On Mon, Jul 29, 2013 at 04:19:48PM +0800, Lai Jiangshan wrote:
> On 07/27/2013 07:19 AM, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
> >
> > This commit adds the state machine that takes the per-CPU idle data
> > as input and produces a full-system-idle indication as output. This
> > state machine is driven out of RCU's quiescent-state-forcing
> > mechanism, which invokes rcu_sysidle_check_cpu() to collect per-CPU
> > idle state and then rcu_sysidle_report() to drive the state machine.
> >
> > The full-system-idle state is sampled using rcu_sys_is_idle(), which
> > also drives the state machine if RCU is idle (and does so by forcing
> > RCU to become non-idle). This function returns true if all but the
> > timekeeping CPU (tick_do_timer_cpu) are idle and have been idle long
> > enough to avoid memory contention on the full_sysidle_state state
> > variable. The rcu_sysidle_force_exit() may be called externally
> > to reset the state machine back into non-idle state.
> >
> > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> > Cc: Frederic Weisbecker <fweis...@gmail.com>
> > Cc: Steven Rostedt <rost...@goodmis.org>
> > ---
> > include/linux/rcupdate.h | 18 +++
> > kernel/rcutree.c | 16 ++-
> > kernel/rcutree.h | 5 +
> > kernel/rcutree_plugin.h | 284
> > ++++++++++++++++++++++++++++++++++++++++++++++-
> > 4 files changed, 316 insertions(+), 7 deletions(-)
> >
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 48f1ef9..1aa8d8c 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -1011,4 +1011,22 @@ static inline bool rcu_is_nocb_cpu(int cpu) { return
> > false; }
> > #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
> >
> >
> > +/* Only for use by adaptive-ticks code. */
> > +#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
> > +extern bool rcu_sys_is_idle(void);
> > +extern void rcu_sysidle_force_exit(void);
> > +#else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
> > +
> > +static inline bool rcu_sys_is_idle(void)
> > +{
> > + return false;
> > +}
> > +
> > +static inline void rcu_sysidle_force_exit(void)
> > +{
> > +}
> > +
> > +#endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
> > +
> > +
> > #endif /* __LINUX_RCUPDATE_H */
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 725524e..aa6d96e 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -718,6 +718,7 @@ static int dyntick_save_progress_counter(struct
> > rcu_data *rdp,
> > bool *isidle, unsigned long *maxj)
> > {
> > rdp->dynticks_snap = atomic_add_return(0, &rdp->dynticks->dynticks);
> > + rcu_sysidle_check_cpu(rdp, isidle, maxj);
> > return (rdp->dynticks_snap & 0x1) == 0;
> > }
> >
> > @@ -1356,11 +1357,17 @@ int rcu_gp_fqs(struct rcu_state *rsp, int
> > fqs_state_in)
> > rsp->n_force_qs++;
> > if (fqs_state == RCU_SAVE_DYNTICK) {
> > /* Collect dyntick-idle snapshots. */
> > + if (is_sysidle_rcu_state(rsp)) {
> > + isidle = 1;
>
> isidle = true;
> the type of isidle is bool
>
> > + maxj = jiffies - ULONG_MAX / 4;
> > + }
> > force_qs_rnp(rsp, dyntick_save_progress_counter,
> > &isidle, &maxj);
> > + rcu_sysidle_report_gp(rsp, isidle, maxj);
> > fqs_state = RCU_FORCE_QS;
> > } else {
> > /* Handle dyntick-idle and offline CPUs. */
> > + isidle = 0;
>
> isidle = false;
>
> > force_qs_rnp(rsp, rcu_implicit_dynticks_qs, &isidle, &maxj);
> > }
> > /* Clear flag to prevent immediate re-entry. */
> > @@ -2087,9 +2094,12 @@ static void force_qs_rnp(struct rcu_state *rsp,
> > cpu = rnp->grplo;
> > bit = 1;
> > for (; cpu <= rnp->grphi; cpu++, bit <<= 1) {
> > - if ((rnp->qsmask & bit) != 0 &&
> > - f(per_cpu_ptr(rsp->rda, cpu), isidle, maxj))
> > - mask |= bit;
> > + if ((rnp->qsmask & bit) != 0) {
> > + if ((rnp->qsmaskinit & bit) != 0)
> > + *isidle = 0;
>
> *isidle = false
All good catches, fixed.
