linux-kernel - Re: [git pull] scheduler fixes

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Date:	Sun, 1 Feb 2009 00:43:49 +0300
From:	Alexey Zaytsev <alexey.zaytsev@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [git pull] scheduler fixes

On Sat, Jan 31, 2009 at 20:54, Ingo Molnar <mingo@...e.hu> wrote:
>
> * Alexey Zaytsev <alexey.zaytsev@...il.com> wrote:
>
>> On Sat, Jan 31, 2009 at 20:23, Peter Zijlstra <peterz@...radead.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?
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