Re: [patch 1/2]scsi: scsi_run_queue() doesn't use local list tohandle starved sdev

[Shaohua Li]

On Thu, 2011-12-22 at 19:17 -0600, James Bottomley wrote:
> On Fri, 2011-12-23 at 08:40 +0800, Shaohua Li wrote:
> > On Thu, 2011-12-22 at 18:27 +0000, James Bottomley wrote:
> > > On Thu, 2011-12-22 at 11:10 +0800, Shaohua Li wrote:
> > > > scsi_run_queue() picks off all sdev from host starved_list to a local list,
> > > > then handle them. If there are multiple threads running scsi_run_queue(),
> > > > the starved_list will get messed. This is quite common, because request
> > > > rq_affinity is on by default.
> > > > 
> > > > Signed-off-by: Shaohua Li <shaohua.li@xxxxxxxxx>
> > > > ---
> > > >  drivers/scsi/scsi_lib.c |   21 ++++++++++++++-------
> > > >  1 file changed, 14 insertions(+), 7 deletions(-)
> > > > 
> > > > Index: linux/drivers/scsi/scsi_lib.c
> > > > ===================================================================
> > > > --- linux.orig/drivers/scsi/scsi_lib.c	2011-12-21 16:56:23.000000000 +0800
> > > > +++ linux/drivers/scsi/scsi_lib.c	2011-12-22 09:33:09.000000000 +0800
> > > > @@ -401,9 +401,8 @@ static inline int scsi_host_is_busy(stru
> > > >   */
> > > >  static void scsi_run_queue(struct request_queue *q)
> > > >  {
> > > > -	struct scsi_device *sdev = q->queuedata;
> > > > +	struct scsi_device *sdev = q->queuedata, *head_sdev = NULL;
> > > >  	struct Scsi_Host *shost;
> > > > -	LIST_HEAD(starved_list);
> > > >  	unsigned long flags;
> > > >  
> > > >  	/* if the device is dead, sdev will be NULL, so no queue to run */
> > > > @@ -415,9 +414,8 @@ static void scsi_run_queue(struct reques
> > > >  		scsi_single_lun_run(sdev);
> > > >  
> > > >  	spin_lock_irqsave(shost->host_lock, flags);
> > > > -	list_splice_init(&shost->starved_list, &starved_list);
> > > >  
> > > > -	while (!list_empty(&starved_list)) {
> > > > +	while (!list_empty(&shost->starved_list)) {
> > > 
> > > The original reason for working from a copy instead of the original list
> > > was that the device can end up back on the starved list because of a
> > > variety of conditions in the HBA and so this would cause the loop not to
> > > exit, so this piece of the patch doesn't look right to me.
> > +               /*
> > +                * the head sdev is no longer starved and removed from the
> > +                * starved list, select a new sdev as head.
> > +                */
> > +               if (head_sdev == sdev && list_empty(&sdev->starved_entry))
> > +                       head_sdev = NULL;
> > I had this in the loop, which is to guarantee the loop will exit if a
> > device is removed from the starved list.
> 
> And the non-head sdevs? which can also get put back onto the list
yes, it will be put back to the list tail. Note we always handle the
head sdev first. move the cursor of list iterating to next dev and next,
we will hit head_sdev == sdev eventually, then the loop exit. or the
cursor isn't moving, which means we have scsi_host_is_busy(). in both
cases, the loop will exit.

> What's the reason for not just traversing the list once using list
> splice?
> 
> Basically, the changelog doesn't seem to explain what you're doing and
> the logic looks flawed.
The main reason is we have multiple CPU trying to get the list to its
local list and then handle it, then putback unprocessed entries back. we
have multiple cpu handles blk softirq, because request rq_affinity is on
default. And the putback always happen, because scsi_run_queue is called
after every request finish. When it's called, scsi_host has just one
free slot. The in-coordination of multiple threads handle of the starved
list will mess the list. I observed dev which doesn't get chance to
dispatch request is put to starved list tail, while dev which dispatches
request is still at head. eg, this will cause unfairness. This means
some devices will get starved.

Thanks,
Shaohua 

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