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Date:   Wed, 28 Jun 2023 12:50:21 +0800
From:   Ming Lei <ming.lei@redhat.com>
To:     Chengming Zhou <chengming.zhou@linux.dev>
Cc:     axboe@kernel.dk, tj@kernel.org, hch@lst.de,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        zhouchengming@bytedance.com
Subject: Re: [PATCH 1/4] blk-mq: use percpu csd to remote complete instead of
 per-rq csd
Message-ID: <ZJu8DQgug3/UjpUJ@ovpn-8-21.pek2.redhat.com>
References: <20230627120854.971475-1-chengming.zhou@linux.dev>
 <20230627120854.971475-2-chengming.zhou@linux.dev>
 <ZJuY0OZu42H0oBa7@ovpn-8-21.pek2.redhat.com>
 <9c038fe8-0361-c6fb-9b4e-7b74e0591ae9@linux.dev>
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In-Reply-To: <9c038fe8-0361-c6fb-9b4e-7b74e0591ae9@linux.dev>
Precedence: bulk

On Wed, Jun 28, 2023 at 11:28:20AM +0800, Chengming Zhou wrote:
> On 2023/6/28 10:20, Ming Lei wrote:
> > On Tue, Jun 27, 2023 at 08:08:51PM +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.
> >>
> >> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> >> ---
> >>  block/blk-mq.c         | 12 ++++++++----
> >>  include/linux/blk-mq.h |  5 +----
> >>  2 files changed, 9 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/block/blk-mq.c b/block/blk-mq.c
> >> index decb6ab2d508..a36822479b94 100644
> >> --- a/block/blk-mq.c
> >> +++ b/block/blk-mq.c
> >> @@ -43,6 +43,7 @@
> >>  #include "blk-ioprio.h"
> >>  
> >>  static DEFINE_PER_CPU(struct llist_head, blk_cpu_done);
> >> +static DEFINE_PER_CPU(struct __call_single_data, blk_cpu_csd);
> > 
> > It might be better to use call_single_data, given:
> > 
> > /* Use __aligned() to avoid to use 2 cache lines for 1 csd */
> >   typedef struct __call_single_data call_single_data_t
> >   	__aligned(sizeof(struct __call_single_data));
> > 
> 
> Good, I will change to use this.
> 
> >>  
> >>  static void blk_mq_insert_request(struct request *rq, blk_insert_t flags);
> >>  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;
> >> +	struct __call_single_data *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);
> >>  }
> > 
> > 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.
> > 
> 
> If this llist_add() see the llist is empty, the consumer function in the softirq
> on the remote CPU must have consumed the llist, so smp_call_function_single_async()
> won't return -EBUSY ?

block softirq can be scheduled from other code path, such as blk_mq_raise_softirq()
for single queue's remote completion, where no percpu csd schedule is needed, so
two smp_call_function_single_async() could be called, and the 2nd one
may return -EBUSY.

Not mention csd_unlock() could be called after the callback returns, see
__flush_smp_call_function_queue().

But that is fine, if there is pending block softirq, the llist is
guaranteed to be consumed because the csd callback just raises block
softirq, and request/llist is consumed in softirq handler.

Thanks,
Ming