Re: [PATCH 2/2] md/raid5: reserve stripe cache for user I/O during rebuild

["yu kuai" <yukuai@fygo.io>]

Hi,

在 2026/7/10 21:23, Hiroshi Nishida 写道:
> The resync read-ahead window (RAID5_SYNC_WINDOW) can fill the stripe
> cache with rebuild stripes and starve concurrent user I/O, producing a
> burst-starvation flip-flop between rebuild and application throughput.
>
> Add two yield points to the window-submission loop:
>   - stop the window immediately if any thread is waiting for a stripe
>     (waitqueue_active(&conf->wait_for_stripe)); the check is intentionally
>     racy -- a waiter appearing just after is serviced by the next
>     sync_request call, so no barrier is needed.
>   - stop expanding once active_stripes reaches half the cache
>     (max_nr_stripes / RAID5_SYNC_HWMARK), but only when
>     preread_active_stripes > 0, i.e. user write I/O is actually competing.
>     Sync stripes never set STRIPE_PREREAD_ACTIVE, so during a pure rebuild
>     the counter stays zero and the window fills freely; rebuild-only
>     throughput is unchanged.
>
> This bounds the share of the stripe cache a rebuild may hold while user
> I/O is present, so application latency no longer collapses during the
> read-ahead bursts, without throttling a rebuild that has the array to
> itself.
>
> Signed-off-by: Hiroshi Nishida <nishidafmly@gmail.com>
> ---
>   drivers/md/raid5.c | 21 +++++++++++++++++++++
>   drivers/md/raid5.h |  1 +
>   2 files changed, 22 insertions(+)
>
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 574880e4f23f..6575bdb84bb2 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -6656,6 +6656,27 @@ static inline sector_t raid5_sync_request(struct mddev *mddev, sector_t sector_n
>   	     submitted < RAID5_SYNC_WINDOW && win_sector < max_sector &&
>   	     win_sector < mddev->resync_max;
>   	     submitted++, win_sector += RAID5_STRIPE_SECTORS(conf)) {
> +		/*
> +		 * Yield to user I/O: stop the read-ahead if anyone is waiting
> +		 * for a stripe.  The check is intentionally racy -- a waiter
> +		 * appearing just after is serviced by the next sync_request
> +		 * call, so no barrier is needed.
> +		 */
> +		if (waitqueue_active(&conf->wait_for_stripe))
> +			break;
> +		/*
> +		 * Reserve cache for user I/O only when it is actually competing.
> +		 * preread_active_stripes counts stripes queued for write I/O
> +		 * (including the read phase of RMW); sync stripes never set
> +		 * STRIPE_PREREAD_ACTIVE, so during a pure rebuild it stays zero
> +		 * and the window fills freely.  Competing user reads do not bump
> +		 * the counter but are caught by the waitqueue_active() check
> +		 * above.
> +		 */
> +		if (atomic_read(&conf->preread_active_stripes) > 0 &&
> +		    atomic_read(&conf->active_stripes) >=
> +		    conf->max_nr_stripes / RAID5_SYNC_HWMARK)
> +			break;

Please check md_do_sync(), if any user IO is active, sync speed and sync IO depth is
limited there. Above is not the right place to throttle sync IO for user IO, when the
above checking failed, the array is already under heavy sync IO load.

>   		sh = raid5_get_active_stripe(conf, NULL, win_sector,
>   					     R5_GAS_NOBLOCK);
>   		if (!sh)
> diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
> index 63b630118782..263330af4e7d 100644
> --- a/drivers/md/raid5.h
> +++ b/drivers/md/raid5.h
> @@ -492,6 +492,7 @@ struct disk_info {
>   #define HASH_MASK		(NR_HASH - 1)
>   #define MAX_STRIPE_BATCH	8
>   #define RAID5_SYNC_WINDOW	32	/* stripes to pre-submit per sync_request call */
> +#define RAID5_SYNC_HWMARK	2	/* rebuild uses at most 1/N of stripe cache */
>   
>   /* NOTE NR_STRIPE_HASH_LOCKS must remain below 64.
>    * This is because we sometimes take all the spinlocks

-- 
Thanks,
Kuai