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SubjectRe: [patch] severe softirq handling performance bug, fix, 2.4.5

Ingo Molnar writes:
> (unlike bottom halves, soft-IRQs do not preempt kernel code.)
...

Since when do we have this rule? :-)

> the two error cases are:
>
> #1 hard-IRQ interrupts user-space code, activates softirq, and returns to
> user-space code
>
> #2 hard-IRQ interrupts the idle task, activates softirq and returns to
> the idle task.
>
> category #1 is easy to fix, in entry.S we have to check active softirqs
> not only the exception and ret-from-syscall cases, but also in the
> IRQ-ret-to-userspace case.
>
> category #2 is subtle, because the idle process is kernel code, so
> returning to it we do not execute active softirqs. The main two types of
> idle handlers both had a window do 'miss' softirq execution:

Ingo, I don't think this is the fix.

You should check Softirqs on return from every single IRQ.
In do_softirq() it will make sure that we won't run softirqs
while already doing so or being already nested in a hard-IRQ.

Every port works this way, I don't know where you got this "soft-IRQs
cannot run when returning to kernel code" rule, it simply doesn't
exist.

And looking at the x86 code, I don't even understand how your fixes
can make a difference, what about the do_softirq() call in
arch/i386/kernel/irq.c:do_IRQ()??? That should be taking care of all
these "error cases" right?

Later,
David S. Miller
davem@redhat.com
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