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io_uring/sqpoll: wait on sqd->wait for thread parking

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io_sqd_handle_event() just does a mutex unlock/lock dance when it's
supposed to park, somewhat relying on full ordering with the thread
trying to park it which does a similar unlock/lock dance on sqd->lock.
However, with adaptive spinning on mutexes, this can waste an awful
lot of time. Normally this isn't very noticeable, as parking and
unparking the thread isn't a common (or fast path) occurence. However,
in testing ring resizing, it's testing exactly that, as each resize
will require the SQPOLL to safely park and unpark.

Have io_sq_thread_park() explicitly wait on sqd->park_pending being
zero before attempting to grab the sqd->lock again.

In a resize test, this brings the runtime of SQPOLL down from about
60 seconds to a few seconds, just like the !SQPOLL tests. And saves
a ton of spinning time on the mutex, on both sides.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Jens Axboe 2024-10-28 13:18:27 -06:00
parent 79cfe9e59c
commit b898b8c99e
1 changed files with 2 additions and 1 deletions
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3
io_uring/sqpoll.c
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@ -40,6 +40,7 @@ void io_sq_thread_unpark(struct io_sq_data *sqd)
if (atomic_dec_return(&sqd->park_pending))
set_bit(IO_SQ_THREAD_SHOULD_PARK, &sqd->state);
mutex_unlock(&sqd->lock);
wake_up(&sqd->wait);
}
void io_sq_thread_park(struct io_sq_data *sqd)
@ -215,7 +216,7 @@ static bool io_sqd_handle_event(struct io_sq_data *sqd)
mutex_unlock(&sqd->lock);
if (signal_pending(current))
did_sig = get_signal(&ksig);
cond_resched();
wait_event(sqd->wait, !atomic_read(&sqd->park_pending));
mutex_lock(&sqd->lock);
sqd->sq_cpu = raw_smp_processor_id();
}