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Date: Wed, 27 Apr 2022 10:06:58 +0800
From: Guoqing Jiang <guoqing.jiang@...ux.dev>
To: Logan Gunthorpe <logang@...tatee.com>,
linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
Song Liu <song@...nel.org>
Cc: Christoph Hellwig <hch@...radead.org>,
Stephen Bates <sbates@...thlin.com>,
Martin Oliveira <Martin.Oliveira@...eticom.com>,
David Sloan <David.Sloan@...eticom.com>
Subject: Re: [PATCH v2 12/12] md/raid5: Pivot raid5_make_request()
On 4/21/22 3:54 AM, Logan Gunthorpe wrote:
> The number of times the lock is taken can be reduced by pivoting
> raid5_make_request() so that it loops through every stripe and then
> loops through every disk in that stripe to see if the bio must be
> added. This reduces the number of times the lock must be taken by
> a factor equal to the number of data disks.
>
> To accomplish this, store the minimum and maxmimum disk sector that
> has already been finished and continue to the next logical sector if
> it is found that the disk sector has already been done. Then add a
> add_all_stripe_bios() to check all the bios for overlap and add them
> all if none of them overlap.
>
> Signed-off-by: Logan Gunthorpe<logang@...tatee.com>
> ---
> drivers/md/raid5.c | 92 +++++++++++++++++++++++++++++++++++++++++++---
> drivers/md/raid5.h | 1 +
> 2 files changed, 88 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 40a25c4b80bd..f86866cb15be 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -3571,6 +3571,48 @@ static bool add_stripe_bio(struct stripe_head *sh, struct bio *bi,
> return true;
> }
>
> +static int add_all_stripe_bios(struct stripe_head *sh, struct bio *bi,
> + sector_t first_logical_sector, sector_t last_sector,
> + int forwrite, int previous)
> +{
> + int dd_idx;
> + int ret = 1;
> +
> + spin_lock_irq(&sh->stripe_lock);
> +
> + for (dd_idx = 0; dd_idx < sh->disks; dd_idx++) {
> + struct r5dev *dev = &sh->dev[dd_idx];
> +
> + clear_bit(R5_BioReady, &dev->flags);
> +
> + if (dd_idx == sh->pd_idx)
> + continue;
> +
> + if (dev->sector < first_logical_sector ||
> + dev->sector >= last_sector)
> + continue;
> +
> + if (stripe_bio_overlaps(sh, bi, dd_idx, forwrite)) {
> + set_bit(R5_Overlap, &dev->flags);
> + ret = 0;
> + continue;
> + }
> +
> + set_bit(R5_BioReady, &dev->flags);
Is it possible to just call __add_stripe_bio here? And change above
"continue"
to "return",
> + }
> +
> + if (!ret)
> + goto out;
> +
> + for (dd_idx = 0; dd_idx < sh->disks; dd_idx++)
> + if (test_bit(R5_BioReady, &sh->dev[dd_idx].flags))
> + __add_stripe_bio(sh, bi, dd_idx, forwrite, previous);
then we don't need another loop here, also no need to introduce another
flag.
But I am not sure it is feasible, so just FYI.
> +
> +out:
> + spin_unlock_irq(&sh->stripe_lock);
> + return ret;
> +}
> +
> static void end_reshape(struct r5conf *conf);
>
> static void stripe_set_idx(sector_t stripe, struct r5conf *conf, int previous,
> @@ -5869,6 +5911,10 @@ enum stripe_result {
> struct stripe_request_ctx {
> bool do_flush;
> struct stripe_head *batch_last;
> + sector_t disk_sector_done;
> + sector_t start_disk_sector;
> + bool first_wrap;
> + sector_t last_sector;
> };
Could you add some comments to above members if possible?
