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Message-ID: <1f6b254f-9cb9-412e-b249-8efe1d4157d3@linux.dev>
Date: Thu, 1 Aug 2024 15:40:16 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Takero Funaki <flintglass@...il.com>, Johannes Weiner
<hannes@...xchg.org>, Yosry Ahmed <yosryahmed@...gle.com>,
Nhat Pham <nphamcs@...il.com>, Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/2] mm: zswap: fix global shrinker error handling
logic
On 2024/7/31 08:49, Takero Funaki wrote:
> This patch fixes the zswap global shrinker, which did not shrink the
> zpool as expected.
>
> The issue addressed is that shrink_worker() did not distinguish between
> unexpected errors and expected errors, such as failed writeback from an
> empty memcg. The shrinker would stop shrinking after iterating through
> the memcg tree 16 times, even if there was only one empty memcg.
>
> With this patch, the shrinker no longer considers encountering an empty
> memcg, encountering a memcg with writeback disabled, or reaching the end
> of a memcg tree walk as a failure, as long as there are memcgs that are
> candidates for writeback. Systems with one or more empty memcgs will now
> observe significantly higher zswap writeback activity after the zswap
> pool limit is hit.
>
> To avoid an infinite loop when there are no writeback candidates, this
> patch tracks writeback attempts during memcg tree walks and limits
> reties if no writeback candidates are found.
>
> To handle the empty memcg case, the helper function shrink_memcg() is
> modified to check if the memcg is empty and then return -ENOENT.
>
> Fixes: a65b0e7607cc ("zswap: make shrinking memcg-aware")
> Signed-off-by: Takero Funaki <flintglass@...il.com>
Looks good to me:
Reviewed-by: Chengming Zhou <chengming.zhou@...ux.dev>
Thanks.
> ---
> mm/zswap.c | 40 +++++++++++++++++++++++++++++++++-------
> 1 file changed, 33 insertions(+), 7 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 3c16a1192252..d46caa42ed4f 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1287,10 +1287,10 @@ static struct shrinker *zswap_alloc_shrinker(void)
>
> static int shrink_memcg(struct mem_cgroup *memcg)
> {
> - int nid, shrunk = 0;
> + int nid, shrunk = 0, scanned = 0;
>
> if (!mem_cgroup_zswap_writeback_enabled(memcg))
> - return -EINVAL;
> + return -ENOENT;
>
> /*
> * Skip zombies because their LRUs are reparented and we would be
> @@ -1304,21 +1304,34 @@ static int shrink_memcg(struct mem_cgroup *memcg)
>
> shrunk += list_lru_walk_one(&zswap_list_lru, nid, memcg,
> &shrink_memcg_cb, NULL, &nr_to_walk);
> + scanned += 1 - nr_to_walk;
> }
> +
> + if (!scanned)
> + return -ENOENT;
> +
> return shrunk ? 0 : -EAGAIN;
> }
>
> static void shrink_worker(struct work_struct *w)
> {
> struct mem_cgroup *memcg;
> - int ret, failures = 0;
> + int ret, failures = 0, attempts = 0;
> unsigned long thr;
>
> /* Reclaim down to the accept threshold */
> thr = zswap_accept_thr_pages();
>
> /*
> - * Global reclaim will select cgroup in a round-robin fashion.
> + * Global reclaim will select cgroup in a round-robin fashion from all
> + * online memcgs, but memcgs that have no pages in zswap and
> + * writeback-disabled memcgs (memory.zswap.writeback=0) are not
> + * candidates for shrinking.
> + *
> + * Shrinking will be aborted if we encounter the following
> + * MAX_RECLAIM_RETRIES times:
> + * - No writeback-candidate memcgs found in a memcg tree walk.
> + * - Shrinking a writeback-candidate memcg failed.
> *
> * We save iteration cursor memcg into zswap_next_shrink,
> * which can be modified by the offline memcg cleaner
> @@ -1356,9 +1369,14 @@ static void shrink_worker(struct work_struct *w)
> spin_unlock(&zswap_shrink_lock);
>
> if (!memcg) {
> - if (++failures == MAX_RECLAIM_RETRIES)
> + /*
> + * Continue shrinking without incrementing failures if
> + * we found candidate memcgs in the last tree walk.
> + */
> + if (!attempts && ++failures == MAX_RECLAIM_RETRIES)
> break;
>
> + attempts = 0;
> goto resched;
> }
>
> @@ -1366,8 +1384,16 @@ static void shrink_worker(struct work_struct *w)
> /* drop the extra reference */
> mem_cgroup_put(memcg);
>
> - if (ret == -EINVAL)
> - break;
> + /*
> + * There are no writeback-candidate pages in the memcg.
> + * This is not an issue as long as we can find another memcg
> + * with pages in zswap. Skip this without incrementing attempts
> + * and failures.
> + */
> + if (ret == -ENOENT)
> + continue;
> + ++attempts;
> +
> if (ret && ++failures == MAX_RECLAIM_RETRIES)
> break;
> resched:
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