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

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Date:   Thu, 11 Jan 2018 19:38:45 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        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 (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.


- if the patch's goal is to bound (not necessarily to watchdog's threshold)
the amount of time we spend in console_unlock(), then the patch is kinda
overcomplicated. but no further questions in this case.

- but if the patch's goal is to bound (to lockup threshold) the amount of
time spent in console_unlock() in order to avoid lockups [uh, a reason],
then the patch is rather oversimplified.


claiming that for any given A, B, C the following is always true

				A * B < C

where
	A is the amount of data to print in the worst case
	B the time call_console_drivers() needs to print a single
	  char to all registered and enabled consoles
	C the watchdog's threshold

is not really a step forward.

and the "last console_sem owner prints all pending messages" rule
is still there.


> 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...

	-ss