Messages in this thread | | | Subject | Re: [PATCH] locking/hung_task: Defer showing held locks | From | Tetsuo Handa <> | Date | Tue, 20 Dec 2016 22:34:47 +0900 |
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Vegard Nossum wrote: > On 13 December 2016 at 15:45, Tetsuo Handa > <penguin-kernel@i-love.sakura.ne.jp> wrote: > > When I was running my testcase which may block hundreds of threads > > on fs locks, I got lockup due to output from debug_show_all_locks() > > added by commit b2d4c2edb2e4f89a ("locking/hung_task: Show all locks"). > > > > I think we don't need to call debug_show_all_locks() on each blocked > > thread. Let's defer calling debug_show_all_locks() till before panic() > > or leaving for_each_process_thread() loop. > > First of all, sorry for not answering earlier.
No problem.
> > I'm not sure I fully understand the problem, you say the "output from > debug_show_all_locks()" caused a lockup, but was the problem simply > that the amount of output caused it to stall for a long time?
In Linux 4.9, in order to tell administrator that something might be wrong with memory allocation, warn_alloc() which calls printk() periodically when memory allocation is stalling for too long was added. However, since printk() waits until all pending data is sent to console using cond_resched(), printk() continues waiting as long as somebody else calls printk() when cond_resched() is called. This is problematic under OOM situation.
Since the OOM killer calls printk() with oom_lock held, it happened that printk() called from the OOM killer is forever unable to return because warn_alloc() periodically calls printk() since the OOM killer is holding oom_lock.
And it happened that khungtaskd is another source which calls printk() periodically when threads are blocked on fs locks waiting for memory allocation. debug_show_all_locks() generates far more amount of output compared to warn_alloc() if debug_show_all_locks() is called on each thread blocked on fs locks waiting for memory allocation. Therefore, we should avoid calling debug_show_all_locks() on each blocked thread.
Full story starts at http://lkml.kernel.org/r/1481020439-5867-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp but I appreciate if you can join on http://lkml.kernel.org/r/1478416501-10104-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp .
> > Could we instead > > 1) move the debug_show_all_locks() into the if > (sysctl_hung_task_panic) bit unconditionally > > 2) call something (touch_nmi_watchdog()?) inside debug_show_all_locks() > > 3) in another way make debug_show_all_locks() more robust so it doesn't "lockup" > > ?
Yes, that might be an improvement. But not needed for this patch.
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