Messages in this thread |  | | Date | Wed, 6 Feb 2013 15:23:46 +0100 | From | Jan Kara <> | Subject | Re: [PATCH v2] printk: Avoid softlockups in console_unlock() |
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On Tue 05-02-13 12:38:38, Andrew Morton wrote: > On Mon, 4 Feb 2013 23:17:10 +0100 > Jan Kara <jack@suse.cz> wrote: > > > A CPU can be caught in console_unlock() for a long time (tens of seconds are > > reported by our customers) when other CPUs are using printk heavily and serial > > console makes printing slow. Despite serial console drivers are calling > > touch_nmi_watchdog() this triggers softlockup warnings because > > interrupts are disabled for the whole time console_unlock() runs (e.g. > > vprintk() calls console_unlock() with interrupts disabled). Thus IPIs > > cannot be processed and other CPUs get stuck spinning in calls like > > smp_call_function_many(). Also RCU eventually starts reporting lockups. > > > > In my artifical testing I also managed to trigger a situation when disk > > disappeared from the system apparently because commands to / from it > > could not be delivered for long enough. This is why just silencing > > watchdogs isn't a reliable solution to the problem and we simply have to > > avoid spending too long in console_unlock(). > > > > We fix the issue by limiting the time we spend in console_unlock() to > > watchdog_thresh() / 4 (unless we are in an early boot stage or oops is > > happening). The rest of the buffer will be printed either by further > > callers to printk() or by a queued work. > > I still hate the patch :( > > > ... > > > > +void console_unlock(void) > > +{ > > + if (__console_unlock()) { > > + /* Let worker do the rest of printing */ > > + schedule_work(&printk_work); > > + } > > } > > This creates another place from where we cannot call printk(): anywhere > where worker_pool.lock is held. > > And as schedule_work() can do a wakeup it creates a third reason why > the sched code cannot call printk (along with rq->lock taken by > wake_up(klogd) and rq->lock taken by up(&console_sem). Hence > printk_sched(). See the lkml thread "[GIT PULL] printk: Support for > full dynticks mode". > > We already have machinery for doing async tickling in printk: the > printk_pending stuff. Did you consider adding another > PRINTK_PENDING_foo in some fashion? Yes, I noticed that thread just yesterday and also though that using similar trick might be viable. I'll experiment if we could use the same method for handling lockup problems I hit. Steven seems to have already tweaked PRINTK_PENDING stuff to be usable more easily...
Honza
Honza -- Jan Kara <jack@suse.cz> SUSE Labs, CR
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