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Message-ID: <d1778341-549e-6f88-0282-3096bfcd6614@molgen.mpg.de>
Date:   Fri, 23 Jun 2023 12:03:40 +0200
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Subject: Re: [PATCH 1/3] md/raid10: optimize fix_read_error
To:     linan666@huaweicloud.com
Cc:     song@kernel.org, linux-raid@vger.kernel.org,
        linux-kernel@vger.kernel.org, linan122@huawei.com,
        yukuai3@huawei.com, yi.zhang@huawei.com, houtao1@huawei.com,
        yangerkun@huawei.com
References: <20230623173236.2513554-1-linan666@huaweicloud.com>
 <20230623173236.2513554-2-linan666@huaweicloud.com>
Content-Language: en-US
From:   Paul Menzel <pmenzel@molgen.mpg.de>
In-Reply-To: <20230623173236.2513554-2-linan666@huaweicloud.com>
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Dear Li,


Thank you for your patch.

Am 23.06.23 um 19:32 schrieb linan666@huaweicloud.com:
> From: Li Nan <linan122@huawei.com>
> 
> We dereference r10_bio->read_slot too many times in fix_read_error().
> Optimize it by using a variable to store read_slot.

I am always cautious reading about optimizations without any benchmarks 
or object code analysis. Although your explanation makes sense, did you 
check, that performance didn’t decrease in some way? (Maybe the compiler 
even generates the same code.)


Kind regards,

Paul


> Signed-off-by: Li Nan <linan122@huawei.com>
> ---
>   drivers/md/raid10.c | 20 ++++++++++----------
>   1 file changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index 381c21f7fb06..94ae294c8a3c 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -2725,10 +2725,10 @@ static int r10_sync_page_io(struct md_rdev *rdev, sector_t sector,
>   static void fix_read_error(struct r10conf *conf, struct mddev *mddev, struct r10bio *r10_bio)
>   {
>   	int sect = 0; /* Offset from r10_bio->sector */
> -	int sectors = r10_bio->sectors;
> +	int sectors = r10_bio->sectors, slot = r10_bio->read_slot;
>   	struct md_rdev *rdev;
>   	int max_read_errors = atomic_read(&mddev->max_corr_read_errors);
> -	int d = r10_bio->devs[r10_bio->read_slot].devnum;
> +	int d = r10_bio->devs[slot].devnum;
>   
>   	/* still own a reference to this rdev, so it cannot
>   	 * have been cleared recently.
> @@ -2749,13 +2749,13 @@ static void fix_read_error(struct r10conf *conf, struct mddev *mddev, struct r10
>   		pr_notice("md/raid10:%s: %pg: Failing raid device\n",
>   			  mdname(mddev), rdev->bdev);
>   		md_error(mddev, rdev);
> -		r10_bio->devs[r10_bio->read_slot].bio = IO_BLOCKED;
> +		r10_bio->devs[slot].bio = IO_BLOCKED;
>   		return;
>   	}
>   
>   	while(sectors) {
>   		int s = sectors;
> -		int sl = r10_bio->read_slot;
> +		int sl = slot;
>   		int success = 0;
>   		int start;
>   
> @@ -2790,7 +2790,7 @@ static void fix_read_error(struct r10conf *conf, struct mddev *mddev, struct r10
>   			sl++;
>   			if (sl == conf->copies)
>   				sl = 0;
> -		} while (!success && sl != r10_bio->read_slot);
> +		} while (!success && sl != slot);
>   		rcu_read_unlock();
>   
>   		if (!success) {
> @@ -2798,16 +2798,16 @@ static void fix_read_error(struct r10conf *conf, struct mddev *mddev, struct r10
>   			 * as bad on the first device to discourage future
>   			 * reads.
>   			 */
> -			int dn = r10_bio->devs[r10_bio->read_slot].devnum;
> +			int dn = r10_bio->devs[slot].devnum;
>   			rdev = conf->mirrors[dn].rdev;
>   
>   			if (!rdev_set_badblocks(
>   				    rdev,
> -				    r10_bio->devs[r10_bio->read_slot].addr
> +				    r10_bio->devs[slot].addr
>   				    + sect,
>   				    s, 0)) {
>   				md_error(mddev, rdev);
> -				r10_bio->devs[r10_bio->read_slot].bio
> +				r10_bio->devs[slot].bio
>   					= IO_BLOCKED;
>   			}
>   			break;
> @@ -2816,7 +2816,7 @@ static void fix_read_error(struct r10conf *conf, struct mddev *mddev, struct r10
>   		start = sl;
>   		/* write it back and re-read */
>   		rcu_read_lock();
> -		while (sl != r10_bio->read_slot) {
> +		while (sl != slot) {
>   			if (sl==0)
>   				sl = conf->copies;
>   			sl--;
> @@ -2850,7 +2850,7 @@ static void fix_read_error(struct r10conf *conf, struct mddev *mddev, struct r10
>   			rcu_read_lock();
>   		}
>   		sl = start;
> -		while (sl != r10_bio->read_slot) {
> +		while (sl != slot) {
>   			if (sl==0)
>   				sl = conf->copies;
>   			sl--;