On Wed, 10 Feb 2021, Song Bao Hua (Barry Song) wrote:
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/
The article you cited does not refute what I said about prioritized
interrupts.
The article is about eliminating the distinction between fast and slow
interrupt handlers.
The article says that some developers convinced Linus that, although
minimal interrupt latency is desirable, is isn't strictly necessary.
The article also warns of stack overflow from arbitrarily deep slow
interrupt nesting, but that's not what m68k does.
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:
I'm not going to guess whether other platforms might be affected -- you're
supporting this patch so you will have to show that it is correct.
"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.
Interrupt handlers on m68k do not enable interrupts. If they did, you
would see that warning fire. It doesn't fire. Try it.
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
