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Date:   Fri, 3 May 2019 10:51:16 +0200
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Daniel Vetter <daniel.vetter@ffwll.ch>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        "Paul E. McKenney" <paulmck@linux.ibm.com>,
        Valdis Kletnieks <valdis.kletnieks@vt.edu>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        "Liu, Chuansheng" <chuansheng.liu@intel.com>
Subject: Re: [PATCH] RFC: hung_task: taint kernel
Message-ID: <20190503085116.GK3271@phenom.ffwll.local>
Mail-Followup-To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        "Paul E. McKenney" <paulmck@linux.ibm.com>,
        Valdis Kletnieks <valdis.kletnieks@vt.edu>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        "Liu, Chuansheng" <chuansheng.liu@intel.com>
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On Fri, May 03, 2019 at 09:47:03AM +0900, Tetsuo Handa wrote:
> On 2019/05/03 5:46, Daniel Vetter wrote:
> > There's the hung_task_panic sysctl, but that's a bit an extreme measure.
> > As a fallback taint at least the machine.
> > 
> > Our CI uses this to decide when a reboot is necessary, plus to figure
> > out whether the kernel is still happy.
> 
> Why your CI can't watch for "blocked for more than" message instead of
> setting the taint flag? How does your CI decide a reboot is necessary?

We spam an awful lot into dmesg, and at least historically had
occasionally trouble capturing it all (we're better than that now I
think). Plus the thing that parses dmesg isn't the thing that runs
testcases, hence why we started to use taint flags (or procfs lockdep
status) and similar things to check the kernel is still alive enough.

> There is no need to set the tainted flag when some task was just blocked
> for a while. It might be due to memory pressure, it might be due to setting
> very short timeout (e.g. a few seconds), it might be due to busy CPUs doing
> something else...

Yeah I realize that this probably doesn't have much use outside of our CI,
but maybe there's someone how likes the idea.

Wrt spurious taints: You can disable the hung_tasks checker outright,
which also stops the tainting.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch