linux-kernel - Re: [PATCH RFC nohz_full 6/7] nohz_full: Add full-system-idle state machine

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Date:	Wed, 24 Jul 2013 20:09:04 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, sbw@....edu
Subject: Re: [PATCH RFC nohz_full 6/7] nohz_full: Add full-system-idle state
 machine

On Thu, Jul 18, 2013 at 10:06:25PM -0700, Paul E. McKenney wrote:
> > Lets summarize the last sequence, the following happens ordered by time:
> > 
> >         CPU 0                          CPU 1
> > 
> >      cmpxchg(&full_sysidle_state,
> >              RCU_SYSIDLE_SHORT,
> >              RCU_SYSIDLE_LONG);
> > 
> >      smp_mb() //cmpxchg
> > 
> >      atomic_read(rdtp(1)->dynticks_idle)
> > 
> >      //CPU 0 goes to sleep
> >                                        //CPU 1 wakes up
> >                                        atomic_inc(rdtp(1)->dynticks_idle)
> > 
> >                                        smp_mb()
> > 
> >                                        ACCESS_ONCE(full_sysidle_state)
> > 
> > 
> > Are you suggesting that because the CPU 1 executes its atomic_inc() _after_ (in terms
> > of absolute time) the atomic_read of CPU 0, the ordering settled in both sides guarantees
> > that the value read from CPU 1 is the one from the cmpxchg that precedes the atomic_read,
> > or FULL or FULL_NOTED that happen later.
> > 
> > If so that's a big lesson for me.                                     
> 
> It is not absolute time that matters.  Instead, it is the fact that
> CPU 0, when reading from ->dynticks_idle, read the old value before the
> atomic_inc().  Therefore, anything CPU 0 did before that memory barrier
> preceding CPU 0's read must come before anything CPU 1 did after that
> memory barrier following the atomic_inc().  For this to work, there
> must be some access to the same variable on each CPU.

Aren't we in the following situation?

    CPU 0                          CPU 1

    STORE A                        STORE B
    LOAD B                         LOAD A


If so and referring to your perfbook, this is an "ears to mouth" situation.
And it seems to describe there is no strong guarantee in that situation.

> 
> Or, if you must think in terms of time, you need a separate independent
> timeline for each variable, with no direct mapping from one timeline to
> another, except resulting from memory-barrier interactions.
> 
> 							Thanx, Paul
> 
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