linux-kernel - Re: [PATCH -next RFC v2 8/8] sbitmap: wake up the number of threads based on required tags

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Date:   Fri, 8 Apr 2022 14:13:02 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Yu Kuai <yukuai3@...wei.com>, axboe@...nel.dk,
        andriy.shevchenko@...ux.intel.com, john.garry@...wei.com,
        ming.lei@...hat.com
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com
Subject: Re: [PATCH -next RFC v2 8/8] sbitmap: wake up the number of threads
 based on required tags

On 4/8/22 00:39, Yu Kuai wrote:
> +static unsigned int get_wake_nr(struct sbq_wait_state *ws, unsigned int nr_tags)
> +{
> +	struct sbq_wait *wait;
> +	struct wait_queue_entry *entry;
> +	unsigned int nr = 1;
> +
> +	spin_lock_irq(&ws->wait.lock);
> +	list_for_each_entry(entry, &ws->wait.head, entry) {
> +		wait = container_of(entry, struct sbq_wait, wait);
> +		if (nr_tags <= wait->nr_tags)
> +			break;
> +
> +		nr++;
> +		nr_tags -= wait->nr_tags;
> +	}
> +	spin_unlock_irq(&ws->wait.lock);
> +
> +	return nr;
> +}
> +
>   static bool __sbq_wake_up(struct sbitmap_queue *sbq)
>   {
>   	struct sbq_wait_state *ws;
> @@ -648,7 +668,7 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
>   	smp_mb__before_atomic();
>   	atomic_set(&ws->wait_cnt, wake_batch);
>   	sbq_update_preemption(sbq, wake_batch);
> -	wake_up_nr(&ws->wait, wake_batch);
> +	wake_up_nr(&ws->wait, get_wake_nr(ws, wake_batch));
>   
>   	return true;
>   }

An additional comment: my understanding is that __sbq_wake_up() should 
wake up exactly `wake_batch` waiters. The above patch changes that into 
waking up at most `wake_batch` waiters. I think that's wrong.

Bart.