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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Tue, 5 Dec 2023 23:36:27 -0800
Message-ID: <CAJD7tkYkgRxDgZzCs2su7e4ocr5=zz2fjjr81+t35d_sp0E0gQ@mail.gmail.com>
Subject: Re: [PATCH v8 6/6] zswap: shrinks zswap pool based on memory pressure
To:     Chengming Zhou <chengming.zhou@linux.dev>
Cc:     Nhat Pham <nphamcs@gmail.com>, akpm@linux-foundation.org,
        hannes@cmpxchg.org, cerasuolodomenico@gmail.com,
        sjenning@redhat.com, ddstreet@ieee.org, vitaly.wool@konsulko.com,
        mhocko@kernel.org, roman.gushchin@linux.dev, shakeelb@google.com,
        muchun.song@linux.dev, chrisl@kernel.org, linux-mm@kvack.org,
        kernel-team@meta.com, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kselftest@vger.kernel.org, shuah@kernel.org
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On Tue, Dec 5, 2023 at 10:43=E2=80=AFPM Chengming Zhou <chengming.zhou@linu=
x.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, struc=
t 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 =3D mem_cgroup_lruvec(sc->memcg, NODE_DAT=
A(sc->nid));
> >>> +     unsigned long shrink_ret, nr_protected, lru_size;
> >>> +     struct zswap_pool *pool =3D shrinker->private_data;
> >>> +     bool encountered_page_in_swapcache =3D false;
> >>> +
> >>> +     nr_protected =3D
> >>> +             atomic_long_read(&lruvec->zswap_lruvec_state.nr_zswap_p=
rotected);
> >>> +     lru_size =3D list_lru_shrink_count(&pool->list_lru, sc);
> >>> +
> >>> +     /*
> >>> +      * Abort if the shrinker is disabled or if we are shrinking int=
o the
> >>> +      * protected region.
> >>> +      *
> >>> +      * This short-circuiting is necessary because if we have too ma=
ny multiple
> >>> +      * concurrent reclaimers getting the freeable zswap object coun=
ts 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 >=3D lru_size - sc-=
>nr_to_scan) {
> >>> +             sc->nr_scanned =3D 0;
> >>> +             return SHRINK_STOP;
> >>> +     }
> >>> +
> >>> +     shrink_ret =3D list_lru_shrink_walk(&pool->list_lru, sc, &shrin=
k_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 =3D shrinker->private_data;
> >>> +     struct mem_cgroup *memcg =3D sc->memcg;
> >>> +     struct lruvec *lruvec =3D 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 =3D memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_S=
HIFT;
> >>> +     nr_stored =3D memcg_page_state(memcg, MEMCG_ZSWAPPED);
> >>> +#else
> >>> +     /* use pool stats instead of memcg stats */
> >>> +     nr_backing =3D get_zswap_pool_size(pool) >> PAGE_SHIFT;
> >>> +     nr_stored =3D atomic_read(&pool->nr_stored);
> >>> +#endif
> >>> +
> >>> +     if (!zswap_shrinker_enabled || !nr_stored)
> >> When I tested with this series, with !zswap_shrinker_enabled in the de=
fault case,
> >> I found the performance is much worse than that without this patch.
> >>
> >> Testcase: memory.max=3D2G, zswap enabled, kernel build -j32 in a tmpfs=
 directory.
> >>
> >> The reason seems the above cgroup_rstat_flush(), caused much rstat loc=
k contention
> >> to the zswap_store() path. And if I put the "zswap_shrinker_enabled" c=
heck 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!
>