Re: [git pull] scheduler fixes

Ingo Molnar
2009-01-31T22:15:41Z

* Alexey Zaytsev <alexey.zaytsev@gmail.com> wrote:

> On Sat, Jan 31, 2009 at 20:54, Ingo Molnar <mingo@elte.hu> wrote:
> >
> > * Alexey Zaytsev <alexey.zaytsev@gmail.com> wrote:
> >
> >> On Sat, Jan 31, 2009 at 20:23, Peter Zijlstra <peterz@infradead.org> wrote:
> >> > On Sat, 2009-01-31 at 18:11 +0100, Peter Zijlstra wrote:
> >> >> > diff --git a/kernel/sched.c b/kernel/sched.c
> >> >> > index 52bbf1c..5686bb5 100644
> >> >> > --- a/kernel/sched.c
> >> >> > +++ b/kernel/sched.c
> >> >> > @@ -4440,7 +4450,7 @@ void __kprobes sub_preempt_count(int val)
> >> >> > /*
> >> >> > * Underflow?
> >> >> > */
> >> >> > - if (DEBUG_LOCKS_WARN_ON(val > preempt_count()))
> >> >> > + if (DEBUG_LOCKS_WARN_ON(val > preempt_count() - (!!kernel_locked())))
> >> >> > return;
> >> >> > /*
> >> >> > * Is the spinlock portion underflowing?
> >> >
> >> > Since the commit msg of 01e3eb8 says:
> >> >
> >> > kernel_locked() is not a valid test in IRQ context (we update the
> >> > BKL's ->lock_depth and the preempt count separately and non-atomicalyy),
> >> > so we cannot put it into the generic preempt debugging checks which
> >> > can run in IRQ contexts too.
> >> >
> >>
> >> Is the comment actually valid? From arch/x86/kernel/irq_32.c:
> >> do_softirq() actually does
> >> curctx = current_thread_info();
> >> irqctx = softirq_ctx[smp_processor_id()];
> >> irqctx->tinfo.task = curctx->task;
> >>
> >> and so does execute_on_irq_stack().
> >> So kernel_locked() should be valid. It corresponds to the thread
> >> that is being interrupted.
> >>
> >> And answering an earlier question, this happens only on i386 and only
> >> with 4K stacks because x86_64 dosn't have a separate softirq stack,
> >> so the preempt count diring the soft irq is at least IRQ_EXIT_OFFSET.
> >>
> >> (If I understood the things correctly)
> >
> > Correct, on 64-bit we use the hardirq stack for softirqs too:
>
> Is there actually a reason for a separate softirq stack on i386-4K, or
> any other architecture?

Yes - it's just 4K so we have separate stacks for hardirqs, softirqs and
for normal syscall activities. On 64-bit the IRQ stack is 16K - so it can
embedd a softirq just fine.

Anyway, changing any detail there is a highly critical change not to be
changed so late in the -rc cycles.

Ingo