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Date: Mon, 1 Jul 2013 11:10:40 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Frederic Weisbecker <fweisbec@...il.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 v2 6/7] nohz_full: Add full-system-idle
state machine
On Mon, Jul 01, 2013 at 06:35:31PM +0200, Frederic Weisbecker wrote:
> On Fri, Jun 28, 2013 at 01:10:21PM -0700, Paul E. McKenney wrote:
> > /*
> > + * Unconditionally force exit from full system-idle state. This is
> > + * invoked when a normal CPU exits idle, but must be called separately
> > + * for the timekeeping CPU (tick_do_timer_cpu). The reason for this
> > + * is that the timekeeping CPU is permitted to take scheduling-clock
> > + * interrupts while the system is in system-idle state, and of course
> > + * rcu_sysidle_exit() has no way of distinguishing a scheduling-clock
> > + * interrupt from any other type of interrupt.
> > + */
> > +void rcu_sysidle_force_exit(void)
> > +{
> > + int oldstate = ACCESS_ONCE(full_sysidle_state);
> > + int newoldstate;
> > +
> > + /*
> > + * Each pass through the following loop attempts to exit full
> > + * system-idle state. If contention proves to be a problem,
> > + * a trylock-based contention tree could be used here.
> > + */
> > + while (oldstate > RCU_SYSIDLE_SHORT) {
> > + newoldstate = cmpxchg(&full_sysidle_state,
> > + oldstate, RCU_SYSIDLE_NOT);
> > + if (oldstate == newoldstate &&
> > + oldstate == RCU_SYSIDLE_FULL_NOTED) {
> > + rcu_kick_nohz_cpu(tick_do_timer_cpu);
> > + return; /* We cleared it, done! */
> > + }
> > + oldstate = newoldstate;
> > + }
> > + smp_mb(); /* Order initial oldstate fetch vs. later non-idle work. */
> > +}
> > +
> > +/*
> > * Invoked to note entry to irq or task transition from idle. Note that
> > * usermode execution does -not- count as idle here! The caller must
> > * have disabled interrupts.
> > @@ -2474,6 +2506,214 @@ static void rcu_sysidle_exit(struct rcu_dynticks *rdtp, int irq)
> > atomic_inc(&rdtp->dynticks_idle);
> > smp_mb__after_atomic_inc();
> > WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks_idle) & 0x1));
> > +
> > + /*
> > + * If we are the timekeeping CPU, we are permitted to be non-idle
> > + * during a system-idle state. This must be the case, because
> > + * the timekeeping CPU has to take scheduling-clock interrupts
> > + * during the time that the system is transitioning to full
> > + * system-idle state. This means that the timekeeping CPU must
> > + * invoke rcu_sysidle_force_exit() directly if it does anything
> > + * more than take a scheduling-clock interrupt.
> > + */
> > + if (smp_processor_id() == tick_do_timer_cpu)
> > + return;
> > +
> > + /* Update system-idle state: We are clearly no longer fully idle! */
> > + rcu_sysidle_force_exit();
> > +}
> > +
> > +/*
> > + * Check to see if the current CPU is idle. Note that usermode execution
> > + * does not count as idle. The caller must have disabled interrupts.
> > + */
> > +static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
> > + unsigned long *maxj)
> > +{
> > + int cur;
> > + int curnmi;
> > + unsigned long j;
> > + struct rcu_dynticks *rdtp = rdp->dynticks;
> > +
> > + /*
> > + * If some other CPU has already reported non-idle, if this is
> > + * not the flavor of RCU that tracks sysidle state, or if this
> > + * is an offline or the timekeeping CPU, nothing to do.
> > + */
> > + if (!*isidle || rdp->rsp != rcu_sysidle_state ||
> > + cpu_is_offline(rdp->cpu) || rdp->cpu == tick_do_timer_cpu)
> > + return;
> > + /* WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu); */
> > +
> > + /*
> > + * Pick up current idle and NMI-nesting counters, check. We check
> > + * for NMIs using RCU's main ->dynticks counter. This works because
> > + * any time ->dynticks has its low bit set, ->dynticks_idle will
> > + * too -- unless the only reason that ->dynticks's low bit is set
> > + * is due to an NMI from idle. Which is exactly the case we need
> > + * to account for.
> > + */
> > + cur = atomic_read(&rdtp->dynticks_idle);
> > + curnmi = atomic_read(&rdtp->dynticks);
> > + if ((cur & 0x1) || (curnmi & 0x1)) {
>
> I think you wanted to ignore NMIs this time because they don't read walltime?
>
> By the way they can still read jiffies, but unlike irq_enter(), nmi_enter()
> don't catch up with missing jiffies update. So the behaviour doesn't change
> compared to !NO_HZ_FULL.
You are right, I missed this when ripping out NMI handling. Will fix!
> > + *isidle = 0; /* We are not idle! */
> > + return;
> > + }
> > + smp_mb(); /* Read counters before timestamps. */
> > +
> > + /* Pick up timestamps. */
> > + j = ACCESS_ONCE(rdtp->dynticks_idle_jiffies);
> > + /* If this CPU entered idle more recently, update maxj timestamp. */
> > + if (ULONG_CMP_LT(*maxj, j))
> > + *maxj = j;
>
> So I'm a bit confused with the ordering so I'm probably going to ask a silly question.
>
> What makes sure that we are not reading a stale value of rdtp->dynticks_idle
> in the following scenario:
>
> CPU 0 CPU 1
>
> //CPU 1 idle
> //rdtp(1)->dynticks_idle == 0
>
> sysidle_check_cpu(CPU 1) {
> rdtp(1)->dynticks_idle == 0
> }
> cmpxchg(full_sysidle_state,
> ...RCU_SYSIDLE_SHORT)
> rcu_irq_exit() {
rcu_irq_enter(), right?
> rdtp(1)->dynticks_idle = 1
> smp_mb()
> rcu_sysidle_force_exit() {
> full_sysidle_state == RCU_SYSIDLE_SHORT
> // no cmpxchg
> smp_mb()
> ...
>
> [1]
> sysidle_check_cpu(CPU 1) {
> rdtp(1)->dynticks_idle == 0
> }
>
> cmpxchg(RCU_SYSIDLE_FULL, ...)
You know, I had an RCU_SYSIDLE_LONG state for this purpose, but later
convinced myself that I didn't need it. :-/
Time to go put it back in, and thank you for your careful review!
Thanx, Paul
> [2]
> sysidle_check_cpu(CPU 1) {
> rdtp(1)->dynticks_idle == 0
> }
>
> cmpxchg(RCU_SYSIDLE_FULL_NOTED, ...)
>
>
> I mean in [1] and [2] I can't see something in the ordering that guarantees that we see
> the new value rdtp(1)->dynticks_idle == 1.
>
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