On 6/14/25 18:25, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> Before this patch, each context will hold a global lock to dispatch one
> request at a time, which introduce intense lock competition:
How so ? If there is only a single context issuing IOs, there will not be any
contention on the lock.
> lock
> ops.dispatch_request
> unlock
>
> Hence support dispatch a batch of requests while holding the lock to
> reduce lock contention.
Lock contention would happen only if you have multiple processes issuing I/Os.
For a single context case, this simply reduces the overhead of dispatching
commands by avoiding the lock+unlock per request. So please explain that clearly.
>
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
> block/blk-mq-sched.c | 55 ++++++++++++++++++++++++++++++++++++++++----
> block/blk-mq.h | 21 +++++++++++++++++
> 2 files changed, 72 insertions(+), 4 deletions(-)
>
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index 990d0f19594a..d7cb88c8e8c7 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -101,6 +101,49 @@ static bool elevator_can_dispatch(struct sched_dispatch_ctx *ctx)
> return true;
> }
>
> +static void elevator_dispatch_requests(struct sched_dispatch_ctx *ctx)
> +{
> + struct request *rq;
> + int budget_token[BUDGET_TOKEN_BATCH];
> + int count;
> + int i;
These 2 can be declared on the same line.
> +
> + while (true) {
> + if (!elevator_can_dispatch(ctx))
> + return;
> +
> + count = blk_mq_get_dispatch_budgets(ctx->q, budget_token);
> + if (count <= 0)
> + return;
> +
> + elevator_lock(ctx->e);
> + for (i = 0; i < count; ++i) {
> + rq = ctx->e->type->ops.dispatch_request(ctx->hctx);
> + if (!rq) {
> + ctx->run_queue = true;
> + goto err_free_budgets;
> + }
> +
> + blk_mq_set_rq_budget_token(rq, budget_token[i]);
> + list_add_tail(&rq->queuelist, &ctx->rq_list);
> + ctx->count++;
> + if (rq->mq_hctx != ctx->hctx)
> + ctx->multi_hctxs = true;
> +
> + if (!blk_mq_get_driver_tag(rq)) {
> + i++;
> + goto err_free_budgets;
> + }
> + }
> + elevator_unlock(ctx->e);
> + }
> +
> +err_free_budgets:
> + elevator_unlock(ctx->e);
> + for (; i < count; ++i)> + blk_mq_put_dispatch_budget(ctx->q, budget_token[i]);
> +}
--
Damien Le Moal
Western Digital Research
