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block: fix that blk_time_get_ns() doesn't update time after schedule
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While monitoring the throttle time of IO from iocost, it's found that
such time is always zero after the io_schedule() from ioc_rqos_throttle,
for example, with the following debug patch:

+       printk("%s-%d: %s enter %llu\n", current->comm, current->pid, __func__, blk_time_get_ns());
        while (true) {
                set_current_state(TASK_UNINTERRUPTIBLE);
                if (wait.committed)
                        break;
                io_schedule();
        }
+       printk("%s-%d: %s exit  %llu\n", current->comm, current->pid, __func__, blk_time_get_ns());

It can be observerd that blk_time_get_ns() always return the same time:

[ 1068.096579] fio-1268: ioc_rqos_throttle enter 1067901962288
[ 1068.272587] fio-1268: ioc_rqos_throttle exit  1067901962288
[ 1068.274389] fio-1268: ioc_rqos_throttle enter 1067901962288
[ 1068.472690] fio-1268: ioc_rqos_throttle exit  1067901962288
[ 1068.474485] fio-1268: ioc_rqos_throttle enter 1067901962288
[ 1068.672656] fio-1268: ioc_rqos_throttle exit  1067901962288
[ 1068.674451] fio-1268: ioc_rqos_throttle enter 1067901962288
[ 1068.872655] fio-1268: ioc_rqos_throttle exit  1067901962288

And I think the root cause is that 'PF_BLOCK_TS' is always cleared
by blk_flush_plug() before scheduel(), hence blk_plug_invalidate_ts()
will never be called:

blk_time_get_ns
 plug->cur_ktime = ktime_get_ns();
 current->flags |= PF_BLOCK_TS;

io_schedule:
 io_schedule_prepare
  blk_flush_plug
   __blk_flush_plug
    /* the flag is cleared, while time is not */
    current->flags &= ~PF_BLOCK_TS;
 schedule
 sched_update_worker
  /* the flag is not set, hence plug->cur_ktime is not cleared */
  if (tsk->flags & PF_BLOCK_TS)
   blk_plug_invalidate_ts()

blk_time_get_ns
 /* got the time stashed before schedule */
 return plug->cur_ktime;

Fix the problem by clearing cached time in __blk_flush_plug().

Fixes: 06b23f9 ("block: update cached timestamp post schedule/preemption")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240411032349.3051233-2-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
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Yu Kuai authored and Jens Axboe committed Apr 12, 2024
1 parent 013ee5a commit 3ec4848
Showing 1 changed file with 1 addition and 0 deletions.
1 change: 1 addition & 0 deletions block/blk-core.c
Original file line number Diff line number Diff line change
Expand Up @@ -1197,6 +1197,7 @@ void __blk_flush_plug(struct blk_plug *plug, bool from_schedule)
if (unlikely(!rq_list_empty(plug->cached_rq)))
blk_mq_free_plug_rqs(plug);

plug->cur_ktime = 0;
current->flags &= ~PF_BLOCK_TS;
}

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