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From: Bart Van Assche <bvanassche@acm.org>
To: Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org, Hannes Reinecke <hare@suse.de>
Subject: Re: [PATCH 4/9] sbitmap: test bit before calling test_and_set_bit()
Date: Thu, 14 Oct 2021 11:41:51 -0700	[thread overview]
Message-ID: <a345227d-5b93-e78b-3078-b8056007b671@acm.org> (raw)
In-Reply-To: <20211013165416.985696-5-axboe@kernel.dk>

On 10/13/21 9:54 AM, Jens Axboe wrote:
> If we come across bits that are already set, then it's quicker to test
> that first and gate the test_and_set_bit() operation on the result of
> the bit test.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
>   lib/sbitmap.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index c6e2f1f2c4d2..11b244a8d00f 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -166,7 +166,7 @@ static int __sbitmap_get_word(unsigned long *word, unsigned long depth,
>   			return -1;
>   		}
>   
> -		if (!test_and_set_bit_lock(nr, word))
> +		if (!test_bit(nr, word) && !test_and_set_bit_lock(nr, word))
>   			break;
>   
>   		hint = nr + 1;

Hi Jens,

Are numbers available about how much this change improves performance? 
I'm asking this since my understanding is that this patch only helps if 
bit 'nr' is set after find_next_zero_bit() finished and before 
test_and_set_bit() is called. Isn't that a very short time window? Am I 
perhaps missing something?

Thanks,

Bart.



  parent reply	other threads:[~2021-10-14 18:41 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-13 16:54 [PATCHSET v2 0/9] Batched completions Jens Axboe
2021-10-13 16:54 ` [PATCH 1/9] block: define io_batch structure Jens Axboe
2021-10-14  5:45   ` Christoph Hellwig
2021-10-14 15:50     ` Jens Axboe
2021-10-14 16:06       ` Christoph Hellwig
2021-10-14 18:14         ` Jens Axboe
2021-10-13 16:54 ` [PATCH 2/9] block: add a struct io_batch argument to fops->iopoll() Jens Axboe
2021-10-13 16:54 ` [PATCH 3/9] sbitmap: add helper to clear a batch of tags Jens Axboe
2021-10-13 16:54 ` [PATCH 4/9] sbitmap: test bit before calling test_and_set_bit() Jens Axboe
2021-10-14  7:20   ` Hannes Reinecke
2021-10-14 13:01     ` Jens Axboe
2021-10-14 18:41   ` Bart Van Assche [this message]
2021-10-13 16:54 ` [PATCH 5/9] block: add support for blk_mq_end_request_batch() Jens Axboe
2021-10-14  7:32   ` Christoph Hellwig
2021-10-14 15:27     ` Jens Axboe
2021-10-13 16:54 ` [PATCH 6/9] nvme: add support for batched completion of polled IO Jens Axboe
2021-10-14  7:43   ` Christoph Hellwig
2021-10-14 15:30     ` Jens Axboe
2021-10-14 15:34       ` Jens Axboe
2021-10-14 16:07       ` Christoph Hellwig
2021-10-14 16:11         ` Jens Axboe
2021-10-13 16:54 ` [PATCH 7/9] block: assign batch completion handler in blk_poll() Jens Axboe
2021-10-14  7:48   ` Christoph Hellwig
2021-10-14 15:43     ` Jens Axboe
2021-10-13 16:54 ` [PATCH 8/9] io_uring: utilize the io_batch infrastructure for more efficient polled IO Jens Axboe
2021-10-14  8:03   ` Christoph Hellwig
2021-10-14 15:45     ` Jens Axboe
2021-10-14 16:08       ` Christoph Hellwig
2021-10-14 18:14         ` Jens Axboe
2021-10-16  4:29           ` Christoph Hellwig
2021-10-16 14:33             ` Jens Axboe
2021-10-13 16:54 ` [PATCH 9/9] nvme: wire up completion batching for the IRQ path Jens Axboe
2021-10-14  7:53   ` Christoph Hellwig
2021-10-14 15:49     ` Jens Axboe

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