Re: [PATCH v6 2/2] block: cancel all throttled bios in del_gendisk()

[yukuai (C)]

在 2022/01/14 11:05, Ming Lei 写道:
> On Thu, Jan 13, 2022 at 04:46:18PM +0800, yukuai (C) wrote:
> > 在 2022/01/12 11:05, Ming Lei 写道:
> > > Hello Yu Kuai,
> > >
> > > On Mon, Jan 10, 2022 at 09:47:58PM +0800, Yu Kuai wrote:
> > > > Throttled bios can't be issued after del_gendisk() is done, thus
> > > > it's better to cancel them immediately rather than waiting for
> > > > throttle is done.
> > > >
> > > > For example, if user thread is throttled with low bps while it's
> > > > issuing large io, and the device is deleted. The user thread will
> > > > wait for a long time for io to return.
> > > >
> > > > Noted this patch is mainly from revertion of commit 32e3374304c7
> > > > ("blk-throttle: remove tg_drain_bios") and commit b77412372b68
> > > > ("blk-throttle: remove blk_throtl_drain").
> > > >
> > > > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > > > ---
> > > >    block/blk-throttle.c | 77 ++++++++++++++++++++++++++++++++++++++++++++
> > > >    block/blk-throttle.h |  2 ++
> > > >    block/genhd.c        |  2 ++
> > > >    3 files changed, 81 insertions(+)
> > >
> > > Just wondering why not take the built-in way in throtl_upgrade_state() for
> > > canceling throttled bios? Something like the following, then we can avoid
> > > to re-invent the wheel.
> > >
> > >    block/blk-throttle.c | 38 +++++++++++++++++++++++++++++++-------
> > >    block/blk-throttle.h |  2 ++
> > >    block/genhd.c        |  3 +++
> > >    3 files changed, 36 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> > > index cf7e20804f1b..17e56b2e44c4 100644
> > > --- a/block/blk-throttle.c
> > > +++ b/block/blk-throttle.c
> > > @@ -1816,16 +1816,11 @@ static void throtl_upgrade_check(struct throtl_grp *tg)
> > >              throtl_upgrade_state(tg->td);
> > >    }
> > > -static void throtl_upgrade_state(struct throtl_data *td)
> > > +static void __throtl_cancel_bios(struct throtl_data *td)
> > >    {
> > >      struct cgroup_subsys_state *pos_css;
> > >      struct blkcg_gq *blkg;
> > > -   throtl_log(&td->service_queue, "upgrade to max");
> > > -   td->limit_index = LIMIT_MAX;
> > > -   td->low_upgrade_time = jiffies;
> > > -   td->scale = 0;
> > > -   rcu_read_lock();
> > >      blkg_for_each_descendant_post(blkg, pos_css, td->queue->root_blkg) {
> > >              struct throtl_grp *tg = blkg_to_tg(blkg);
> > >              struct throtl_service_queue *sq = &tg->service_queue;
> > > @@ -1834,12 +1829,41 @@ static void throtl_upgrade_state(struct throtl_data *td)
> > >              throtl_select_dispatch(sq);
> > >              throtl_schedule_next_dispatch(sq, true);
> > Hi, Ming Lei
> >
> > I'm confused that how can bios be canceled here?
> > tg->iops and tg->bps stay untouched, how can throttled bios
> > dispatch?
>
> I thought that throttled bios will be canceled by 'tg->disptime = jiffies - 1;'
> and the following dispatch schedule.
>
> But looks it isn't enough, since tg_update_disptime() updates
> ->disptime. However,
> this problem can be solved easily by not updating ->disptime in case that we are
> canceling.
>
> > >      }
> > > -   rcu_read_unlock();
> > >      throtl_select_dispatch(&td->service_queue);
> > >      throtl_schedule_next_dispatch(&td->service_queue, true);
> > >      queue_work(kthrotld_workqueue, &td->dispatch_work);
> > >    }
> > > +void blk_throtl_cancel_bios(struct request_queue *q)
> > > +{
> > > +   struct cgroup_subsys_state *pos_css;
> > > +   struct blkcg_gq *blkg;
> > > +
> > > +   rcu_read_lock();
> > > +   spin_lock_irq(&q->queue_lock);
> > > +   __throtl_cancel_bios(q->td);
> > > +   spin_unlock_irq(&q->queue_lock);
> > > +   rcu_read_unlock();
> > > +
> > > +   blkg_for_each_descendant_post(blkg, pos_css, q->root_blkg)
> > > +           del_timer_sync(&blkg_to_tg(blkg)->service_queue.pending_timer);
> > > +   del_timer_sync(&q->td->service_queue.pending_timer);
> >
> > By the way, I think delete timer will end up io hung here if there are
> > some bios still be throttled.
>
> Firstly ->queue_lock is held by blk_throtl_cancel_bios(), so no new bios
> will be throttled.
>
> Also if we don't update ->disptime, any new bios throttled after releasing
> ->queue_lock will be dispatched soon.

Hi, Ming Lei

Just to be curiosity, I'm still trying to understand the logic here:

For example, if bps is set to 1k, and a io with size 16k is just
dispatched, then io throtle should wait for 16s untill new io can be 
dispatched. (details in tg_with_in_bps_limit）.

How does such mechanism bypassed here?

Thanks,
Kuai