On 4/8/2022 3:39 PM, Yu Kuai wrote:
> Currently, same waitqueue might be woken up continuously:
>
> __sbq_wake_up __sbq_wake_up
> sbq_wake_ptr -> assume 0
> sbq_wake_ptr -> 0
> atomic_dec_return
> atomic_dec_return
> atomic_cmpxchg -> succeed
> atomic_cmpxchg -> failed
> return true
>
> __sbq_wake_up
> sbq_wake_ptr
> atomic_read(&sbq->wake_index) -> still 0
> sbq_index_atomic_inc -> inc to 1
> if (waitqueue_active(&ws->wait))
> if (wake_index != atomic_read(&sbq->wake_index))
> atomic_set -> reset from 1 to 0
> wake_up_nr -> wake up first waitqueue
> // continue to wake up in first waitqueue
>
> What's worse, io hung is possible in theory because wake up might be
> missed. For example, 2 * wake_batch tags are put, while only wake_batch
> threads are worken:
>
> __sbq_wake_up
> atomic_cmpxchg -> reset wait_cnt
> __sbq_wake_up -> decrease wait_cnt
> ...
> __sbq_wake_up -> wait_cnt is decreased to 0 again
> atomic_cmpxchg
> sbq_index_atomic_inc -> increase wake_index
> wake_up_nr -> wake up and waitqueue might be empty
> sbq_index_atomic_inc -> increase again, one waitqueue is skipped
> wake_up_nr -> invalid wake up because old wakequeue might be empty
>
> To fix the problem, refactor to make sure waitqueues will be woken up
> one by one, and also choose the next waitqueue by the number of threads
> that are waiting to keep waitqueues balanced.
Hi, do you think that updating wake_index before atomic_cmpxchg(ws->wait_cnt) also can solve these two problems?
like this:
__sbq_wake_up()
{
....
if (wait_cnt <= 0) {
ret = atomic_cmpxchg(sbq->wake_index, old_wake_index, next_wake_index);
if (ret == old_wake_index) {
ret = atomic_cmpxchg(ws->wait_cnt, wait_cnt, wake_batch);
if (ret == wait_cnt)
wake_up_nr(ws->wait, wake_batch);
}
}
}
Your solution is picking the waitqueue with the largest waiters_cnt as the next one to be waked up, I think that waitqueue is possible to starve.
if lots of threads in a same waitqueue stop waiting before sbq wakes them up, it will cause the waiters_cnt of waitqueue is much less than others, looks like sbq_update_wake_index() would never pick this waitqueue. What do you think? is it possible?
>
> Test cmd: nr_requests is 64, and queue_depth is 32
> [global]
> filename=/dev/sda
> ioengine=libaio
> direct=1
> allow_mounted_write=0
> group_reporting
>
> [test]
> rw=randwrite
> bs=4k
> numjobs=512
> iodepth=2
>
> Before this patch, waitqueues can be extremly unbalanced, for example:
> ws_active=484
> ws={
> {.wait_cnt=8, .waiters_cnt=117},
> {.wait_cnt=8, .waiters_cnt=59},
> {.wait_cnt=8, .waiters_cnt=76},
> {.wait_cnt=8, .waiters_cnt=0},
> {.wait_cnt=5, .waiters_cnt=24},
> {.wait_cnt=8, .waiters_cnt=12},
> {.wait_cnt=8, .waiters_cnt=21},
> {.wait_cnt=8, .waiters_cnt=175},
> }
>
> With this patch, waitqueues is always balanced, for example:
> ws_active=477
> ws={
> {.wait_cnt=8, .waiters_cnt=59},
> {.wait_cnt=6, .waiters_cnt=62},
> {.wait_cnt=8, .waiters_cnt=61},
> {.wait_cnt=8, .waiters_cnt=60},
> {.wait_cnt=8, .waiters_cnt=63},
> {.wait_cnt=8, .waiters_cnt=56},
> {.wait_cnt=8, .waiters_cnt=59},
> {.wait_cnt=8, .waiters_cnt=57},
> }
>
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
> lib/sbitmap.c | 81 ++++++++++++++++++++++++++-------------------------
