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

[Joseph Qi <jiangqi903@gmail.com>]

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@alibaba-inc.com>
>>
>> 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.
Since both throtl_pending_timer_fn and throtl_upgrade_state won't
handle the queued bios, io stall happens.

Thanks,
Joseph