Re: [PATCH RFC] scsi: core: remove unsed 'restarts' from scsi_device

[Yu Kuai]

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@xxxxxxxxxx>
> > >
> > > 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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Attachment: 6.1.0-rc6-patched.svg
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