Re: [PATCH v8 6/6] zswap: shrinks zswap pool based on memory pressure

On Tue, Dec 5, 2023 at 10:00 PM Yosry Ahmed yosryahmed@google.com wrote:

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@@ -526,6 +582,102 @@ static struct zswap_entry *zswap_entry_find_get(struct rb_root *root, return entry; }

+/********************************* +* shrinker functions +**********************************/ +static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_one *l,

•                                spinlock_t *lock, void *arg);
  

+static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,

•         struct shrink_control *sc)
  

+{

• struct lruvec *lruvec = mem_cgroup_lruvec(sc->memcg, NODE_DATA(sc->nid));
  

• unsigned long shrink_ret, nr_protected, lru_size;
  

• struct zswap_pool *pool = shrinker->private_data;
  

• bool encountered_page_in_swapcache = false;
  

• nr_protected =
  

•         atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
  

• lru_size = list_lru_shrink_count(&pool->list_lru, sc);
  

• /*
  

•  * Abort if the shrinker is disabled or if we are shrinking into the
  

•  * protected region.
  

•  *
  

•  * This short-circuiting is necessary because if we have too many multiple
  

•  * concurrent reclaimers getting the freeable zswap object counts at the
  

•  * same time (before any of them made reasonable progress), the total
  

•  * number of reclaimed objects might be more than the number of unprotected
  

•  * objects (i.e the reclaimers will reclaim into the protected area of the
  

•  * zswap LRU).
  

•  */
  

• if (!zswap_shrinker_enabled || nr_protected >= lru_size - sc->nr_to_scan) {
  

•         sc->nr_scanned = 0;
  

•         return SHRINK_STOP;
  

• }
  

• shrink_ret = list_lru_shrink_walk(&pool->list_lru, sc, &shrink_memcg_cb,
  

•         &encountered_page_in_swapcache);
  

• if (encountered_page_in_swapcache)
  

•         return SHRINK_STOP;
  

• return shrink_ret ? shrink_ret : SHRINK_STOP;
  

+}

+static unsigned long zswap_shrinker_count(struct shrinker *shrinker,

•         struct shrink_control *sc)
  

+{

• struct zswap_pool *pool = shrinker->private_data;
  

• struct mem_cgroup *memcg = sc->memcg;
  

• struct lruvec *lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(sc->nid));
  

• unsigned long nr_backing, nr_stored, nr_freeable, nr_protected;
  

+#ifdef CONFIG_MEMCG_KMEM

• cgroup_rstat_flush(memcg->css.cgroup);
  

• nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT;
  

• nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);
  

+#else

• /* use pool stats instead of memcg stats */
  

• nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;
  

• nr_stored = atomic_read(&pool->nr_stored);
  

+#endif

• if (!zswap_shrinker_enabled || !nr_stored)
  

When I tested with this series, with !zswap_shrinker_enabled in the default case, I found the performance is much worse than that without this patch.

Testcase: memory.max=2G, zswap enabled, kernel build -j32 in a tmpfs directory.

The reason seems the above cgroup_rstat_flush(), caused much rstat lock contention to the zswap_store() path. And if I put the "zswap_shrinker_enabled" check above the cgroup_rstat_flush(), the performance become much better.

Maybe we can put the "zswap_shrinker_enabled" check above cgroup_rstat_flush()?

Yes, we should do nothing if !zswap_shrinker_enabled. We should also use mem_cgroup_flush_stats() here like other places unless accuracy is crucial, which I doubt given that reclaim uses mem_cgroup_flush_stats().

Ah, good points on both suggestions. We should not do extra work for non-user. And, this is a best-effort approximation of the memory saving factor, so as long as it is not *too* far off I think it's acceptable.

mem_cgroup_flush_stats() has some thresholding to make sure we don't do flushes unnecessarily, and I have a pending series in mm-unstable that makes that thresholding per-memcg. Keep in mind that adding a call to mem_cgroup_flush_stats() will cause a conflict in mm-unstable, because the series there adds a memcg argument to mem_cgroup_flush_stats(). That should be easily amenable though, I can post a fixlet for my series to add the memcg argument there on top of users if needed.

Hmm so how should we proceed from here? How about this:

a) I can send a fixlet to move the enablement check above the stats flushing + use mem_cgroup_flush_stats b) Then maybe, you can send a fixlet to update this new callsite?

Does that sound reasonable?

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Thanks!

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•         return 0;
  

• nr_protected =
  

•         atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_protected);
  

• nr_freeable = list_lru_shrink_count(&pool->list_lru, sc);
  

• /*
  

•  * Subtract the lru size by an estimate of the number of pages
  

•  * that should be protected.
  

•  */
  

• nr_freeable = nr_freeable > nr_protected ? nr_freeable - nr_protected : 0;
  

• /*
  

•  * Scale the number of freeable pages by the memory saving factor.
  

•  * This ensures that the better zswap compresses memory, the fewer
  

•  * pages we will evict to swap (as it will otherwise incur IO for
  

•  * relatively small memory saving).
  

•  */
  

• return mult_frac(nr_freeable, nr_backing, nr_stored);
  

+}

+static void zswap_alloc_shrinker(struct zswap_pool *pool) +{

• pool->shrinker =
  

•         shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE, "mm-zswap");
  

• if (!pool->shrinker)
  

•         return;
  

• pool->shrinker->private_data = pool;
  

• pool->shrinker->scan_objects = zswap_shrinker_scan;
  

• pool->shrinker->count_objects = zswap_shrinker_count;
  

• pool->shrinker->batch = 0;
  

• pool->shrinker->seeks = DEFAULT_SEEKS;
  

+}

/*********************************

• per-cpu code

**********************************/

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