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: Thursday, February 11, 2021 2:12 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 Wed, 10 Feb 2021, Song Bao Hua (Barry Song) wrote:
> 
> > > On Wed, 10 Feb 2021, Song Bao Hua (Barry Song) wrote:
> > >
> > > > > On Wed, 10 Feb 2021, Song Bao Hua (Barry Song) wrote:
> > > > >
> > > > > > > There is no warning from m68k builds. That's because
> > > > > > > arch_irqs_disabled() returns true when the IPL is non-zero.
> > > > > >
> > > > > > So for m68k, the case is arch_irqs_disabled() is true, but
> > > > > > interrupts can still come?
> > > > > >
> > > > > > Then it seems it is very confusing. If prioritized interrupts
> > > > > > can still come while arch_irqs_disabled() is true,
> > > > >
> > > > > Yes, on m68k CPUs, an IRQ having a priority level higher than the
> > > > > present priority mask will get serviced.
> > > > >
> > > > > Non-Maskable Interrupt (NMI) is not subject to this rule and gets
> > > > > serviced regardless.
> > > > >
> > > > > > how could spin_lock_irqsave() block the prioritized interrupts?
> > > > >
> > > > > It raises the the mask level to 7. Again, please see
> > > > > arch/m68k/include/asm/irqflags.h
> > > >
> > > > Hi Finn,
> > > > Thanks for your explanation again.
> > > >
> > > > TBH, that is why m68k is so confusing. irqs_disabled() on m68k
> > > > should just reflect the status of all interrupts have been disabled
> > > > except NMI.
> > > >
> > > > irqs_disabled() should be consistent with the calling of APIs such
> > > > as local_irq_disable, local_irq_save, spin_lock_irqsave etc.
> > > >
> > >
> > > When irqs_disabled() returns true, we cannot infer that
> > > arch_local_irq_disable() was called. But I have not yet found driver
> > > code or core kernel code attempting that inference.
> > >
> > > > >
> > > > > > Isn't arch_irqs_disabled() a status reflection of irq disable
> > > > > > API?
> > > > > >
> > > > >
> > > > > Why not?
> > > >
> > > > If so, arch_irqs_disabled() should mean all interrupts have been
> > > > masked except NMI as NMI is unmaskable.
> > > >
> > >
> > > Can you support that claim with a reference to core kernel code or
> > > documentation? (If some arch code agrees with you, that's neither here
> > > nor there.)
> >
> > I think those links I share you have supported this. Just you don't
> > believe :-)
> >
> 
> Your links show that the distinction between fast and slow handlers was
> removed. Your links don't support your claim that "arch_irqs_disabled()
> should mean all interrupts have been masked". Where is the code that makes
> that inference? Where is the documentation that supports your claim?

(1)
https://lwn.net/Articles/380931/
Looking at all these worries, one might well wonder if a system which *disabled
interrupts for all handlers* would function well at all. So it is interesting
to note one thing: any system which has the lockdep locking checker enabled
has been running all handlers that way for some years now. Many developers
and testers run lockdep-enabled kernels, and they are available for some of
the more adventurous distributions (Rawhide, for example) as well. So we
have quite a bit of test coverage for this mode of operation already.

(2)
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b738a50a

"We run all handlers *with interrupts disabled* and expect them not to
enable them. Warn when we catch one who does."

(3) 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e58aa3d2d0cc
genirq: Run irq handlers *with interrupts disabled*

Running interrupt handlers with interrupts enabled can cause stack
overflows. That has been observed with multiqueue NICs delivering all
their interrupts to a single core. We might band aid that somehow by
checking the interrupt stacks, but the real safe fix is to *run the irq
handlers with interrupts disabled*.


All these documents say we are running irq handler with interrupts
disabled. but it seems you think high-prio interrupts don't belong
to "interrupts" in those documents :-)

that is why we can't get agreement. I think "interrupts" mean
all except NMI in these documents, but you insist high-prio IRQ
is an exception.

