block, bfq: don't break merge chain in bfq_split_bfqq()

[ Upstream commit 42c306ed723321af4003b2a41bb73728cab54f85 ]

Consider the following scenario:

    Process 1       Process 2       Process 3       Process 4
     (BIC1)          (BIC2)          (BIC3)          (BIC4)
      Λ               |               |                |
       \-------------\ \-------------\ \--------------\|
                      V               V                V
      bfqq1--------->bfqq2---------->bfqq3----------->bfqq4
ref    0              1               2                4

If Process 1 issue a new IO and bfqq2 is found, and then bfq_init_rq()
decide to spilt bfqq2 by bfq_split_bfqq(). Howerver, procress reference
of bfqq2 is 1 and bfq_split_bfqq() just clear the coop flag, which will
break the merge chain.

Expected result: caller will allocate a new bfqq for BIC1

    Process 1       Process 2       Process 3       Process 4
     (BIC1)          (BIC2)          (BIC3)          (BIC4)
                      |               |                |
                       \-------------\ \--------------\|
                                      V                V
      bfqq1--------->bfqq2---------->bfqq3----------->bfqq4
ref    0              0               1                3

Since the condition is only used for the last bfqq4 when the previous
bfqq2 and bfqq3 are already splited. Fix the problem by checking if
bfqq is the last one in the merge chain as well.

Fixes: 36eca8948323 ("block, bfq: add Early Queue Merge (EQM)")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240902130329.3787024-4-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index e68881f71ff137f1c80758b35dff9e9bfa76fea0..515e3c1a547590b17485bf3b9af320c17647671e 100644 (file)
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -6013,7 +6013,7 @@ bfq_split_bfqq(struct bfq_io_cq *bic, struct bfq_queue *bfqq)
 {
        bfq_log_bfqq(bfqq->bfqd, bfqq, "splitting queue");
 
-       if (bfqq_process_refs(bfqq) == 1) {
+       if (bfqq_process_refs(bfqq) == 1 && !bfqq->new_bfqq) {
                bfqq->pid = current->pid;
                bfq_clear_bfqq_coop(bfqq);
                bfq_clear_bfqq_split_coop(bfqq);