On Tue, Sep 26, 2017 at 09:06:57AM +0800, Joseph Qi wrote:

Hi Shaohua,

On 17/9/26 01:22, Shaohua Li wrote:

quoted

On Mon, Sep 25, 2017 at 06:46:42PM +0800, Joseph Qi wrote:

quoted

From: Joseph Qi <redacted>

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?