Re: [RFC][PATCH v2 1/2] printk: Make printk() completely async

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Sergey Senozhatsky wrote:
> Hello,
> 
> On (03/07/16 09:22), Jan Kara wrote:
> [..]
> > > hm, just for note, none of system-wide wqs seem to have a ->rescuer thread
> > > (WQ_MEM_RECLAIM).
> > > 
> > > [..]
> > > > Even if you use printk_wq with WQ_MEM_RECLAIM for printing_work work item,
> > > > printing_work_func() will not be called until current work item calls
> > > > schedule_timeout_*(). That will be an undesirable random delay. If you use
> > > > a dedicated kernel thread rather than a dedicated workqueue with WQ_MEM_RECLAIM,
> > > > we can avoid this random delay.
> > > 
> > > hm. yes, seems that it may take some time until workqueue wakeup() a ->rescuer thread.
> > > need to look more.
> > 
> > Yes, it takes some time (0.1s or 2 jiffies) before workqueue code gives up
> > creating a worker process and wakes up rescuer thread. However I don't see
> > that as a problem...
> 
> yes, that's why I asked Tetsuo whether his concern was a wq's MAYDAY timer
> delay. the two commits that Tetsuo pointed at earlier in he loop (373ccbe59270
> and 564e81a57f97) solved the problem by switching to WQ_MEM_RECLAIM wq.
> I've slightly tested OOM-kill on my desktop system and haven't spotted any
> printk delays (well, a test on desktop is not really representative, of
> course).

I wanted to tell that if kworker is running a buggy function that calls
cond_resched() but does not call schedule_timeout_*() for very long time,
such delay can become many seconds. WQ_MEM_RECLAIM is a requirement for
waking up when kworker called schedule_timeout_*(). WQ_MEM_RECLAIM wq can
still cause huge delay if kworker does not call schedule_timeout_*().
Not specific to OOM-killer or vmstat.