Re: Perfromance drop on SCSI hard disk

["Alex,Shi" <alex.shi@intel.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);
>