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

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Date:	Wed, 31 Mar 2010 08:36:56 -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 09:03:39AM -0700, Paul E. McKenney wrote:
> 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?

And I was in fact missing something.  The rcu_preempt_note_context_switch()
function currently combines some work that needs to happen only at
context-switch time with work that needs to happen all the time.

At first glance, it appears that the big "if" statement in
rcu_preempt_note_context_switch() need only happen for context switches.
The remaining lines must happen unconditionally for context switches,
and should be executed from rcu_check_callbacks() only if the current
CPU is not in an RCU read-side critical section.

							Thanx, Paul
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