Date:   Tue, 5 Dec 2023 23:36:27 -0800
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Chengming Zhou <chengming.zhou@...ux.dev>
Cc:     Nhat Pham <nphamcs@...il.com>, akpm@...ux-foundation.org,
        hannes@...xchg.org, cerasuolodomenico@...il.com,
        sjenning@...hat.com, ddstreet@...e.org, vitaly.wool@...sulko.com,
        mhocko@...nel.org, roman.gushchin@...ux.dev, shakeelb@...gle.com,
        muchun.song@...ux.dev, chrisl@...nel.org, linux-mm@...ck.org,
        kernel-team@...a.com, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...r.kernel.org, shuah@...nel.org
Subject: Re: [PATCH v8 6/6] zswap: shrinks zswap pool based on memory pressure

On Tue, Dec 5, 2023 at 10:43 PM Chengming Zhou <chengming.zhou@...ux.dev> wrote:
>
> On 2023/12/6 13:59, Yosry Ahmed wrote:
> > [..]
> >>> @@ -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().
> >
>
> Yes. After changing to use mem_cgroup_flush_stats() here, the performance
> become much better.
>
> > 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,
>
> My test branch is linux-next 20231205, and it's all good after changing
> to use mem_cgroup_flush_stats(memcg).

Thanks for reporting back. We should still move the
zswap_shrinker_enabled check ahead, no need to even call
mem_cgroup_flush_stats() if we will do nothing anyway.

>
> > 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.
> >
>
> It's great. Thanks!
>

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