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SubjectRe: BLKSECDISCARD ioctl and hung tasks
On Fri, Feb 14, 2020 at 1:50 PM Bart Van Assche <bvanassche@acm.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

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