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Wed, 26 Aug 2020 09:44:47 -0700 (PDT) Date: Wed, 26 Aug 2020 12:43:32 -0400 From: Johannes Weiner To: Xunlei Pang Cc: Michal Hocko , Andrew Morton , Vladimir Davydov , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v2] mm: memcg: Fix memcg reclaim soft lockup Message-ID: <20200826164332.GB995045@cmpxchg.org> References: <1598449622-108748-1-git-send-email-xlpang@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1598449622-108748-1-git-send-email-xlpang@linux.alibaba.com> X-Rspamd-Queue-Id: BCA2B1004DD92 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam03 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 Wed, Aug 26, 2020 at 09:47:02PM +0800, Xunlei Pang wrote: > We've met softlockup with "CONFIG_PREEMPT_NONE=y", when > the target memcg doesn't have any reclaimable memory. > > It can be easily reproduced as below: > watchdog: BUG: soft lockup - CPU#0 stuck for 111s![memcg_test:2204] > CPU: 0 PID: 2204 Comm: memcg_test Not tainted 5.9.0-rc2+ #12 > Call Trace: > shrink_lruvec+0x49f/0x640 > shrink_node+0x2a6/0x6f0 > do_try_to_free_pages+0xe9/0x3e0 > try_to_free_mem_cgroup_pages+0xef/0x1f0 > try_charge+0x2c1/0x750 > mem_cgroup_charge+0xd7/0x240 > __add_to_page_cache_locked+0x2fd/0x370 > add_to_page_cache_lru+0x4a/0xc0 > pagecache_get_page+0x10b/0x2f0 > filemap_fault+0x661/0xad0 > ext4_filemap_fault+0x2c/0x40 > __do_fault+0x4d/0xf9 > handle_mm_fault+0x1080/0x1790 > > It only happens on our 1-vcpu instances, because there's no chance > for oom reaper to run to reclaim the to-be-killed process. > > Add cond_resched() at the upper shrink_node_memcgs() to solve this > issue, and any other possible issue like meomry.min protection. > > Suggested-by: Michal Hocko > Signed-off-by: Xunlei Pang This generally makes sense to me but really should have a comment: /* * This loop can become CPU-bound when there are thousands * of cgroups that aren't eligible for reclaim - either * because they don't have any pages, or because their * memory is explicitly protected. Avoid soft lockups. */ cond_resched(); The placement in the middle of the multi-part protection checks is a bit odd too. It would be better to have it either at the top of the loop, or at the end, by replacing the continues with goto next.