Re: [PATCH v2 1/4] blk-mq: use percpu csd to remote complete instead of per-rq csd

Christoph Hellwig
2023-07-06T13:07:42Z

On Thu, Jun 29, 2023 at 07:03:56PM +0800, chengming.zhou@linux.dev wrote:
> From: Chengming Zhou <zhouchengming@bytedance.com>
>
> If request need to be completed remotely, we insert it into percpu llist,
> and smp_call_function_single_async() if llist is empty previously.
>
> We don't need to use per-rq csd, percpu csd is enough. And the size of
> struct request is decreased by 24 bytes.
>
> This way is cleaner, and looks correct, given block softirq is guaranteed to be
> scheduled to consume the list if one new request is added to this percpu list,
> either smp_call_function_single_async() returns -EBUSY or 0.

Please trim your commit logs to 73 characters per line so that they
are readable in git log output.

> static void blk_mq_request_bypass_insert(struct request *rq,
> @@ -1156,13 +1157,13 @@ static void blk_mq_complete_send_ipi(struct request *rq)
> {
> struct llist_head *list;
> unsigned int cpu;
> + call_single_data_t *csd;
>
> cpu = rq->mq_ctx->cpu;
> list = &per_cpu(blk_cpu_done, cpu);
> - if (llist_add(&rq->ipi_list, list)) {
> - INIT_CSD(&rq->csd, __blk_mq_complete_request_remote, rq);
> - smp_call_function_single_async(cpu, &rq->csd);
> - }
> + csd = &per_cpu(blk_cpu_csd, cpu);
> + if (llist_add(&rq->ipi_list, list))
> + smp_call_function_single_async(cpu, csd);
> }

No need for the list and csd variables here as they are only used
once.

But I think this code has a rpboem when it is preemptd between
the llist_add and smp_call_function_single_async. We either need a
get_cpu/put_cpu around them, or instroduce a structure with the list
and csd, and then you can use one pointer from per_cpu and still ensure
the list and csd are for the same CPU.