> 1 file changed, 42 insertions(+), 39 deletions(-)
>
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 393f2b71647a..176fba0252d7 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -575,68 +575,71 @@ void sbitmap_queue_min_shallow_depth(struct sbitmap_queue *sbq,
> }
> EXPORT_SYMBOL_GPL(sbitmap_queue_min_shallow_depth);
>
> -static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
> +/* always choose the 'ws' with the max waiters */
> +static void sbq_update_wake_index(struct sbitmap_queue *sbq,
> + int old_wake_index)
> {
> - int i, wake_index;
> + int index, wake_index;
> + int max_waiters = 0;
>
> - if (!atomic_read(&sbq->ws_active))
> - return NULL;
> + if (old_wake_index != atomic_read(&sbq->wake_index))
> + return;
>
> - wake_index = atomic_read(&sbq->wake_index);
> - for (i = 0; i < SBQ_WAIT_QUEUES; i++) {
> - struct sbq_wait_state *ws = &sbq->ws[wake_index];
> + for (wake_index = 0; wake_index < SBQ_WAIT_QUEUES; wake_index++) {
> + struct sbq_wait_state *ws;
> + int waiters;
>
> - if (waitqueue_active(&ws->wait)) {
> - if (wake_index != atomic_read(&sbq->wake_index))
> - atomic_set(&sbq->wake_index, wake_index);
> - return ws;
> - }
> + if (wake_index == old_wake_index)
> + continue;
>
> - wake_index = sbq_index_inc(wake_index);
> + ws = &sbq->ws[wake_index];
> + waiters = atomic_read(&ws->waiters_cnt);
> + if (waiters > max_waiters) {
> + max_waiters = waiters;
> + index = wake_index;
> + }
> }
>
> - return NULL;
> + if (max_waiters)
> + atomic_cmpxchg(&sbq->wake_index, old_wake_index, index);
> }
>
> static bool __sbq_wake_up(struct sbitmap_queue *sbq)
> {
> struct sbq_wait_state *ws;
> unsigned int wake_batch;
> - int wait_cnt;
> + int wait_cnt, wake_index;
>
> - ws = sbq_wake_ptr(sbq);
> - if (!ws)
> + if (!atomic_read(&sbq->ws_active))
> return false;
>
> + wake_index = atomic_read(&sbq->wake_index);
> + ws = &sbq->ws[wake_index];
> wait_cnt = atomic_dec_return(&ws->wait_cnt);
> - if (wait_cnt <= 0) {
> - int ret;
> -
> - wake_batch = READ_ONCE(sbq->wake_batch);
> -
> - /*
> - * Pairs with the memory barrier in sbitmap_queue_resize() to
> - * ensure that we see the batch size update before the wait
> - * count is reset.
> - */
> - smp_mb__before_atomic();
> -
> + if (wait_cnt > 0) {
> + return false;
> + } else if (wait_cnt < 0) {
> /*
> - * For concurrent callers of this, the one that failed the
> - * atomic_cmpxhcg() race should call this function again
> + * Concurrent callers should call this function again
> * to wakeup a new batch on a different 'ws'.
> */
> - ret = atomic_cmpxchg(&ws->wait_cnt, wait_cnt, wake_batch);
> - if (ret == wait_cnt) {
> - sbq_index_atomic_inc(&sbq->wake_index);
> - wake_up_nr(&ws->wait, wake_batch);
> - return false;
> - }
> -
> + sbq_update_wake_index(sbq, wake_index);
> return true;
> }
>
> - return false;
> + sbq_update_wake_index(sbq, wake_index);
> + wake_batch = READ_ONCE(sbq->wake_batch);
> +
> + /*
> + * Pairs with the memory barrier in sbitmap_queue_resize() to
> + * ensure that we see the batch size update before the wait
> + * count is reset.
> + */
> + smp_mb__before_atomic();
> + atomic_set(&ws->wait_cnt, wake_batch);
> + wake_up_nr(&ws->wait, wake_batch);
> +
> + return true;
> }
>
> void sbitmap_queue_wake_up(struct sbitmap_queue *sbq)
