on 7/21/2023 5:57 PM, Ming Lei wrote:
> From: David Jeffery <djeffery@xxxxxxxxxx>
>
> Current code supposes that it is enough to provide forward progress by just
> waking up one wait queue after one completion batch is done.
>
> Unfortunately this way isn't enough, cause waiter can be added to
> wait queue just after it is woken up.
>
Sorry for missed this. The change looks good and makes code simpler. But the
issue is not supposed to heppen in real world.
> Follows one example(64 depth, wake_batch is 8)
>
> 1) all 64 tags are active
>
> 2) in each wait queue, there is only one single waiter
>
> 3) each time one completion batch(8 completions) wakes up just one waiter in each wait
> queue, then immediately one new sleeper is added to this wait queue
>
> 4) after 64 completions, 8 waiters are wakeup, and there are still 8 waiters in each
> wait queue>
As we only wait when all tags are consumed (After sbitmap_prepare_to_wait,
we will try sbitmap_queue_get before sleep), there will be 64 active tags again
when any new sleeper is added. Then the menthioned issue should not exist.
If there was no any gain with old way to wakeup, this looks good. Otherwise
we may need to reconsider this.
Wish this helps, thanks!
> 5) after another 8 active tags are completed, only one waiter can be wakeup, and the other 7
> can't be waken up anymore.
>
> Turns out it isn't easy to fix this problem, so simply wakeup enough waiters for
> single batch.
>
> Cc: David Jeffery <djeffery@xxxxxxxxxx>
> Cc: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
> Cc: Gabriel Krisman Bertazi <krisman@xxxxxxx>
> Cc: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
> ---
> lib/sbitmap.c | 15 +++++++--------
> 1 file changed, 7 insertions(+), 8 deletions(-)
>
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index eff4e42c425a..d0a5081dfd12 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -550,7 +550,7 @@ EXPORT_SYMBOL_GPL(sbitmap_queue_min_shallow_depth);
>
> static void __sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr)
> {
> - int i, wake_index;
> + int i, wake_index, woken;
>
> if (!atomic_read(&sbq->ws_active))
> return;
> @@ -567,13 +567,12 @@ static void __sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr)
> */
> wake_index = sbq_index_inc(wake_index);
>
> - /*
> - * It is sufficient to wake up at least one waiter to
> - * guarantee forward progress.
> - */
> - if (waitqueue_active(&ws->wait) &&
> - wake_up_nr(&ws->wait, nr))
> - break;
> + if (waitqueue_active(&ws->wait)) {
> + woken = wake_up_nr(&ws->wait, nr);
> + if (woken == nr)
> + break;
> + nr -= woken;
> + }
> }
>
> if (wake_index != atomic_read(&sbq->wake_index))
>
