linux-kernel - Re:Re: [PATCH V2] workqueue: watchdog: update wq_watchdog

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Date:   Mon, 22 Mar 2021 09:47:32 +0800 (GMT+08:00)
From:   王擎 <wangqing@...o.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Guilherme G. Piccoli" <gpiccoli@...onical.com>,
        Andrey Ignatov <rdna@...com>, Vlastimil Babka <vbabka@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Santosh Sivaraj <santosh@...six.org>,
        linux-kernel@...r.kernel.org
Subject: Re:Re: [PATCH V2] workqueue: watchdog: update wq_watchdog_touched for unbound lockup checking

>On Fri, Mar 19, 2021 at 04:00:36PM +0800, Wang Qing wrote:
>> When touch_softlockup_watchdog() is called, only wq_watchdog_touched_cpu 
>> updated, while the unbound worker_pool running on its core uses 
>> wq_watchdog_touched to determine whether locked up. This may be mischecked.
>
>Can you please elaborate here, preferably with a concrete scenario where the
>new code is better?

The previous code is problematic for judging whether the unbound pool is locked up:
When the expected single cpu stall occurs, only wq_watchdog_touched_cpu is updated, 
However, the unbound pool uses wq_watchdog_touched to determine whether it is 
locked up, so when the unbound pool is running in a scenario where the cpu stall is 
expected, a misjudgment will occur, because wq_watchdog_touched only be update 
when all the cpu stall as expect.

So we need to update wq_watchdog_touched in touch_softlockup_watchdog(), and it is 
only used for unbound pool,  it will not affect the bound pool in any way.

Thanks,

Qing

>
>Thanks.
>
>-- 
>tejun