linux-kernel - Re: BLKSECDISCARD ioctl and hung tasks

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Date:   Fri, 14 Feb 2020 17:22:57 +0800
From:   Ming Lei <tom.leiming@...il.com>
To:     Bart Van Assche <bvanassche@....org>
Cc:     Salman Qazi <sqazi@...gle.com>, Ming Lei <ming.lei@...hat.com>,
        Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        Gwendal Grignou <gwendal@...gle.com>,
        Jesse Barnes <jsbarnes@...gle.com>
Subject: Re: BLKSECDISCARD ioctl and hung tasks

On Fri, Feb 14, 2020 at 1:50 PM Bart Van Assche <bvanassche@....org> wrote:
>
> On 2020-02-13 11:21, Salman Qazi wrote:
> > AFAICT, This is not actually sufficient, because the issuer of the bio
> > is waiting for the entire bio, regardless of how it is split later.
> > But, also there isn't a good mapping between the size of the secure
> > discard and how long it will take.  If given the geometry of a flash
> > device, it is not hard to construct a scenario where a relatively
> > small secure discard (few thousand sectors) will take a very long time
> > (multiple seconds).
> >
> > Having said that, I don't like neutering the hung task timer either.
>
> Hi Salman,
>
> How about modifying the block layer such that completions of bio
> fragments are considered as task activity? I think that bio splitting is
> rare enough for such a change not to affect performance of the hot path.

Are you sure that the task hung warning won't be triggered in case of
non-splitting?

>
> How about setting max_discard_segments such that a discard always
> completes in less than half the hung task timeout? This may make
> discards a bit slower for one particular block driver but I think that's
> better than hung task complaints.

I am afraid you can't find a golden setting max_discard_segments working
for every drivers. Even it is found, the performance  may have been affected.

So just wondering why not take the simple approach used in blk_execute_rq()?

Thanks,
Ming Lei