Re: [PATCH] mm/page_alloc: Wait for oom_lock before retrying.

[Petr Mladek <pmladek@suse.com>]

On Tue 2016-12-13 21:06:57, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > On Mon 12-12-16 13:55:35, Michal Hocko wrote:
> > > On Mon 12-12-16 21:12:06, Tetsuo Handa wrote:
> > > > Michal Hocko wrote:
> > [...]
> > > > > > I think this warn_alloc() is too much noise. When something went
> > > > > > wrong, multiple instances of Thread-2 tend to call warn_alloc()
> > > > > > concurrently. We don't need to report similar memory information.
> > > > > 
> > > > > That is why we have ratelimitting. It is needs a better tunning then
> > > > > just let's do it.
> > > > 
> > > > I think that calling show_mem() once per a series of warn_alloc() threads is
> > > > sufficient. Since the amount of output by dump_stack() and that by show_mem()
> > > > are nearly equals, we can save nearly 50% of output if we manage to avoid
> > > > the same show_mem() calls.
> > > 
> > > I do not mind such an update. Again, that is what we have the
> > > ratelimitting for. The fact that it doesn't throttle properly means that
> > > we should tune its parameters.
> > 
> > What about the following? Does this help?
> 
> I don't think it made much difference.
> 
> I noticed that one of triggers which cause a lot of
> "** XXX printk messages dropped **" is show_all_locks() added by
> commit b2d4c2edb2e4f89a ("locking/hung_task: Show all locks"). When there are
> a lot of threads being blocked on fs locks, show_all_locks() on each blocked
> thread generates incredible amount of messages periodically. Therefore,
> I temporarily set /proc/sys/kernel/hung_task_timeout_secs to 0 to disable
> hung task warnings for testing this patch.
> 
> http://I-love.SAKURA.ne.jp/tmp/serial-20161213.txt.xz is a console log with
> this patch applied. Due to hung task warnings disabled, amount of messages
> are significantly reduced.
> 
> Uptime > 400 are testcases where the stresser was invoked via "taskset -c 0".
> Since there are some "** XXX printk messages dropped **" messages, I can't
> tell whether the OOM killer was able to make forward progress. But guessing
>  from the result that there is no corresponding "Killed process" line for
> "Out of memory: " line at uptime = 450 and the duration of PID 14622 stalled,
> I think it is OK to say that the system got stuck because the OOM killer was
> not able to make forward progress.

I am afraid that as long as you see "** XXX printk messages dropped
**" then there is something that is able to keep warn_alloc() busy,
never leave the printk()/console_unlock() and and block OOM killer
progress.

> ----------
> [  450.767693] Out of memory: Kill process 14642 (a.out) score 999 or sacrifice child
> [  450.769974] Killed process 14642 (a.out) total-vm:4168kB, anon-rss:84kB, file-rss:0kB, shmem-rss:0kB
> [  450.776538] oom_reaper: reaped process 14642 (a.out), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB
> [  450.781170] Out of memory: Kill process 14643 (a.out) score 999 or sacrifice child
> [  450.783469] Killed process 14643 (a.out) total-vm:4168kB, anon-rss:84kB, file-rss:0kB, shmem-rss:0kB
> [  450.787912] oom_reaper: reaped process 14643 (a.out), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB
> [  450.792630] Out of memory: Kill process 14644 (a.out) score 999 or sacrifice child
> [  450.964031] a.out: page allocation stalls for 10014ms, order:0, mode:0x24280ca(GFP_HIGHUSER_MOVABLE|__GFP_ZERO)
> [  450.964033] CPU: 0 PID: 14622 Comm: a.out Tainted: G        W       4.9.0+ #99
> (...snipped...)
> [  740.984902] a.out: page allocation stalls for 300003ms, order:0, mode:0x24280ca(GFP_HIGHUSER_MOVABLE|__GFP_ZERO)
> [  740.984905] CPU: 0 PID: 14622 Comm: a.out Tainted: G        W       4.9.0+ #99
> ----------
> 
> Although it is fine to make warn_alloc() less verbose, this is not
> a problem which can be avoided by simply reducing printk(). Unless
> we give enough CPU time to the OOM killer and OOM victims, it is
> trivial to lockup the system.

You could try to use printk_deferred() in warn_alloc(). It will not
handle console. It will help to be sure that the blocked printk()
is the main problem.

Best Regards,
Petr

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