Re: [RFC][PATCH] irq_work

Peter Zijlstra
2010-06-25T10:39:45Z

On Thu, 2010-06-24 at 14:38 +0200, Andi Kleen wrote:
> The sleepable
> soft irq would have avoided that (that's not a show stopper)

I'm still not convinced sleepable softirq is a workable thing.

Softirqs:
A) are non-preemptible
B) are per-cpu because of A
C) can be ran from ksoftirqd context
D) generic kernel infrastructure with identical semantics on all archs

If you were to make something like a sleepable softirq, you'd loose A
(per definition), B (sleepable implies migratable to cpus_allowed) and
possibly D (unless you want to touch all architectures).

Now from your 'requirements':

> I have one case that needs to sleep (but only when interrupting user code)

> TIF works for user space, but it's a bit ugly because it requires adding
> more data to the task_struct because CPUs can change.

Which I read as:

1) needs to run in the task context of the task that got 'interrupted'
2) needs to stay on the cpu it got interrupted on.

So C is out of the window too, at which point there's nothing resembling
softirqs left.

To boot, x86_64 runs softirqs from the hardirq stack:

/* Call softirq on interrupt stack. Interrupts are off. */
ENTRY(call_softirq)
CFI_STARTPROC
push %rbp
CFI_ADJUST_CFA_OFFSET 8
CFI_REL_OFFSET rbp,0
mov %rsp,%rbp
CFI_DEF_CFA_REGISTER rbp
incl PER_CPU_VAR(irq_count)
cmove PER_CPU_VAR(irq_stack_ptr),%rsp
push %rbp # backlink for old unwinder
call __do_softirq
leaveq
CFI_DEF_CFA_REGISTER rsp
CFI_ADJUST_CFA_OFFSET -8
decl PER_CPU_VAR(irq_count)
ret
CFI_ENDPROC
END(call_softirq)

Also, -rt has something that could be considered sleepable softirqs,
although we call them preemptible softirqs. It runs all softirqs from
cpu bound kthreads, which again doesn't match your requirements.

So no, I don't think your idea of sleepable softirqs is sound.