From: Tetsuo Handa <firstname.lastname@example.org> To: Michal Hocko <email@example.com> Cc: Andrew Morton <firstname.lastname@example.org>, Johannes Weiner <email@example.com>, David Rientjes <firstname.lastname@example.org>, email@example.com, Yong-Taek Lee <firstname.lastname@example.org>, Paul McKenney <email@example.com>, Linus Torvalds <firstname.lastname@example.org> Subject: Re: [PATCH] mm, oom: Tolerate processes sharing mm with different view of oom_score_adj. Date: Wed, 16 Jan 2019 22:32:50 +0900 [thread overview] Message-ID: <email@example.com> (raw) In-Reply-To: <20190116121915.GJ24149@dhcp22.suse.cz> On 2019/01/16 21:19, Michal Hocko wrote: > On Wed 16-01-19 20:30:25, Tetsuo Handa wrote: >> On 2019/01/16 20:09, Michal Hocko wrote: >>> On Wed 16-01-19 19:55:21, Tetsuo Handa wrote: >>>> This patch reverts both commit 44a70adec910d692 ("mm, oom_adj: make sure >>>> processes sharing mm have same view of oom_score_adj") and commit >>>> 97fd49c2355ffded ("mm, oom: kill all tasks sharing the mm") in order to >>>> close a race and reduce the latency at __set_oom_adj(), and reduces the >>>> warning at __oom_kill_process() in order to minimize the latency. >>>> >>>> Commit 36324a990cf578b5 ("oom: clear TIF_MEMDIE after oom_reaper managed >>>> to unmap the address space") introduced the worst case mentioned in >>>> 44a70adec910d692. But since the OOM killer skips mm with MMF_OOM_SKIP set, >>>> only administrators can trigger the worst case. >>>> >>>> Since 44a70adec910d692 did not take latency into account, we can hold RCU >>>> for minutes and trigger RCU stall warnings by calling printk() on many >>>> thousands of thread groups. Even without calling printk(), the latency is >>>> mentioned by Yong-Taek Lee . And I noticed that 44a70adec910d692 is >>>> racy, and trying to fix the race will require a global lock which is too >>>> costly for rare events. >>>> >>>> If the worst case in 44a70adec910d692 happens, it is an administrator's >>>> request. Therefore, tolerate the worst case and speed up __set_oom_adj(). >>> >>> I really do not think we care about latency. I consider the overal API >>> sanity much more important. Besides that the original report you are >>> referring to was never exaplained/shown to represent real world usecase. >>> oom_score_adj is not really a an interface to be tweaked in hot paths. >> >> I do care about the latency. Holding RCU for more than 2 minutes is insane. > > Creating 8k threads could be considered insane as well. But more > seriously. I absolutely do not insist on holding a single RCU section > for the whole operation. But that doesn't really mean that we want to > revert these changes. for_each_process is by far not only called from > this path. Unlike check_hung_uninterruptible_tasks() where failing to resume after breaking RCU section is tolerable, failing to resume after breaking RCU section for __set_oom_adj() is not tolerable; it leaves the possibility of different oom_score_adj. Unless it is inevitable (e.g. SysRq-t), I think that calling printk() on each thread from RCU section is a poor choice. What if thousands of threads concurrently called __set_oom_adj() when each __set_oom_adj() call involves printk() on thousands of threads which can take more than 2 minutes? How long will it take to complete?
next prev parent reply other threads:[~2019-01-16 13:33 UTC|newest] Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-01-16 10:55 Tetsuo Handa 2019-01-16 11:09 ` Michal Hocko 2019-01-16 11:30 ` Tetsuo Handa 2019-01-16 12:19 ` Michal Hocko 2019-01-16 13:32 ` Tetsuo Handa [this message] 2019-01-16 13:41 ` Michal Hocko 2019-01-17 10:40 ` Tetsuo Handa 2019-01-17 15:51 ` Michal Hocko 2019-01-30 22:49 ` [PATCH v2] " Tetsuo Handa 2019-01-31 7:11 ` Michal Hocko 2019-01-31 20:59 ` Tetsuo Handa 2019-02-01 9:14 ` Michal Hocko 2019-02-02 11:06 ` Tetsuo Handa 2019-02-11 15:07 ` Michal Hocko
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