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

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Date:	Mon, 7 Mar 2016 23:36:00 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	sergey.senozhatsky.work@...il.com, akpm@...ux-foundation.org,
	jack@...e.com, pmladek@...e.com, tj@...nel.org,
	linux-kernel@...r.kernel.org, sergey.senozhatsky@...il.com
Subject: Re: [RFC][PATCH v2 1/2] printk: Make printk() completely async

On (03/07/16 20:10), Tetsuo Handa wrote:
[..]
> > > > 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.

your point is taken. thanks.

	-ss