Re: [PATCH] blk-throttle: fix possible io stall when doing upgrade

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On Tue, Sep 26, 2017 at 09:06:57AM +0800, Joseph Qi wrote:
> Hi Shaohua,
> 
> On 17/9/26 01:22, Shaohua Li wrote:
> > On Mon, Sep 25, 2017 at 06:46:42PM +0800, Joseph Qi wrote:
> >> From: Joseph Qi <qijiang.qj@xxxxxxxxxxxxxxx>
> >>
> >> Currently it will try to dispatch bio in throtl_upgrade_state. This may
> >> lead to io stall in the following case.
> >> Say the hierarchy is like:
> >> /-test1
> >>   |-subtest1
> >> and subtest1 has 32 queued bios now.
> >>
> >> throtl_pending_timer_fn            throtl_upgrade_state
> >> ------------------------------------------------------------------------
> >>                                    upgrade to max
> >>                                    throtl_select_dispatch
> >>                                    throtl_schedule_next_dispatch
> >> throtl_select_dispatch
> >> throtl_schedule_next_dispatch
> >>
> >> Since throtl_select_dispatch will move queued bios from subtest1 to
> >> test1 in throtl_upgrade_state, it will then just do nothing in
> >> throtl_pending_timer_fn. As a result, queued bios won't be dispatched
> >> any more if no proper timer scheduled.
> > 
> > Sorry, didn't get it. If throtl_pending_timer_fn does nothing (because
> > throtl_upgrade_state already moves bios to parent), there is no pending
> > blkcg/bio, not rearming the timer wouldn't lose anything. Am I missing
> > anything? could you please describe the failure in details?
> > 
> > Thanks,
> > Shaohua
> >In normal case, throtl_pending_timer_fn tries to move bios from
> subtest1 to test1, and finally do the real issueing work when reach
> the top-level.
> But int the case above, throtl_select_dispatch in
> throtl_pending_timer_fn returns 0, because the work is done by
> throtl_upgrade_state. Then throtl_pending_timer_fn *thinks* there is
> nothing to do, but the queued bios are still in service queue of
> test1.

Still didn't get, sorry. If there are pending bios in test1, why
throtl_schedule_next_dispatch in throtl_pending_timer_fn doesn't setup the
timer?



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