Re: [PATCH] scsi: storvsc: Prevent running tasklet for long

[Saurabh Singh Sengar]

On Wed, Jul 06, 2022 at 11:09:43AM +0000, David Laight wrote:
> From: Praveen Kumar
> > Sent: 06 July 2022 10:15
> > 
> > On 05-07-2022 21:02, Saurabh Sengar wrote:
> > > There can be scenarios where packets in ring buffer are continuously
> > > getting queued from upper layer and dequeued from storvsc interrupt
> > > handler, such scenarios can hold the foreach_vmbus_pkt loop (which is
> > > executing as a tasklet) for a long duration. Theoretically its possible
> > > that this loop executes forever. Add a condition to limit execution of
> > > this tasklet for finite amount of time to avoid such hazardous scenarios.
> 
> Does this really make much difference?
> 
> I'd guess the tasklet gets immediately rescheduled as soon as
> the upper layer queues another packet?
> 
> Or do you get a different 'bug' where it is never woken again
> because the ring is stuck full?
> 
> 	David

My initial understanding was that staying in a tasklet for "too long" may not be a
good idea, however I was not sure what the "too long" value be, thus we are thinking
to provide this parameter as a configurable sysfs entry. I couldn't find any linux
doc justifying this, so please correct me here if I am mistaken.
We have also considered the networking drivers NAPI budget feature while deciding
this approach, where softirq exits once the budget is crossed. This budget feature
act as a performance tuning parameter for driver, and also can help with ring buffer
overflow. I believe similar reasons are true for scsi softirq as well.

NAPI budget Ref : https://wiki.linuxfoundation.org/networking/napi.

- Saurabh


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