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

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Finn Thain [mailto:fthain@xxxxxxxxxxxxxxxxxxx]
> Sent: Wednesday, February 10, 2021 1:29 PM
> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>
> Cc: tanxiaofei <tanxiaofei@xxxxxxxxxx>; jejb@xxxxxxxxxxxxx;
> martin.petersen@xxxxxxxxxx; linux-scsi@xxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; linuxarm@xxxxxxxxxxxxx;
> linux-m68k@xxxxxxxxxxxxxxx
> Subject: RE: [Linuxarm] Re: [PATCH for-next 00/32] spin lock usage optimization
> for SCSI drivers
> 
> 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?
> 

I doubt this is true as it has been already thought as unnecessary
in Linux:
https://lwn.net/Articles/380931/

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

I doubt that is true as genirq is running understand the consumption
that hardIRQ is running in irq-disabled context:
"We run all handlers with interrupts disabled and expect them not to
enable them. Warn when we catch one who does."
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b738a50a

If it is, m68k is against the assumption of genirq.

> 
> > > 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