Re: [PATCH (resend)] mm,oom: Defer dump_tasks() output.

[Michal Hocko <mhocko@kernel.org>]

On Mon 09-09-19 21:40:24, Tetsuo Handa wrote:
> On 2019/09/09 20:36, Michal Hocko wrote:
> > On Sat 07-09-19 19:54:32, Tetsuo Handa wrote:
> >> (Resending to LKML as linux-mm ML dropped my posts.)
> >>
> >> If /proc/sys/vm/oom_dump_tasks != 0, dump_header() can become very slow
> >> because dump_tasks() synchronously reports all OOM victim candidates, and
> >> as a result ratelimit test for dump_header() cannot work as expected.
> >>
> >> This patch defers dump_tasks() output till oom_lock is released. As a
> >> result of this patch, the latency between out_of_memory() is called and
> >> SIGKILL is sent (and the OOM reaper starts reclaiming memory) will be
> >> significantly reduced.
> >>
> >> Since CONFIG_PRINTK_CALLER was introduced, concurrent printk() became less
> >> problematic. But we still need to correlate synchronously printed messages
> >> and asynchronously printed messages if we defer dump_tasks() messages.
> >> Thus, this patch also prefixes OOM killer messages using "OOM[$serial]:"
> >> format. As a result, OOM killer messages would look like below.
> >>
> >>   [   31.935015][   T71] OOM[1]: kworker/4:1 invoked oom-killer: gfp_mask=0xcc0(GFP_KERNEL), order=-1, oom_score_adj=0
> >>   (...snipped....)
> >>   [   32.052635][   T71] OOM[1]: oom-kill:constraint=CONSTRAINT_NONE,nodemask=(null),global_oom,task_memcg=/,task=firewalld,pid=737,uid=0
> >>   [   32.056886][   T71] OOM[1]: Out of memory: Killed process 737 (firewalld) total-vm:358672kB, anon-rss:22640kB, file-rss:12328kB, shmem-rss:0kB, UID:0 pgtables:421888kB oom_score_adj:0
> >>   [   32.064291][   T71] OOM[1]: Tasks state (memory values in pages):
> >>   [   32.067807][   T71] OOM[1]: [  pid  ]   uid  tgid total_vm      rss pgtables_bytes swapents oom_score_adj name
> >>   [   32.070057][   T54] oom_reaper: reaped process 737 (firewalld), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB
> >>   [   32.072417][   T71] OOM[1]: [    548]     0   548     9772     1172   110592        0             0 systemd-journal
> >>   (...snipped....)
> >>   [   32.139566][   T71] OOM[1]: [    737]     0   737    89668     8742   421888        0             0 firewalld
> >>   (...snipped....)
> >>   [   32.221990][   T71] OOM[1]: [   1300]    48  1300    63025     1788   532480        0             0 httpd
> >>
> >> This patch might affect panic behavior triggered by panic_on_oom or no
> >> OOM-killable tasks, for dump_header(oc, NULL) will not report OOM victim
> >> candidates if there are not-yet-reported OOM victim candidates from past
> >> rounds of OOM killer invocations. I don't know if that matters.
> >>
> >> For now this patch embeds "struct oom_task_info" into each
> >> "struct task_struct". In order to avoid bloating "struct task_struct",
> >> future patch might detach from "struct task_struct" because one
> >> "struct oom_task_info" for one "struct signal_struct" will be enough.
> > 
> > This is not an improvement. It detaches the oom report and tasks_dump
> > for an arbitrary amount of time because the worder context might be
> > stalled for an arbitrary time. Even long after the oom is resolved.
> 
> A new worker thread is created if all existing worker threads are busy
> because this patch solves OOM situation quickly when a new worker thread
> cannot be created due to OOM situation.
> 
> Also, if a worker thread cannot run due to CPU starvation, the same thing
> applies to dump_tasks(). In other words, dump_tasks() cannot complete due
> to CPU starvation, which results in more costly and serious consequences.
> Being able to send SIGKILL and reclaim memory as soon as possible is
> an improvement.

There might be zillion workers waiting to make a forward progress and
you cannot expect any timing here. Just remember your own experiments
with xfs and low memory conditions.

> > Not to mention that 1:1 (oom to tasks) information dumping is
> > fundamentally broken. Any task might be on an oom list of different
> > OOM contexts in different oom scopes (think of OOM happening in disjunct
> > NUMA sets).
> 
> I can't understand what you are talking about. This patch just defers
> printk() from /proc/sys/vm/oom_dump_tasks != 0. Please look at the patch
> carefully. If you are saying that it is bad that OOM victim candidates for
> OOM domain B, C, D ... cannot be printed if printing of OOM victim candidates
> for OOM domain A has not finished, I can update this patch to print them.

You would have to track each ongoing oom context separately. And not
only those from different oom scopes because as a matter of fact a new
OOM might trigger before the previous dump_tasks managed to be handled.

> > This is just adding more kludges and making the code more complex
> > without trying to address an underlying problems. So
> > Nacked-by: Michal Hocko <mhocko@suse.com>
> 
> Since I'm sure that you are misunderstanding, this Nacked-by is invalid.

Thank you very much for your consideration and evaluation of my review.
It seems that I am only burning my time responding to your emails. As
you seem to know the best, right?
-- 
Michal Hocko
SUSE Labs