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

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Date:   Fri, 8 Jul 2022 03:42:03 -0700
From:   Saurabh Singh Sengar <ssengar@...ux.microsoft.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Praveen Kumar' <kumarpraveen@...ux.microsoft.com>,
        "kys@...rosoft.com" <kys@...rosoft.com>,
        "haiyangz@...rosoft.com" <haiyangz@...rosoft.com>,
        "sthemmin@...rosoft.com" <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "decui@...rosoft.com" <decui@...rosoft.com>,
        "jejb@...ux.ibm.com" <jejb@...ux.ibm.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "ssengar@...rosoft.com" <ssengar@...rosoft.com>,
        "mikelley@...rosoft.com" <mikelley@...rosoft.com>
Subject: Re: [PATCH] scsi: storvsc: Prevent running tasklet for long

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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