc0
>