> 
> > >
> > > > >
> > > > > Are all interrupts (including NMI) masked whenever
> > > > > arch_irqs_disabled() returns true on your platforms?
> > > >
> > > > On my platform, once irqs_disabled() is true, all interrupts are
> > > > masked except NMI. NMI just ignore spin_lock_irqsave or
> > > > local_irq_disable.
> > > >
> > > > On ARM64, we also have high-priority interrupts, but they are
> > > > running as PESUDO_NMI:
> > > > https://lwn.net/Articles/755906/
> > > >
> > >
> > > A glance at the ARM GIC specification suggests that your hardware
> > > works much like 68000 hardware.
> > >
> > >    When enabled, a CPU interface takes the highest priority pending
> > >    interrupt for its connected processor and determines whether the
> > >    interrupt has sufficient priority for it to signal the interrupt
> > >    request to the processor. [...]
> > >
> > >    When the processor acknowledges the interrupt at the CPU interface,
> > >    the Distributor changes the status of the interrupt from pending to
> > >    either active, or active and pending. At this point the CPU
> > >    interface can signal another interrupt to the processor, to preempt
> > >    interrupts that are active on the processor. If there is no pending
> > >    interrupt with sufficient priority for signaling to the processor,
> > >    the interface deasserts the interrupt request signal to the
> > >    processor.
> > >
> > > https://developer.arm.com/documentation/ihi0048/b/
> > >
> > > Have you considered that Linux/arm might benefit if it could fully
> > > exploit hardware features already available, such as the interrupt
> > > priority masking feature in the GIC in existing arm systems?
> >
> > I guess no:-) there are only two levels: IRQ and NMI. Injecting a
> > high-prio IRQ level between them makes no sense.
> >
> > To me, arm64's design is quite clear and has no any confusion.
> >
> 
> Are you saying that the ARM64 hardware design is confusing because it
> implements a priority mask, and that's why you had to simplify it with a
> pseudo-nmi scheme in software?

No, I was not saying this. I think both m68k and arm64 have good hardware
design. Just Linux's implementation is running irq-handlers with interrupts
disabled. So ARM64's pseudo-nmi is adapted to Linux better.

> 
> > >
> > > > On m68k, it seems you mean：
> > > > irq_disabled() is true, but high-priority interrupts can still come;
> > > > local_irq_disable() can disable high-priority interrupts, and at that
> > > > time, irq_disabled() is also true.
> > > >
> > > > TBH, this is wrong and confusing on m68k.
> > > >
> > >
> > > Like you, I was surprised when I learned about it. But that doesn't mean
> > > it's wrong. The fact that it works should tell you something.
> > >
> >
> > The fact is that m68k lets arch_irq_disabled() return true to pretend
> > all IRQs are disabled while high-priority IRQ is still open, thus "pass"
> > all sanitizing check in genirq and kernel core.
> >
> 
> The fact is that m68k has arch_irq_disabled() return false when all IRQs
> are enabled. So there is no bug.

But it has arch_irq_disabled() return true while some interrupts(not NMI)
are still open.

> 
> > > Things could always be made simpler. But discarding features isn't
> > > necessarily an improvement.
> >
> > This feature could be used by calling local_irq_enable_in_hardirq() in
> > those IRQ handlers who hope high-priority interrupts to preempt it for a
> > while.
> >
> 
> So, if one handler is sensitive to interrupt latency, all other handlers
> should be modified? I don't think that's workable.

I think we just enable preempt_rt or force threaded_irq, and then improve
the priority of the irq thread who is sensitive to latency. No need to
touch all threads.

I also understand your point, we let one high-prio interrupt preempt
low priority interrupt, then we don't need to change the whole system.
But I think Linux prefers the method of threaded_irq or preempt_rt
for this kind of problems.

> 
> In anycase, what you're describing is a completely different nested
> interrupt scheme that would defeat the priority level mechanism that the
> hardware provides us with.

Yes. Indeed.

> 
> > It shouldn't hide somewhere and make confusion.
> >
> 
> The problem is hiding so well that no-one has found it! I say it doesn't
> exist.

Long long ago(before 2.6.38), we had a kernel supporting IRQF_DISABLED and
nested interrupts were widely supported, but system also ran well in most
cases. That means nested interrupts don't really matter in most cases.
That is why m68k is also running well even though it is still nesting.

> 
> > On the other hand, those who care about realtime should use threaded IRQ
> > and let IRQ threads preempt each other.
> >
> 
> Yes. And those threads also have priority levels.

Finn, I am not a m68k guy, would you help check if this could activate a
warning on m68k. maybe we can discuss this question in genirq maillist from
this warning if you are happy. Thanks very much.

diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
index 7c9d6a2d7e90..b8ca27555c76 100644
--- a/include/linux/hardirq.h
+++ b/include/linux/hardirq.h
@@ -32,6 +32,7 @@ static __always_inline void rcu_irq_enter_check_tick(void)
  */
 #define __irq_enter()                                  \
        do {                                            \
+               WARN_ONCE(in_hardirq() && irqs_disabled(), "nested
interrupts\n"); \
                preempt_count_add(HARDIRQ_OFFSET);      \
                lockdep_hardirq_enter();                \
                account_hardirq_enter(current);         \
@@ -44,6 +45,7 @@ static __always_inline void rcu_irq_enter_check_tick(void)
  */
 #define __irq_enter_raw()                              \
        do {                                            \
+               WARN_ONCE(in_hardirq() && irqs_disabled(), "nested
interrupts\n"); \
                preempt_count_add(HARDIRQ_OFFSET);      \
                lockdep_hardirq_enter();                \
        } while (0)

Best Regards
Barry