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blk-mq: optimise blk_mq_flush_plug_list() 

Instead of using list_del_init() in a loop, that generates a lot of
unnecessary memory read/writes, iterate from the first request of a
batch and cut out a sublist with list_cut_before().

Apart from removing the list node initialisation part, this is more
register-friendly, and the assembly uses the stack less intensively.

list_empty() at the beginning is done with hope, that the compiler can
optimise out the same check in the following list_splice_init().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Pavel Begunkov 2019-11-29 00:11:55 +03:00 committed by Jens Axboe
parent 28ca0d6d39
commit 95ed0c5b12
1 changed files with 18 additions and 37 deletions
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55
block/blk-mq.c
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@ -1678,14 +1678,10 @@ static int plug_rq_cmp(void *priv, struct list_head *a, struct list_head *b)
void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
{
struct blk_mq_hw_ctx *this_hctx;
struct blk_mq_ctx *this_ctx;
struct request_queue *this_q;
struct request *rq;
LIST_HEAD(list);
LIST_HEAD(rq_list);
unsigned int depth;
if (list_empty(&plug->mq_list))
return;
list_splice_init(&plug->mq_list, &list);
if (plug->rq_count > 2 && plug->multiple_queues)
@ -1693,42 +1689,27 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
plug->rq_count = 0;
this_q = NULL;
this_hctx = NULL;
this_ctx = NULL;
depth = 0;
do {
struct list_head rq_list;
struct request *rq, *head_rq = list_entry_rq(list.next);
struct list_head *pos = &head_rq->queuelist; /* skip first */
struct blk_mq_hw_ctx *this_hctx = head_rq->mq_hctx;
struct blk_mq_ctx *this_ctx = head_rq->mq_ctx;
unsigned int depth = 1;
while (!list_empty(&list)) {
rq = list_entry_rq(list.next);
list_del_init(&rq->queuelist);
BUG_ON(!rq->q);
if (rq->mq_hctx != this_hctx || rq->mq_ctx != this_ctx) {
if (this_hctx) {
trace_block_unplug(this_q, depth, !from_schedule);
blk_mq_sched_insert_requests(this_hctx, this_ctx,
&rq_list,
from_schedule);
}
this_q = rq->q;
this_ctx = rq->mq_ctx;
this_hctx = rq->mq_hctx;
depth = 0;
list_for_each_continue(pos, &list) {
rq = list_entry_rq(pos);
BUG_ON(!rq->q);
if (rq->mq_hctx != this_hctx || rq->mq_ctx != this_ctx)
break;
depth++;
}
depth++;
list_add_tail(&rq->queuelist, &rq_list);
}
/*
* If 'this_hctx' is set, we know we have entries to complete
* on 'rq_list'. Do those.
*/
if (this_hctx) {
trace_block_unplug(this_q, depth, !from_schedule);
list_cut_before(&rq_list, &list, pos);
trace_block_unplug(head_rq->q, depth, !from_schedule);
blk_mq_sched_insert_requests(this_hctx, this_ctx, &rq_list,
from_schedule);
}
} while(!list_empty(&list));
}
static void blk_mq_bio_to_request(struct request *rq, struct bio *bio,