> static enum stripe_result make_stripe_request(struct mddev *mddev,
> @@ -5908,6 +5954,36 @@ static enum stripe_result make_stripe_request(struct mddev *mddev,
>
> new_sector = raid5_compute_sector(conf, logical_sector, previous,
> &dd_idx, NULL);
> +
> + /*
> + * This is a tricky algorithm to figure out which stripe_heads that
> + * have already been visited and exit early if the stripe_head has
> + * already been done. (Seeing all disks are added to a stripe_head
> + * once in add_all_stripe_bios().
> + *
> + * To start with, the disk sector of the last stripe that has been
> + * completed is stored in ctx->disk_sector_done. If the new_sector is
> + * less than this value, the stripe_head has already been done.
> + *
> + * There's one issue with this: if the request starts in the middle of
> + * a chunk, all the stripe heads before the starting offset will be
> + * missed. To account for this, set the first_wrap boolean to true
> + * if new_sector is less than the starting sector. Clear the
> + * boolean once the start sector is hit for the second time.
> + * When first_wrap is set, ignore the disk_sector_done.
> + */
> + if (ctx->start_disk_sector == MaxSector) {
> + ctx->start_disk_sector = new_sector;
> + } else if (new_sector < ctx->start_disk_sector) {
> + ctx->first_wrap = true;
> + } else if (new_sector == ctx->start_disk_sector) {
> + ctx->first_wrap = false;
> + ctx->start_disk_sector = 0;
> + return STRIPE_SUCCESS;
> + } else if (!ctx->first_wrap && new_sector <= ctx->disk_sector_done) {
> + return STRIPE_SUCCESS;
> + }
> +
Hmm, with above tricky algorithm, I guess the point is that we can avoid
to call below
stripe_add_to_batch_list where has hash_lock contention. If so, maybe we
can change
stripe_can_batch for the purpose.
> if (stripe_can_batch(sh)) {
> stripe_add_to_batch_list(conf, sh, ctx->batch_last);
> if (ctx->batch_last)
> @@ -5977,8 +6057,10 @@ static enum stripe_result make_stripe_request(struct mddev *mddev,
> static bool raid5_make_request(struct mddev *mddev, struct bio * bi)
> {
> struct r5conf *conf = mddev->private;
> - sector_t logical_sector, last_sector;
> - struct stripe_request_ctx ctx = {};
> + sector_t logical_sector;
> + struct stripe_request_ctx ctx = {
> + .start_disk_sector = MaxSector,
> + };
> const int rw = bio_data_dir(bi);
> enum stripe_result res;
> DEFINE_WAIT(w);
> @@ -6021,7 +6103,7 @@ static bool raid5_make_request(struct mddev *mddev, struct bio * bi)
> }
>
> logical_sector = bi->bi_iter.bi_sector & ~((sector_t)RAID5_STRIPE_SECTORS(conf)-1);
> - last_sector = bio_end_sector(bi);
> + ctx.last_sector = bio_end_sector(bi);
> bi->bi_next = NULL;
>
> /* Bail out if conflicts with reshape and REQ_NOWAIT is set */
> @@ -6036,7 +6118,7 @@ static bool raid5_make_request(struct mddev *mddev, struct bio * bi)
> }
> md_account_bio(mddev, &bi);
> prepare_to_wait(&conf->wait_for_overlap, &w, TASK_UNINTERRUPTIBLE);
> - while (logical_sector < last_sector) {
> + while (logical_sector < ctx.last_sector) {
> res = make_stripe_request(mddev, conf, &ctx, logical_sector,
> bi);
> if (res == STRIPE_FAIL) {
> diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
> index 638d29863503..e73b58844f83 100644
> --- a/drivers/md/raid5.h
> +++ b/drivers/md/raid5.h
> @@ -308,6 +308,7 @@ enum r5dev_flags {
> R5_Wantwrite,
> R5_Overlap, /* There is a pending overlapping request
> * on this block */
> + R5_BioReady, /* The current bio can be added to this disk */
This doesn't seem right to me, since the comment describes bio status
while others
are probably for r5dev.
Thanks,
Guoqing
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