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Fri, 11 Dec 2020 09:52:54 -0800 (PST) MIME-Version: 1.0 References: <20201202182725.265020-1-shy828301@gmail.com> <20201202182725.265020-6-shy828301@gmail.com> <20201210153356.GE264602@cmpxchg.org> In-Reply-To: From: Yang Shi Date: Fri, 11 Dec 2020 09:52:42 -0800 Message-ID: Subject: Re: [PATCH 5/9] mm: memcontrol: add per memcg shrinker nr_deferred To: Johannes Weiner Cc: Roman Gushchin , Kirill Tkhai , Shakeel Butt , Dave Chinner , Michal Hocko , Andrew Morton , Linux MM , Linux FS-devel Mailing List , Linux Kernel Mailing List Content-Type: text/plain; charset="UTF-8" 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, Dec 10, 2020 at 11:12 AM Yang Shi wrote: > > On Thu, Dec 10, 2020 at 7:36 AM Johannes Weiner wrote: > > > > On Wed, Dec 02, 2020 at 10:27:21AM -0800, Yang Shi wrote: > > > @@ -504,6 +577,34 @@ int memcg_expand_shrinker_maps(int new_id) > > > return ret; > > > } > > > > > > +int memcg_expand_shrinker_deferred(int new_id) > > > +{ > > > + int size, old_size, ret = 0; > > > + struct mem_cgroup *memcg; > > > + > > > + size = (new_id + 1) * sizeof(atomic_long_t); > > > + old_size = memcg_shrinker_deferred_size; > > > + if (size <= old_size) > > > + return 0; > > > + > > > + mutex_lock(&memcg_shrinker_mutex); > > > > The locking is somewhat confusing. I was wondering why we first read > > memcg_shrinker_deferred_size "locklessly", then change it while > > holding the &memcg_shrinker_mutex. > > > > memcg_shrinker_deferred_size only changes under shrinker_rwsem(write), > > correct? This should be documented in a comment, IMO. > > Yes, it is correct. > > > > > memcg_shrinker_mutex looks superfluous then. The memcg allocation path > > is the read-side of memcg_shrinker_deferred_size, and so simply needs > > to take shrinker_rwsem(read) to lock out shrinker (de)registration. > > I see you point. Yes, it seems shrinker_{maps|deferred} allocation > could be synchronized with shrinker registration by shrinker_rwsem. > > memcg_shrinker_mutex is just renamed from memcg_shrinker_map_mutex > which was introduced by shrinker_maps patchset. I'm not quite sure why > this mutex was introduced at the first place, I guess the main purpose > is to *not* exacerbate the contention of shrinker_rwsem? > > If that contention is not a concern, we could remove that dedicated mutex. It seems using shrinker_rwsem instead of dedicated mutex should not exacerbate the contention since we just add one read critical section. Will do it in v2. > > > > > Also, isn't memcg_shrinker_deferred_size just shrinker_nr_max? And > > No, it is variable. It is nr * sizeof(atomit_long_t). The nr is the > current last shrinker ID. If a new shrinker is registered, the nr may > grow. > > > memcg_expand_shrinker_deferred() is only called when size >= old_size > > in the first place (because id >= shrinker_nr_max)? > > Yes.