linux-kernel - Re: [PATCH] mm: don't warn about allocations which stall for too long

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Message-ID: <40ed01d3-1475-cd4a-0dff-f7a6ee24d5e9@suse.cz>
Date:   Wed, 1 Nov 2017 18:42:25 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.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>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        "yuwang.yuwang" <yuwang.yuwang@...baba-inc.com>
Subject: Re: [PATCH] mm: don't warn about allocations which stall for too long

On 11/01/2017 04:33 PM, Steven Rostedt wrote:
> On Wed, 1 Nov 2017 09:30:05 +0100
> Vlastimil Babka <vbabka@...e.cz> wrote:
> 
>>
>> But still, it seems to me that the scheme only works as long as there
>> are printk()'s coming with some reasonable frequency. There's still a
>> corner case when a storm of printk()'s can come that will fill the ring
>> buffers, and while during the storm the printing will be distributed
>> between CPUs nicely, the last unfortunate CPU after the storm subsides
>> will be left with a large accumulated buffer to print, and there will be
>> no waiters to take over if there are no more printk()'s coming. What
>> then, should it detect such situation and defer the flushing?
> 
> No!
> 
> If such a case happened, that means the system is doing something
> really stupid.

Hm, what about e.g. a soft lockup that triggers backtraces from all
CPU's? Yes, having softlockups is "stupid" but sometimes they do happen
and the system still recovers (just some looping operation is missing
cond_resched() and took longer than expected). It would be sad if it
didn't recover because of a printk() issue...

> Btw, each printk that takes over, does one message, so the last one to
> take over, shouldn't have a full buffer anyway.

There might be multiple messages per each CPU, e.g. the softlockup
backtraces.

> But still, if you have such a hypothetical situation, the system should
> just crash. The printk is still bounded by the length of the buffer.
> Although it is slow, it will finish.

Finish, but with single CPU doing the printing, which is wrong?

> Which is not the case with the
> current situation. And the current situation (as which this patch
> demonstrates) does happen today and is not hypothetical.

Yep, so ideally it can be fixed without corner cases :)

Vlastimil

> -- Steve
>