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Wed, 08 Mar 2023 23:06:36 -0800 (PST) Received: from [10.70.252.135] ([139.177.225.245]) by smtp.gmail.com with ESMTPSA id 1-20020aa79241000000b005a7ae8b3a09sm10498506pfp.32.2023.03.08.23.06.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 08 Mar 2023 23:06:35 -0800 (PST) Message-ID: <07078623-b7ef-ceb4-eccc-8872a4067273@bytedance.com> Date: Thu, 9 Mar 2023 15:06:25 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.8.0 Subject: Re: [PATCH v4 7/8] mm: vmscan: remove shrinker_rwsem from synchronize_shrinkers() Content-Language: en-US To: Kirill Tkhai , akpm@linux-foundation.org, hannes@cmpxchg.org, shakeelb@google.com, mhocko@kernel.org, roman.gushchin@linux.dev, muchun.song@linux.dev, david@redhat.com, shy828301@gmail.com, rppt@kernel.org 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, christian.koenig@amd.com References: <20230307065605.58209-1-zhengqi.arch@bytedance.com> <20230307065605.58209-8-zhengqi.arch@bytedance.com> From: Qi Zheng In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 133DC1C000C X-Stat-Signature: b9no1pk9rofznbwfqahu1zdj1ohtp8ms X-HE-Tag: 1678345597-428988 X-HE-Meta: U2FsdGVkX18r6Xf46FRtr5iZojCCCclZG/f7yeFBCc8uATv6vHQTK9KK03V/CCAhX5ij9+5HObE0s8/i6DbO/rHhOhub7EHGvo+ljWuexyLniHeaqKCRv/YPGOF7LD624l4x5DTvtaUUC40FjrRl0HSo63fWDO0xYVyssZfaoU5aB3hVai6/iphGKn35vQklB0J1W7Rifula5O3/LSJu/dPTtHlK5UIa/88FN9AbTaHlEOtiWz99j7qLIxGd0RKp34M4pe+tDnK1tv0cqAERePsTpST+Cj4Q03WW82aeKYwQDY/Ywwc4GePDRDENU2Q44no2BEx2YCX/qFe76NAXZStGS4AskNo30Z0w4fQR5KsQmqSM55vXKR9R5jhi+M5Jk52IxgCcyVLQUmuiQG3MrSMUaC+LPywJ5Nl7RkDrvFT+YbHVi27Wk3gDnp+2lzYGmUXnSWp9RFHBPzpjzeduBSTAz2CMm9SXD4EJ20Dyo75rDy+GWpH+uC7FtlZDr6aBAC4NVOkPP1Etk6NRv0jqKca4gv+w328GCmZL1lnG9aK0vktd1ss4O8JT/Ulggv+TPv7jHsUgm+svZo2OQMEBFO1BWEA/DnCEsPJDW3m0cHvXeexhsekkmeubBGYRAlZ92CXK6aRXAsu00ExX+lvk+NAoQRtwI1jQI0EcJFSSCZKcVC9Whi6yVPj0ZAVQ/ziDPII0Owpx95LbQm8M/Dqsb5sytJuA8B7mcJtyc+yCCaBXZ22Z1WhsPcRsDrw3d+TTpQ3RVTJHdZemo4Ekn3/U9y7cYmy6r6644GSeLRRTQ+k3P3obMxBkAE4gFwSId79aM4zvGafp/JBRXADG5i7VXAvGhx4/qEMvq4anq8aN6mJjaQJH3JjdlGhS3BVxPFaMjjDZnJue0rkgUOwZseW1E6cG0VtGf5K6WZ5TLa5rZ6E/I6i1jsebNAnobBtDdNm+ZOEIPD414RaRS9iaLsU P0ntlHIO Dmg1hF2/VPPdAop/FW8LwxzJunsDlPFadicL3lhTXsZZxZCsKR1upxtoyNk09Yi+oYO88ClWJo0J2Idd+S5D1n61Kyviy1nNYjBz5D6g+ckxuNRZa90CsG9F03Hz/2u2Ey0iWRaGj6l3RNwLj3tVAFADpunubgxhiPoo01o+1wmDSmXdjYorgnxyJzVuG04P3f69Fprm9Jc2Vc6NjbLOfEwPeUtFHukVk1lJy9r/fP9rMC2LZz/dEGcP5dJD1E1zXMcZR0coftsnexPzynPCMLh8U/xXDD6l7EDeFNukhoxkXN02WRq777PxE4c4cbhFnOQt6f0bqfMhlIcSwWnZe3FTOUTYoTM1IWpkVs04pmaKfJd3DSld01e8M6viy5IHeGC66jppmyPRdggPDzTwDSES6MleTbDMbEzd8Fvl9jgrAXsJV1GnzMJ9qXE4RJ2aiqa+NYeUkb8DOEyCxIouD+l6ux7XFckiR4QNQ 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: Hi Kirill, On 2023/3/9 06:39, Kirill Tkhai wrote: > On 07.03.2023 09:56, Qi Zheng wrote: >> Now there are no readers of shrinker_rwsem, so >> synchronize_shrinkers() does not need to hold the >> writer of shrinker_rwsem to wait for all running >> shinkers to complete, synchronize_srcu() is enough. >> >> Signed-off-by: Qi Zheng >> --- >> mm/vmscan.c | 8 ++------ >> 1 file changed, 2 insertions(+), 6 deletions(-) >> >> diff --git a/mm/vmscan.c b/mm/vmscan.c >> index 7aaf6f94ac1b..ac7ab4aa344f 100644 >> --- a/mm/vmscan.c >> +++ b/mm/vmscan.c >> @@ -796,15 +796,11 @@ EXPORT_SYMBOL(unregister_shrinker); >> /** >> * synchronize_shrinkers - Wait for all running shrinkers to complete. >> * >> - * This is equivalent to calling unregister_shrink() and register_shrinker(), >> - * but atomically and with less overhead. This is useful to guarantee that all >> - * shrinker invocations have seen an update, before freeing memory, similar to >> - * rcu. >> + * This is useful to guarantee that all shrinker invocations have seen an >> + * update, before freeing memory. >> */ >> void synchronize_shrinkers(void) >> { >> - down_write(&shrinker_rwsem); >> - up_write(&shrinker_rwsem); >> atomic_inc(&shrinker_srcu_generation); >> synchronize_srcu(&shrinker_srcu); >> } > > Just curious, callers of synchronize_shrinkers() don't want to have parallel register_shrinker() and unregister_shrink() are completed? > Here we only should wait for parallel shrink_slab(), correct? I think yes. The synchronize_shrinkers() is currently only used by TTM pool. In TTM pool, a shrinker named "drm-ttm_pool" is registered, and the scan_objects callback will pick an entry from its own shrinker_list: ttm_pool_shrink --> spin_lock(&shrinker_lock); pt = list_first_entry(&shrinker_list, typeof(*pt), shrinker_list); list_move_tail(&pt->shrinker_list, &shrinker_list); spin_unlock(&shrinker_lock); These entries have been removed from shrinker_list before calling synchronize_shrinkers(): ttm_pool_fini --> ttm_pool_type_fini --> spin_lock(&shrinker_lock); list_del(&pt->shrinker_list); spin_unlock(&shrinker_lock); synchronize_shrinkers So IIUC, we only need to wait for the parallel shrink_slab() here. Like its comment says: /* We removed the pool types from the LRU, but we need to also make sure * that no shrinker is concurrently freeing pages from the pool. */ + CC: Christian König :) Thanks, Qi