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From:   Salman Qazi <sqazi@google.com>
Date:   Wed, 12 Feb 2020 17:20:38 -0800
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Subject: Re: BLKSECDISCARD ioctl and hung tasks
To:     "Theodore Y. Ts'o" <tytso@mit.edu>
Cc:     Jens Axboe <axboe@kernel.dk>, Ming Lei <ming.lei@redhat.com>,
        Bart Van Assche <bvanassche@acm.org>,
        Christoph Hellwig <hch@lst.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-block@vger.kernel.org, Gwendal Grignou <gwendal@google.com>,
        Jesse Barnes <jsbarnes@google.com>
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On Wed, Feb 12, 2020 at 3:07 PM Theodore Y. Ts'o <tytso@mit.edu> wrote:
>
> This is a problem we've been strugging with in other contexts.  For
> example, if you have the hung task timer set to 2 minutes, and the
> system to panic if the hung task timer exceeds that, and an NFS server
> which the client is writing to crashes, and it takes longer for the
> NFS server to come back, that might be a situation where we might want
> to exempt the hung task warning from panic'ing the system.  On the
> other hand, if the process is failing to schedule for other reasons,
> maybe we would still want the hung task timeout to go off.
>
> So I've been meditating over whether the right answer is to just
> globally configure the hung task timer to something like 5 or 10
> minutes (which would require no kernel changes, yay?), or have some
> way of telling the hung task timeout logic that it shouldn't apply, or
> should have a different timeout, when we're waiting for I/O to
> complete.

The problem that I anticipate in our space is that a generous timeout
will make impatient people reboot their chromebooks, losing us
information
about hangs.  But, this can be worked around by having multiple
different timeouts.  For instance, a thread that is expecting to do
something slow, can set a flag
to indicate that it wishes to be held against the more generous
criteria.  This is something I am tempted to do on older kernels where
we might not feel
comfortable backporting io_uring.

>
> It seems to me that perhaps there's a different solution here for your
> specific case, which is what if there is a asynchronous version of
> BLKSECDISCARD, either using io_uring or some other interface?  That
> bypasses the whole issue of how do we modulate the hung task timeout
> when it's a situation where maybe it's OK for a userspace thread to
> block for more than 120 seconds, without having to either completely
> disable the hung task timeout, or changing it globally to some much
> larger value.

This is worth evaluating.

Thanks,

Salman

>
>                                         - Ted