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

[Jeremi Piotrowski]

On Fri, Jul 08, 2022 at 03:42:03AM -0700, Saurabh Singh Sengar wrote:
> 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.

Staying in tasklet for "too long" is only an issue if you have other imporant
work to do. You might be interested in improving fairness/latency of various
kinds of workloads vs. storvsc:
* different storage devices
* storvsc vs. netdevs
* storvsc vs. userspace

Which one are you trying to address? Or is performance the highest concern?
Then you would likely prefer to keep polling as long as possible.

> 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

Reading code here https://elixir.bootlin.com/linux/latest/source/drivers/hv/connection.c#L448,
it looks like if you restricted storvsc to only process a finite amount of
packets per call you would achieve the *budget* effect. You would get called
again if there are more packets to consume and there is already a timeout in
that function. Having two different timeouts at these 2 levels will have weird
interactions.

There is also the irq_poll facility that exists for the block layer and serves
a similar purpose as NAPI. You would need to switch to using HV_CALL_ISR.

Jeremi

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