Re: [git pull] scheduler fixes

Alexey Zaytsev
2009-01-31T21:44:03Z

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?