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[73.249.122.233]) by smtp.gmail.com with ESMTPSA id v185-20020a3793c2000000b0073b9ccb171asm4305649qkd.130.2023.02.23.07.26.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 23 Feb 2023 07:26:47 -0800 (PST) Date: Thu, 23 Feb 2023 10:26:45 -0500 From: Rafael Aquini To: Qi Zheng Cc: akpm@linux-foundation.org, tkhai@ya.ru, hannes@cmpxchg.org, shakeelb@google.com, mhocko@kernel.org, roman.gushchin@linux.dev, muchun.song@linux.dev, david@redhat.com, shy828301@gmail.com, sultan@kerneltoast.com, dave@stgolabs.net, penguin-kernel@i-love.sakura.ne.jp, paulmck@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 2/7] mm: vmscan: make global slab shrink lockless Message-ID: References: <20230223132725.11685-1-zhengqi.arch@bytedance.com> <20230223132725.11685-3-zhengqi.arch@bytedance.com> MIME-Version: 1.0 In-Reply-To: <20230223132725.11685-3-zhengqi.arch@bytedance.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 3DCCB2001E X-Rspam-User: X-Stat-Signature: rdmn9qa6hcjthm7qahh48qxmg4g84trh X-HE-Tag: 1677166010-951243 X-HE-Meta: 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 7WmlG/go 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Feb 23, 2023 at 09:27:20PM +0800, Qi Zheng wrote: > The shrinker_rwsem is a global lock in shrinkers subsystem, > it is easy to cause blocking in the following cases: > > a. the write lock of shrinker_rwsem was held for too long. > For example, there are many memcgs in the system, which > causes some paths to hold locks and traverse it for too > long. (e.g. expand_shrinker_info()) > b. the read lock of shrinker_rwsem was held for too long, > and a writer came at this time. Then this writer will be > forced to wait and block all subsequent readers. > For example: > - be scheduled when the read lock of shrinker_rwsem is > held in do_shrink_slab() > - some shrinker are blocked for too long. Like the case > mentioned in the patchset[1]. > > Therefore, many times in history ([2],[3],[4],[5]), some > people wanted to replace shrinker_rwsem reader with SRCU, > but they all gave up because SRCU was not unconditionally > enabled. > > But now, since commit 1cd0bd06093c ("rcu: Remove CONFIG_SRCU"), > the SRCU is unconditionally enabled. So it's time to use > SRCU to protect readers who previously held shrinker_rwsem. > > [1]. https://lore.kernel.org/lkml/20191129214541.3110-1-ptikhomirov@virtuozzo.com/ > [2]. https://lore.kernel.org/all/1437080113.3596.2.camel@stgolabs.net/ > [3]. https://lore.kernel.org/lkml/1510609063-3327-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp/ > [4]. https://lore.kernel.org/lkml/153365347929.19074.12509495712735843805.stgit@localhost.localdomain/ > [5]. https://lore.kernel.org/lkml/20210927074823.5825-1-sultan@kerneltoast.com/ > > Signed-off-by: Qi Zheng > --- > mm/vmscan.c | 27 +++++++++++---------------- > 1 file changed, 11 insertions(+), 16 deletions(-) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 9f895ca6216c..02987a6f95d1 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -202,6 +202,7 @@ static void set_task_reclaim_state(struct task_struct *task, > > LIST_HEAD(shrinker_list); > DECLARE_RWSEM(shrinker_rwsem); > +DEFINE_SRCU(shrinker_srcu); > > #ifdef CONFIG_MEMCG > static int shrinker_nr_max; > @@ -706,7 +707,7 @@ void free_prealloced_shrinker(struct shrinker *shrinker) > void register_shrinker_prepared(struct shrinker *shrinker) > { > down_write(&shrinker_rwsem); I think you could revert the rwsem back to a simple mutex, now. > - list_add_tail(&shrinker->list, &shrinker_list); > + list_add_tail_rcu(&shrinker->list, &shrinker_list); > shrinker->flags |= SHRINKER_REGISTERED; > shrinker_debugfs_add(shrinker); > up_write(&shrinker_rwsem); > @@ -760,13 +761,15 @@ void unregister_shrinker(struct shrinker *shrinker) > return; > > down_write(&shrinker_rwsem); > - list_del(&shrinker->list); > + list_del_rcu(&shrinker->list); > shrinker->flags &= ~SHRINKER_REGISTERED; > if (shrinker->flags & SHRINKER_MEMCG_AWARE) > unregister_memcg_shrinker(shrinker); > debugfs_entry = shrinker_debugfs_remove(shrinker); > up_write(&shrinker_rwsem); > > + synchronize_srcu(&shrinker_srcu); > + > debugfs_remove_recursive(debugfs_entry); > > kfree(shrinker->nr_deferred); > @@ -786,6 +789,7 @@ void synchronize_shrinkers(void) > { > down_write(&shrinker_rwsem); > up_write(&shrinker_rwsem); > + synchronize_srcu(&shrinker_srcu); > } > EXPORT_SYMBOL(synchronize_shrinkers); > > @@ -996,6 +1000,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid, > { > unsigned long ret, freed = 0; > struct shrinker *shrinker; > + int srcu_idx; > > /* > * The root memcg might be allocated even though memcg is disabled > @@ -1007,10 +1012,10 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid, > if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg)) > return shrink_slab_memcg(gfp_mask, nid, memcg, priority); > > - if (!down_read_trylock(&shrinker_rwsem)) > - goto out; > + srcu_idx = srcu_read_lock(&shrinker_srcu); > > - list_for_each_entry(shrinker, &shrinker_list, list) { > + list_for_each_entry_srcu(shrinker, &shrinker_list, list, > + srcu_read_lock_held(&shrinker_srcu)) { > struct shrink_control sc = { > .gfp_mask = gfp_mask, > .nid = nid, > @@ -1021,19 +1026,9 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid, > if (ret == SHRINK_EMPTY) > ret = 0; > freed += ret; > - /* > - * Bail out if someone want to register a new shrinker to > - * prevent the registration from being stalled for long periods > - * by parallel ongoing shrinking. > - */ > - if (rwsem_is_contended(&shrinker_rwsem)) { > - freed = freed ? : 1; > - break; > - } > } > > - up_read(&shrinker_rwsem); > -out: > + srcu_read_unlock(&shrinker_srcu, srcu_idx); > cond_resched(); > return freed; > } > -- > 2.20.1 > >