Subject: Re: [PATCH 1/4] block: add scalable completion tracking of requests
To: Ming Lei <tom.leiming@gmail.com>
References: <1478034325-28232-1-git-send-email-axboe@fb.com>
 <1478034325-28232-2-git-send-email-axboe@fb.com>
 <CACVXFVME3jpraUR8cp_i-YFYu4rk4gUCZqOi86PpCHGRrh9c_g@mail.gmail.com>
CC: Jens Axboe <axboe@kernel.dk>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-block <linux-block@vger.kernel.org>,
        Christoph Hellwig <hch@lst.de>
From: Jens Axboe <axboe@fb.com>
Message-ID: <bd83d2b6-3e7f-18c0-45d6-7b9f6c015722@fb.com>
Date: Thu, 3 Nov 2016 07:38:55 -0600
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101
 Thunderbird/45.4.0
MIME-Version: 1.0
In-Reply-To: <CACVXFVME3jpraUR8cp_i-YFYu4rk4gUCZqOi86PpCHGRrh9c_g@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"; format=flowed
Content-Transfer-Encoding: 7bit
SpamDiagnosticOutput: 1:99
SpamDiagnosticMetadata: NSPM
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 03 Nov 2016 13:39:01.4293 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-Transport-CrossTenantHeadersStamped: MWHPR15MB1198
X-OriginatorOrg: fb.com
X-Proofpoint-Spam-Reason: safe
X-FB-Internal: Safe
X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2016-11-03_04:,,
 signatures=0
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 1128
Lines: 30

On 11/03/2016 05:17 AM, Ming Lei wrote:
>> diff --git a/block/blk-core.c b/block/blk-core.c
>> index 0bfaa54d3e9f..ca77c725b4e5 100644
>> --- a/block/blk-core.c
>> +++ b/block/blk-core.c
>> @@ -2462,6 +2462,8 @@ void blk_start_request(struct request *req)
>>  {
>>         blk_dequeue_request(req);
>>
>> +       blk_stat_set_issue_time(&req->issue_stat);
>> +
>>         /*
>>          * We are now handing the request to the hardware, initialize
>>          * resid_len to full count and add the timeout handler.
>> @@ -2529,6 +2531,8 @@ bool blk_update_request(struct request *req, int error, unsigned int nr_bytes)
>>
>>         trace_block_rq_complete(req->q, req, nr_bytes);
>>
>> +       blk_stat_add(&req->q->rq_stats[rq_data_dir(req)], req);
>
> blk_update_request() is often called lockless, so it isn't good to
> do it here.

It's not really a concern, not for the legacy path here nor the mq one
where it is per sw context. The collisions are rare enough that it'll
skew the latencies a bit for that short window, but then go away again.
I'd much rather take that, than adding locking for this part.

-- 
Jens Axboe