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Thu, 23 Feb 2023 10:24:50 -0800 (PST) Received: from sultan-box.localdomain ([142.147.89.230]) by smtp.gmail.com with ESMTPSA id a62-20020a639041000000b004cd2eebc551sm7002483pge.62.2023.02.23.10.24.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 23 Feb 2023 10:24:49 -0800 (PST) Date: Thu, 23 Feb 2023 10:24:47 -0800 From: Sultan Alsawaf 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, 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 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230223132725.11685-3-zhengqi.arch@bytedance.com> X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: AC09A1C0002 X-Stat-Signature: gueizj13p77s8dofrcx59y14a4r9w9yg X-HE-Tag: 1677176691-553601 X-HE-Meta: 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 gsq4p3Y1 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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); > - 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 > > Hi Qi, A different problem I realized after my old attempt to use SRCU was that the unregister_shrinker() path became quite slow due to the heavy synchronize_srcu() call. Both register_shrinker() *and* unregister_shrinker() are called frequently these days, and SRCU is too unfair to the unregister path IMO. Although I never got around to submitting it, I made a non-SRCU solution [1] that uses fine-grained locking instead, which is fair to both the register path and unregister path. (The patch I've linked is a version of this adapted to an older 4.14 kernel FYI, but it can be reworked for the current kernel.) What do you think about the fine-grained locking approach? Thanks, Sultan [1] https://github.com/kerneltoast/android_kernel_google_floral/commit/012378f3173a82d2333d3ae7326691544301e76a