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Date: Mon, 1 Jul 2013 18:35:31 +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 v2 6/7] nohz_full: Add full-system-idle
state machine
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.
> + *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() {
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, ...)
[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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