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. Since both throtl_pending_timer_fn and throtl_upgrade_state won't handle the queued bios, io stall happens. Thanks, Joseph
