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McKenney" Cc: Qi Zheng , 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, 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> <20230223183917.GG2948950@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230223183917.GG2948950@paulmck-ThinkPad-P17-Gen-1> X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 1BE1F180016 X-Stat-Signature: bo1giaqd5whcb3q8antxcjbtumxzkcwz X-HE-Tag: 1677179894-758928 X-HE-Meta: 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 9AG36kP3 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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 10:39:17AM -0800, Paul E. McKenney wrote: > On Thu, Feb 23, 2023 at 10:24:47AM -0800, Sultan Alsawaf wrote: > > 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? > > Another approach is to use synchronize_srcu_expedited(), which avoids > the sleeps that are otherwise used to encourage sharing of grace periods > among concurrent requests. It might be possible to use call_srcu(), > but I don't claim to know the shrinker code well enough to say for sure. Hi Paul, I don't believe call_srcu() can be used since shrinker users need to be guaranteed that their shrinkers aren't in use after unregister_shrinker(). Using synchronize_srcu_expedited() sounds like it'd definitely help, though unregistering a single shrinker would ultimately still require waiting for all shrinkers to finish running before the grace period can elapse. There can be many shrinkers and they're not very fast I think. Thanks, Sultan > > Thanx, Paul > > > Thanks, > > Sultan > > > > [1] https://github.com/kerneltoast/android_kernel_google_floral/commit/012378f3173a82d2333d3ae7326691544301e76a >