Re: [PATCH 04/24] shrinker: defer work only to kswapd

Nikolay Borisov
2019-08-04T16:48:05Z

On 1.08.19 г. 5:17 ч., Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> 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.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> mm/vmscan.c | 93 ++++++++++++++++++++++++++++-------------------------
> 1 file changed, 50 insertions(+), 43 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index b7472953b0e6..c583b4efb9bf 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -500,15 +500,15 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
> struct shrinker *shrinker, int priority)
> {
> unsigned long freed = 0;
> - long total_scan;
> int64_t freeable_objects = 0;
> int64_t scan_count;
> - long nr;
> + int64_t scanned_objects = 0;
> + int64_t next_deferred = 0;
> + int64_t deferred_count = 0;
> long 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;
> @@ -519,47 +519,53 @@ 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.
> + * 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.
> + *
> + * 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.
> */
> - nr = atomic_long_xchg(&shrinker->nr_deferred[nid], 0);
> + if (current_is_kswapd()) {
> + int64_t deferred_scan;
>
> - 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;
> + deferred_count = atomic64_xchg(&shrinker->nr_deferred[nid], 0);
>
> - /*
> - * 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.
> - *
> - * Hence only allow the shrinker to scan the entire cache when
> - * a large delta change is calculated directly.
> - */
> - if (scan_count < freeable_objects / 4)
> - total_scan = min_t(long, total_scan, freeable_objects / 2);
> + /* 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.
> + */
> + if (deferred_count > freeable_objects * 2)
> + deferred_count = freeable_objects * 2;

nit : deferred_count = min(deferred_count, freeable_objects * 2).

How can we have more deferred objects than are currently on the LRU?
Aren't deferred objects always some part of freeable objects. Shouldn't
this mean that for a particular shrinker deferred_count <= freeable_objects?

> +
> + }
>
> /*
> * 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_objects * 2)
> - total_scan = freeable_objects * 2;
> + if (scan_count > freeable_objects * 2)
> + scan_count = freeable_objects * 2;

nit: 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
> @@ -583,10 +589,10 @@ 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;
> @@ -596,17 +602,17 @@ 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;
> - else
> - next_deferred = 0;
> + if (deferred_count)
> + next_deferred = deferred_count - scanned_objects;
> + else if (scan_count > 0)
> + next_deferred = scan_count;
> /*
> * move the unused scan count back into the shrinker in a
> * manner that handles concurrent updates. If we exhausted the
> @@ -618,7 +624,8 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
> else
> new_nr = atomic_long_read(&shrinker->nr_deferred[nid]);
>
> - 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;
> }
>
>