linux-kernel - Re: [PATCH RFC] scsi: core: remove unsed 'restarts' from scsi

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Message-ID: <ce7b34e9-1e16-c05a-b96c-3796b9242e58@huaweicloud.com>
Date:   Mon, 28 Nov 2022 10:49:32 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Ming Lei <ming.lei@...hat.com>, Yu Kuai <yukuai1@...weicloud.com>
Cc:     jejb@...ux.ibm.com, martin.petersen@...cle.com,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH RFC] scsi: core: remove unsed 'restarts' from scsi_device

Hi, Ming

在 2022/11/27 17:45, Ming Lei 写道:
> On Sat, Nov 26, 2022 at 04:54:46PM +0800, Yu Kuai wrote:
>> Hi,
>>
>> 在 2022/11/18 19:30, Yu Kuai 写道:
>>> From: Yu Kuai <yukuai3@...wei.com>
>>>
>>> During code review, I found that 'restarts' is not useful anymore after
>>> the following commits:
>>>
>>> 1) commit ab3cee3762e5 ("blk-mq: In blk_mq_dispatch_rq_list() "no budget"
>>> is a reason to kick")
>>> 2) commit d3b38596875d ("blk-mq: run queue no matter whether the request
>>> is the last request")
>>> 3) commit 673235f91531 ("scsi: core: Fix race between handling STS_RESOURCE
>>> and completion")
>>>
>>> Now that if get budget ever failed, block layer will make sure to
>>> trigger new run queue for the hctx. Hence there is no need to run queue
>>> from scsi layer in this case.
>>>
>>
>> Does anyone has suggestions about this patch?
>>
>> More info why I tried to remove this:
>>
>> while testing megaraid with 4 nvme with none elevator, the default
>> queue_depth is 128, while I test it with fio 128 jobs and 1 iodepth,
>> bw is about 4Gib/s, however, if I test with 128 jobs and 2 iodepth,
>> bw is decreased to about 0.8Gib/s, and with this patch applied,
>> bw can stay 4Gib/s in the later case.
> 
> I will look at this patch next week.
> 
> Can you investigate a bit the reason why perf boost is from this patch?

test cmd:

without this patch, perf will show that on cpu time is wasted on
mod_delayed_work_on:

-    3.44%  swapper          [kernel.vmlinux]            [k] 
mod_delayed_work_on
    - mod_delayed_work_on
       - 3.44% kblockd_mod_delayed_work_on
            __blk_mq_delay_run_hw_queue
                  scsi_run_queue_async
                  scsi_end_request
                  scsi_io_completion
                  scsi_finish_command
                + scsi_complete
+    1.05%  ksoftirqd/30     [kernel.vmlinux]            [k] 
mod_delayed_work_on
+    0.54%  fio              [kernel.vmlinux]            [k] 
mod_delayed_work_on
      0.36%  ksoftirqd/22     [kernel.vmlinux]            [k] 
mod_delayed_work_on
      0.35%  ksoftirqd/1      [kernel.vmlinux]            [k] 
mod_delayed_work_on

I also attched the flamegraph.

Thanks,
Kuai
> 
o> Thanks,
> Ming
> 
> 
> .
> 

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