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

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Date:	Fri, 20 May 2011 13:17:43 +0800
From:	"Alex,Shi" <alex.shi@...el.com>
To:	Shaohua Li <shaohua.li@...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

On Fri, 2011-05-20 at 08:40 +0800, Shaohua Li wrote:
> 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.

I understand what's your worried. But not quite clear of the trigger
scenario. anyway, it is still a potential issue of fairness exist. So
forget my patch. 


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