RE: [Linuxarm] Re: [PATCH for-next 00/32] spin lock usage optimization for SCSI drivers

Finn Thain
2021-02-10T02:00:25Z

On Tue, 9 Feb 2021, Song Bao Hua (Barry Song) wrote:

> > On Tue, 9 Feb 2021, Song Bao Hua (Barry Song) wrote:
> >
> > > > On Sun, 7 Feb 2021, Xiaofei Tan wrote:
> > > >
> > > > > Replace spin_lock_irqsave with spin_lock in hard IRQ of SCSI
> > > > > drivers. There are no function changes, but may speed up if
> > > > > interrupt happen too often.
> > > >
> > > > This change doesn't necessarily work on platforms that support
> > > > nested interrupts.
> > > >
> > > > Were you able to measure any benefit from this change on some
> > > > other platform?
> > >
> > > I think the code disabling irq in hardIRQ is simply wrong. Since
> > > this commit
> > >
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e58aa3d2d0cc
> > > genirq: Run irq handlers with interrupts disabled
> > >
> > > interrupt handlers are definitely running in a irq-disabled context
> > > unless irq handlers enable them explicitly in the handler to permit
> > > other interrupts.
> > >
> >
> > Repeating the same claim does not somehow make it true.
>
> Sorry for I didn't realize xiaofei had replied.
>

I was referring to the claim in patch 00/32, i.e. that interrupt handlers
only run when irqs are disabled.

> > If you put your claim to the test, you'll see that that interrupts are
> > not disabled on m68k when interrupt handlers execute.
>
> Sounds like an implementation issue of m68k since IRQF_DISABLED has been
> totally removed.
>

It's true that IRQF_DISABLED could be used to avoid the need for irq locks
in interrupt handlers. So, if you want to remove irq locks from interrupt
handlers, today you can't use IRQF_DISABLED to help you. So what?

> >
> > The Interrupt Priority Level (IPL) can prevent any given irq handler
> > from being re-entered, but an irq with a higher priority level may be
> > handled during execution of a lower priority irq handler.
> >
>
> We used to have IRQF_DISABLED to support so-called "fast interrupt" to
> avoid this.
>
> But the concept has been totally removed. That is interesting if m68k
> still has this issue.
>

Prioritized interrupts are beneficial. Why would you want to avoid them?

Moreover, there's no reason to believe that m68k is the only platform that
supports nested interrupts.

> > sonic_interrupt() uses an irq lock within an interrupt handler to
> > avoid issues relating to this. This kind of locking may be needed in
> > the drivers you are trying to patch. Or it might not. Apparently,
> > no-one has looked.
>
> Thanks
> Barry
>