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 [1]. 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?
