linux-kernel - Re: [PATCH BUGFIX] block, bfq: postpone rq preparation to insert or merge

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Date:   Mon, 07 May 2018 07:56:37 +0200
From:   Mike Galbraith <efault@....de>
To:     Paolo Valente <paolo.valente@...aro.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Mark Brown <broonie@...nel.org>, linus.walleij@...aro.org,
        bfq-iosched@...glegroups.com, oleksandr@...alenko.name
Subject: Re: [PATCH BUGFIX] block, bfq: postpone rq preparation to insert or
 merge

On Sun, 2018-05-06 at 09:42 +0200, Paolo Valente wrote:
> 
> diff --git a/block/bfq-mq-iosched.c b/block/bfq-mq-iosched.c
> index 118f319af7c0..6662efe29b69 100644
> --- a/block/bfq-mq-iosched.c
> +++ b/block/bfq-mq-iosched.c
> @@ -525,8 +525,13 @@ static void bfq_limit_depth(unsigned int op, struct blk_mq_alloc_data *data)
>         if (unlikely(bfqd->sb_shift != bt->sb.shift))
>                 bfq_update_depths(bfqd, bt);
>  
> +#if 0
>         data->shallow_depth =
>                 bfqd->word_depths[!!bfqd->wr_busy_queues][op_is_sync(op)];
                                                            ^^^^^^^^^^^^^

Q: why doesn't the top of this function look like so?

---
 block/bfq-iosched.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -539,7 +539,7 @@ static void bfq_limit_depth(unsigned int
 	struct bfq_data *bfqd = data->q->elevator->elevator_data;
 	struct sbitmap_queue *bt;
 
-	if (op_is_sync(op) && !op_is_write(op))
+	if (!op_is_write(op))
 		return;
 
 	if (data->flags & BLK_MQ_REQ_RESERVED) {

It looks a bit odd that these elements exist...

+       /*
+        * no more than 75% of tags for sync writes (25% extra tags
+        * w.r.t. async I/O, to prevent async I/O from starving sync
+        * writes)
+        */
+       bfqd->word_depths[0][1] = max(((1U<<bfqd->sb_shift) * 3)>>2, 1U);

+       /* no more than ~37% of tags for sync writes (~20% extra tags) */
+       bfqd->word_depths[1][1] = max(((1U<<bfqd->sb_shift) * 6)>>4, 1U);

...yet we index via and log a guaranteed zero.

	-Mike