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

Ming Lei
2022-01-18T13:28:53Z

On Tue, Jan 18, 2022 at 04:48:26PM +0800, yukuai (C) wrote:
> 在 2022/01/17 17:21, Ming Lei 写道:
> > On Fri, Jan 14, 2022 at 04:21:04PM +0800, yukuai (C) wrote:
> > > 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@huawei.com>
> > > > > > > ---
> > > > > > > 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
> >
> > There isn't such wait code in blk-throttle, and the magic is just in
> > how to compute tg->disptime.
> >
> > > dispatched. (details in tg_with_in_bps_limit）.
> > >
> > > How does such mechanism bypassed here?
> >
> > The point is that tg->disptime is always set as one past time, so all
> > throttled IOs will be dispatched immediately if ->disptime is older than
> > jiffies, and I have verified that the following patch can work as expected.
> >
> Hi, Ming Lei
>
> I'm not sure about the logic here yet, however, I tried the following

I believe I have explained the theory already, here we just miss to
patch tg_may_dispatch(), then CPU is spinning in
throtl_pending_timer_fn() until tg_may_dispatch() becomes true.

> patch, and the patch doesn't work as expected in my case:
>
> 1. limit bps to 1k
> 2. issue io with bs=16k
>
> In this workload, each io will wait for 16s to complete.
>
> 3. when an io is just completed, delete the device
>
> After this is done, what I expected is that the user thread will exit
> immediately(with io error), however, with this patch applied, the user
> thread will wait for about 16s to exit, which is the same without this
> patch.

Please try the revised patch, which can return immediately after
deleting the disk.

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 7c462c006b26..912ba25839f2 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -45,6 +45,7 @@ static struct workqueue_struct *kthrotld_workqueue;
enum tg_state_flags {
THROTL_TG_PENDING = 1 << 0, /* on parent's pending tree */
THROTL_TG_WAS_EMPTY = 1 << 1, /* bio_lists[] became non-empty */
+ THROTL_TG_CANCELING = 1 << 2, /* starts to cancel all bios */
};

#define rb_entry_tg(node) rb_entry((node), struct throtl_grp, rb_node)
@@ -870,8 +871,9 @@ static bool tg_may_dispatch(struct throtl_grp *tg, struct bio *bio,
BUG_ON(tg->service_queue.nr_queued[rw] &&
bio != throtl_peek_queued(&tg->service_queue.queued[rw]));

- /* If tg->bps = -1, then BW is unlimited */
- if (bps_limit == U64_MAX && iops_limit == UINT_MAX) {
+ /* If tg->bps = -1 or we are canceling bios, then BW is unlimited */
+ if ((bps_limit == U64_MAX && iops_limit == UINT_MAX) ||
+ (tg->flags & THROTL_TG_CANCELING)) {
if (wait)
*wait = 0;
return true;
@@ -974,6 +976,9 @@ static void tg_update_disptime(struct throtl_grp *tg)
unsigned long read_wait = -1, write_wait = -1, min_wait = -1, disptime;
struct bio *bio;

+ if (tg->flags & THROTL_TG_CANCELING)
+ goto update;
+
bio = throtl_peek_queued(&sq->queued[READ]);
if (bio)
tg_may_dispatch(tg, bio, &read_wait);
@@ -983,6 +988,7 @@ static void tg_update_disptime(struct throtl_grp *tg)
tg_may_dispatch(tg, bio, &write_wait);

min_wait = min(read_wait, write_wait);
+update:
disptime = jiffies + min_wait;

/* Update dispatch time */
@@ -1836,6 +1842,25 @@ static void throtl_upgrade_state(struct throtl_data *td)
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);
+ blkg_for_each_descendant_post(blkg, pos_css, q->root_blkg) {
+ struct throtl_grp *tg = blkg_to_tg(blkg);
+ struct throtl_service_queue *sq = &tg->service_queue;
+
+ tg->disptime = jiffies - 1;
+ blkg_to_tg(blkg)->flags |= THROTL_TG_CANCELING;
+ throtl_schedule_pending_timer(sq, jiffies + 1);
+ }
+ spin_unlock_irq(&q->queue_lock);
+ rcu_read_unlock();
+}
+
static void throtl_downgrade_state(struct throtl_data *td)
{
td->scale /= 2;
diff --git a/block/blk-throttle.h b/block/blk-throttle.h
index 175f03abd9e4..b412a4d7cc1e 100644
--- a/block/blk-throttle.h
+++ b/block/blk-throttle.h
@@ -160,12 +160,14 @@ static inline void blk_throtl_exit(struct request_queue *q) { }
static inline void blk_throtl_register_queue(struct request_queue *q) { }
static inline void blk_throtl_charge_bio_split(struct bio *bio) { }
static inline bool blk_throtl_bio(struct bio *bio) { return false; }
+static inline void blk_throtl_cancel_bios(struct request_queue *q) {}
#else /* CONFIG_BLK_DEV_THROTTLING */
int blk_throtl_init(struct request_queue *q);
void blk_throtl_exit(struct request_queue *q);
void blk_throtl_register_queue(struct request_queue *q);
void blk_throtl_charge_bio_split(struct bio *bio);
bool __blk_throtl_bio(struct bio *bio);
+void blk_throtl_cancel_bios(struct request_queue *q);
static inline bool blk_throtl_bio(struct bio *bio)
{
struct throtl_grp *tg = blkg_to_tg(bio->bi_blkg);
diff --git a/block/genhd.c b/block/genhd.c
index f7577dde18fc..a32d48b87223 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -29,6 +29,7 @@

#include "blk.h"
#include "blk-mq-sched.h"
+#include "blk-throttle.h"

static struct kobject *block_depr;

@@ -576,6 +577,8 @@ void del_gendisk(struct gendisk *disk)
blk_integrity_del(disk);
disk_del_events(disk);

+ blk_throtl_cancel_bios(disk->queue);
+
mutex_lock(&disk->open_mutex);
remove_inode_hash(disk->part0->bd_inode);
blk_drop_partitions(disk);

Thanks,
Ming