Re: [PATCH 1/1 linux-next] hpsa: remove set but unused variable rc

[Sudip Mukherjee]

On Thu, Oct 30, 2014 at 06:28:13AM +0000, Elliott, Robert (Server Storage) wrote:
> 
> 
> > -----Original Message-----
> > From: linux-scsi-owner@xxxxxxxxxxxxxxx [mailto:linux-scsi-
> > owner@xxxxxxxxxxxxxxx] On Behalf Of Sudip Mukherjee
> > Sent: Thursday, October 30, 2014 12:55 AM
> > To: Fabian Frederick
> > Cc: linux-kernel@xxxxxxxxxxxxxxx; Stephen M. Cameron; James E.J.
> > Bottomley; iss_storagedev@xxxxxx; linux-scsi@xxxxxxxxxxxxxxx
> > Subject: Re: [PATCH 1/1 linux-next] hpsa: remove set but unused variable
> > rc
> > 
> > On Wed, Oct 29, 2014 at 04:15:04PM +0100, Fabian Frederick wrote:
> > > Fix -Wunused-but-set-variable warning
> > 
> > you should also mention why you have left the call to
> > irq_set_affinity_hint().
> > i am not sure , but it looks like irq_set_affinity_hint() is only
> > checking if
> > the lock is available or not. It is just locking ,then if lock is
> > successfull then
> > returning 0 or if lock fails then return -EINVAL, and unlocks before
> > returnig.
> > not doing anything else.
> > 
> > thanks
> > sudip
> 
> No, that function sets a mask value that shows up in 
> 	/proc/irq/nnn/affinity_hint
> that a program like irqbalance may use to set the CPU affinity mask
> for each irq via 
> 	/proc/irq/nnn/smp_affinity   (bitmap format)
> 	/proc/irq/nnn/smp_affinity_list   (range format)
> 
> The reason is that in many cases, it is best when all these occur 
> on the same CPU that submitted the IO:
> * LLD submission queues (if multiple are supported)
> * LDD completion queues
> * MSI-X interrupt indicating completion
> * LLD completion interrupt handler
> * block layer completion handler
> 
> Benefits include:
> * cache efficiency - the data structures for the IO aren't
> pulled from CPU to CPU.
> 
> * avoid IPI overhead in the block layer to get the completion 
> processed on the submitting CPU (which is done if using 
> rq_affinity=2 and the interrupt is routed to on another CPU).
> 
> * self-throttle the CPUs - avoid swamping one CPU with 
> completion processing for IOs submitted by many other CPUs
> (which leads to stalls on the victim and timeouts on the
> aggressors).
> 
> You must run irqbalance with an option to honor the hints;
> some versions default to that, others don't.  Or, disable
> the irqbalance service and set them up the affinities
> manually.
> 
thanks , i think this is happening at desc->affinity_hint = m;

thanks
sudip
> 
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