linux-kernel - Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

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Date:   Thu, 11 Jan 2018 12:50:59 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, Tejun Heo <tj@...nel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        Cong Wang <xiyou.wangcong@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        rostedt@...e.goodmis.org, Byungchul Park <byungchul.park@....com>,
        Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

On Thu 2018-01-11 19:38:45, Sergey Senozhatsky wrote:
> On (01/11/18 10:34), Petr Mladek wrote:
> [..]
> > > except that handing off a console_sem to atomic task when there
> > > is   O(logbuf) > watchdog_thresh   is a regression, basically...
> > > it is what it is.
> > 
> > How this could be a regression? Is not the victim that handles
> > other printk's random? What protected the atomic task to
> > handle the other printks before this patch?
> 
> the non-atomic -> atomic context console_sem transfer. we previously
> would have kept the console_sem owner to its non-atomic owner. we now
> will make sure that if printk from atomic context happens then it will
> make it to console_unlock() loop.
> emphasis on O(logbuf) > watchdog_thresh.

Sergey, please, why do you completely and repeatedly ignore that
argument about statistical effects?

Yes, the above scenario is possible. But Steven's patch might also move the
owner from atomic context to a non-atomic one. The chances should be
more or less equal. The main advantage is that the owner is moved.
This should statistically lower the chance of a soft-lockup.

> 
> > Or do you have a system that started to suffer from softlockups
> > with this patchset and did not do this before?
> [..]
> > Do you know about any system where this patch made the softlockup
> > deterministically or statistically more likely, please?
> 
> I have explained many, many times why my boards die just like before.
> why would I bother collecting any numbers...

Is it with your own printk stress tests or during "normal" work?

If it is during a normal work, is there any chance that we
could have a look at the logs?

Best Regards,
Petr