linux-kernel - Re: [PATCH] rcu: don't call rcu_preempt_note_context_switch() in rcu_check

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Message-ID: <20100330160339.GB2513@linux.vnet.ibm.com>
Date:	Tue, 30 Mar 2010 09:03:39 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rcu: don't call rcu_preempt_note_context_switch() in
 rcu_check_callbacks()

On Tue, Mar 30, 2010 at 05:43:33PM +0800, Lai Jiangshan wrote:
> Paul E. McKenney wrote:
> > On Mon, Mar 29, 2010 at 10:47:59AM +0800, Lai Jiangshan wrote:
> >> Content-Type: text/plain; charset=UTF-8
> >> Content-Transfer-Encoding: 7bit
> >>
> >>
> >> Even though in user mode or idle mode, rcu_check_callbacks() is not
> >> context switch, so we don't call rcu_preempt_note_context_switch()
> >> in rcu_check_callbacks().
> >>
> >> Though there is no harm that calls rcu_preempt_note_context_switch()
> >> in rcu_check_callbacks(), but it is waste.
> >>
> >> rcu_check_callbacks()
> >>   rcu_sched_qs()
> >>     rcu_preempt_note_context_switch()
> >>        Now, ->rcu_read_lock_nesting == 0, so we just calls
> >>        rcu_preempt_qs(), but, rcu_preempt_check_callbacks()
> >>        will call it again and set the ->rcu_read_unlock_special
> >>        correct again.
> >>
> >> So let rcu_preempt_check_callbacks() handle things for us.
> > 
> > Nice!!!
> > 
> > But how about naming the new function that invokes
> > rcu_preempt_note_context_switch() something like
> > rcu_sched_note_context_switch(), and then leaving the
> > name of rcu_sched_qs() the same (rather than changing
> > it to __rcu_sched_qs(), as below)?
> > 
> > This way, the names clearly call out what the function
> > is doing.
> > 
> 
> If I understand right, it will become this:
> 
> schedule() / run_ksoftirqd() / rcu_needs_cpu()
>   rcu_sched_note_context_switch()
>     rcu_sched_qs()
>     rcu_preempt_note_context_switch()

Wow!!!  That was a scare!!!  I misread "run_ksoftirqd()" as
"do_softirq().  ;-)

And I am not seeing a call to rcu_sched_qs() in rcu_needs_cpu()...

Here is how I believe it needs to go:

	schedule():
		rcu_sched_note_context_switch()
			rcu_sched_qs()
			rcu_preempt_note_context_switch()

	run_ksoftirqd():
		rcu_sched_qs()

	rcu_check_callbacks():
		rcu_sched_qs() [if idle etc.]
		rcu_bh_qs() [if not in softirq]

The reason we don't need rcu_bh_qs() from run_ksoftirqd() is that
__do_softirq() already calls rcu_bh_qs().

Make sense, or am I missing something?

							Thanx, Paul
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