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Tue, 21 Feb 2023 23:32:09 -0800 (PST) Received: from [10.200.11.31] ([139.177.225.226]) by smtp.gmail.com with ESMTPSA id k13-20020a636f0d000000b004fbb48e3e5csm4092110pgc.77.2023.02.21.23.32.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 21 Feb 2023 23:32:09 -0800 (PST) Message-ID: <2eb276f1-0d81-f207-4cf9-e6586c5e18c8@bytedance.com> Date: Wed, 22 Feb 2023 15:32:00 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.7.2 Subject: Re: [PATCH 2/5] mm: vmscan: make memcg slab shrink lockless To: Kirill Tkhai Cc: 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, Andrew Morton , Johannes Weiner , Shakeel Butt , Michal Hocko , Roman Gushchin , Muchun Song , David Hildenbrand , Yang Shi References: <20230220091637.64865-1-zhengqi.arch@bytedance.com> <20230220091637.64865-3-zhengqi.arch@bytedance.com> <8733cb3c-7a6a-33c4-b84e-4fb981dae765@ya.ru> Content-Language: en-US From: Qi Zheng In-Reply-To: <8733cb3c-7a6a-33c4-b84e-4fb981dae765@ya.ru> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 0231EC0010 X-Rspam-User: X-Stat-Signature: izpyjjwuq3ozaysta5quj55xm8bqp7of X-HE-Tag: 1677051130-936678 X-HE-Meta: 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 LOvM8dvT i1rT+6q/FWQ0geLw3NBLXud2m440Rka/jqAIOBFjNY+twNUJ0Ceib9d5R6Bc7qw4UzfiFUZj8Jp5KmDqEl19f9EwxoHCSqhJvOV5BVrKuL3vHhnhMXA2tTjyV2kc8y4dR5w132nlexhRijykhorF7MCpJ9PPXeSpw1DCJ1ClxaOtbZiYpOQ/cApfNBCl7zwBCwZw4pYC8i9qXbNAN7MTgdeqOCIL0eF+2cQ2iv8PxKrEIYBW/uMcGDwPt1EEntXVhlP06yp2z5ud7JmTlQjFNMGREoQygw6IgXCG6tUhtRK6LOE3Z/GW4RhVhs4Ppjrn9EhQ8eA8kiXVaPrmQA7DKKAuelTdQ/0N4e5c6RTfDaUGcq98RWyMSgq8q2W/ZrLysJGblD5//xjkBr9onruZWYEA3N5lbdcG1wp6DaCRlnwDls+J328cN6PuVbhoeqjIU+HmO 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 2023/2/22 05:28, Kirill Tkhai wrote: > On 20.02.2023 12:16, Qi Zheng wrote: <...> >> >> void reparent_shrinker_deferred(struct mem_cgroup *memcg) >> { >> - int i, nid; >> + int i, nid, srcu_idx; >> long nr; >> struct mem_cgroup *parent; >> struct shrinker_info *child_info, *parent_info; >> @@ -429,16 +443,16 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg) >> parent = root_mem_cgroup; >> >> /* Prevent from concurrent shrinker_info expand */ >> - down_read(&shrinker_rwsem); >> + srcu_idx = srcu_read_lock(&shrinker_srcu); > > Don't we still have to be protected against parallel expand_one_shrinker_info()? > > It looks like parent->nodeinfo[nid]->shrinker_info pointer may be changed in expand* > right after we've dereferenced it here. Hi Kirill, Oh, indeed. We may wrongly reparent the child's nr_deferred to the old parent's nr_deferred (it is about to be freed by call_srcu). The reparent_shrinker_deferred() will only be called on the offline path (not a hotspot path), so we may be able to use shrinker_mutex introduced later for protection. What do you think? Thanks, Qi > >> for_each_node(nid) { >> - child_info = shrinker_info_protected(memcg, nid); >> - parent_info = shrinker_info_protected(parent, nid); >> + child_info = shrinker_info_srcu(memcg, nid); >> + parent_info = shrinker_info_srcu(parent, nid); >> for (i = 0; i < shrinker_nr_max; i++) { >> nr = atomic_long_read(&child_info->nr_deferred[i]); >> atomic_long_add(nr, &parent_info->nr_deferred[i]); >> } >> } >> - up_read(&shrinker_rwsem); >> + srcu_read_unlock(&shrinker_srcu, srcu_idx); >> } >> >> static bool cgroup_reclaim(struct scan_control *sc) >> @@ -891,15 +905,14 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid, >> { >> struct shrinker_info *info; >> unsigned long ret, freed = 0; >> + int srcu_idx; >> int i; >> >> if (!mem_cgroup_online(memcg)) >> return 0; >> >> - if (!down_read_trylock(&shrinker_rwsem)) >> - return 0; >> - >> - info = shrinker_info_protected(memcg, nid); >> + srcu_idx = srcu_read_lock(&shrinker_srcu); >> + info = shrinker_info_srcu(memcg, nid); >> if (unlikely(!info)) >> goto unlock; >> >> @@ -949,14 +962,9 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid, >> set_shrinker_bit(memcg, nid, i); >> } >> freed += ret; >> - >> - if (rwsem_is_contended(&shrinker_rwsem)) { >> - freed = freed ? : 1; >> - break; >> - } >> } >> unlock: >> - up_read(&shrinker_rwsem); >> + srcu_read_unlock(&shrinker_srcu, srcu_idx); >> return freed; >> } >> #else /* CONFIG_MEMCG */ > -- Thanks, Qi