linux-kernel - Re: Perfromance drop on SCSI hard disk

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Date:	Fri, 20 May 2011 08:40:56 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	"Alex,Shi" <alex.shi@...el.com>
Cc:	Jens Axboe <jaxboe@...ionio.com>,
	"James.Bottomley@...senpartnership.com" 
	<James.Bottomley@...senpartnership.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Perfromance drop on SCSI hard disk

2011/5/20 Alex,Shi <alex.shi@...el.com>:
> On Fri, 2011-05-20 at 02:27 +0800, Jens Axboe wrote:
>> On 2011-05-19 10:26, Alex,Shi wrote:
>> >
>> >> I will queue up the combined patch, it looks fine from here as well.
>> >>
>> >
>> > When I have some time to study Jens and shaohua's patch today. I find a
>> > simpler way to resolved the re-enter issue on starved_list. Following
>> > Jens' idea, we can just put the starved_list device into kblockd if it
>> > come from __scsi_queue_insert().
>> > It can resolve the re-enter issue and recover performance totally, and
>> > need not a work_struct in every scsi_device. The logic/code also looks a
>> > bit simpler.
>> > What's your opinion of this?
>>
>> Isn't this _identical_ to my original patch, with the added async run of
>> the queue passed in (which is important, an oversight)?
>
> Not exactly same. It bases on your patch, but added a bypass way for
> starved_list device. If a starved_list device come from
> __scsi_queue_insert(), that may caused by our talking recursion, kblockd
> with take over the process.  Maybe you oversight this point in original
> patch. :)
>
> The different part from yours is below:
> ---
> static void __scsi_run_queue(struct request_queue *q, bool async)
>  {
>        struct scsi_device *sdev = q->queuedata;
>        struct Scsi_Host *shost;
> @@ -435,30 +437,35 @@ static void scsi_run_queue(struct request_queue
> *q)
>                                       &shost->starved_list);
>                        continue;
>                }
> -
> -               spin_unlock(shost->host_lock);
> -               spin_lock(sdev->request_queue->queue_lock);
> -               __blk_run_queue(sdev->request_queue);
> -               spin_unlock(sdev->request_queue->queue_lock);
> -               spin_lock(shost->host_lock);
> +               if (async)
> +                       blk_run_queue_async(sdev->request_queue);
> +               else {
> +                       spin_unlock(shost->host_lock);
> +                       spin_lock(sdev->request_queue->queue_lock);
> +                       __blk_run_queue(sdev->request_queue);
> +                       spin_unlock(sdev->request_queue->queue_lock);
> +                       spin_lock(shost->host_lock);
>>
I don't quite like this approach. blk_run_queue_async() could
introduce fairness issue as I said in previous mail, because we drop
the sdev from starved list but didn't run its queue immediately. The
issue exists before, but it's a bug to me.
Alex, is there any real advantage of your patch?

Thanks,
Shaohua
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