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

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Date:	Sat, 31 Jan 2009 23:15:14 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Alexey Zaytsev <alexey.zaytsev@...il.com>
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


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

> 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?

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
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