> > + if (f(per_cpu_ptr(rsp->rda, cpu), isidle, maxj))
> > + mask |= bit;
> > + }
> > }
> > if (mask != 0) {
> >
> > diff --git a/kernel/rcutree.h b/kernel/rcutree.h
> > index 1895043..e0de5dc 100644
> > --- a/kernel/rcutree.h
> > +++ b/kernel/rcutree.h
> > @@ -555,6 +555,11 @@ static void rcu_kick_nohz_cpu(int cpu);
> > static bool init_nocb_callback_list(struct rcu_data *rdp);
> > static void rcu_sysidle_enter(struct rcu_dynticks *rdtp, int irq);
> > static void rcu_sysidle_exit(struct rcu_dynticks *rdtp, int irq);
> > +static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
> > + unsigned long *maxj);
> > +static bool is_sysidle_rcu_state(struct rcu_state *rsp);
> > +static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,
> > + unsigned long maxj);
> > static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp);
> >
> > #endif /* #ifndef RCU_TREE_NONCORE */
> > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> > index 3edae39..ff84bed 100644
> > --- a/kernel/rcutree_plugin.h
> > +++ b/kernel/rcutree_plugin.h
> > @@ -28,7 +28,7 @@
> > #include <linux/gfp.h>
> > #include <linux/oom.h>
> > #include <linux/smpboot.h>
> > -#include <linux/tick.h>
> > +#include "time/tick-internal.h"
> >
> > #define RCU_KTHREAD_PRIO 1
> >
> > @@ -2395,12 +2395,12 @@ static void rcu_kick_nohz_cpu(int cpu)
> > * most active flavor of RCU.
> > */
> > #ifdef CONFIG_PREEMPT_RCU
> > -static struct rcu_state __maybe_unused *rcu_sysidle_state =
> > &rcu_preempt_state;
> > +static struct rcu_state *rcu_sysidle_state = &rcu_preempt_state;
> > #else /* #ifdef CONFIG_PREEMPT_RCU */
> > -static struct rcu_state __maybe_unused *rcu_sysidle_state =
> > &rcu_sched_state;
> > +static struct rcu_state *rcu_sysidle_state = &rcu_sched_state;
> > #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
> >
> > -static int __maybe_unused full_sysidle_state; /* Current system-idle
> > state. */
> > +static int full_sysidle_state; /* Current system-idle state. */
> > #define RCU_SYSIDLE_NOT 0 /* Some CPU is not idle. */
> > #define RCU_SYSIDLE_SHORT 1 /* All CPUs idle for brief period. */
> > #define RCU_SYSIDLE_LONG 2 /* All CPUs idle for long enough. */
> > @@ -2444,6 +2444,38 @@ static void rcu_sysidle_enter(struct rcu_dynticks
> > *rdtp, int irq)
> > }
> >
> > /*
> > + * 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. */
>
> why we need this mb()?
> which mb() is paired with this?
This one if for the case where we didn't do the cmpxchg() above. The idea
is that if we saw that oldstate was RCU_SYSIDLE_SHORT or RCU_SYSIDLE_NOT,
that anyone attempting to update the value who has seen any of our
later non-idle activity also "sees" the load from full_sysidle_state,
which reduces the state space a bit. The barrier pairs with the various
cmpxchg() operations that advance full_sysidle_state.
Thanx, Paul
> > +}
> > +
> > +/*
> > * 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.
> > @@ -2476,6 +2508,235 @@ 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;
> > + 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 counter and check. */
> > + cur = atomic_read(&rdtp->dynticks_idle);
> > + if (cur & 0x1) {
> > + *isidle = 0; /* We are not idle! */
>
> *isidle = false;
>
> And other places which use "isidle".
>
> > + 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;
> > +}
> > +
> > +/*
> > + * Is this the flavor of RCU that is handling full-system idle?
> > + */
> > +static bool is_sysidle_rcu_state(struct rcu_state *rsp)
> > +{
> > + return rsp == rcu_sysidle_state;
> > +}
> > +
> > +/*
> > + * Return a delay in jiffies based on the number of CPUs, rcu_node
> > + * leaf fanout, and jiffies tick rate. The idea is to allow larger
> > + * systems more time to transition to full-idle state in order to
> > + * avoid the cache thrashing that otherwise occur on the state variable.
> > + * Really small systems (less than a couple of tens of CPUs) should
> > + * instead use a single global atomically incremented counter, and later
> > + * versions of this will automatically reconfigure themselves accordingly.
> > + */
> > +static unsigned long rcu_sysidle_delay(void)
> > +{
> > + if (nr_cpu_ids <= RCU_SYSIDLE_SMALL)
> > + return 0;
> > + return DIV_ROUND_UP(nr_cpu_ids * HZ, rcu_fanout_leaf * 1000);
> > +}
> > +
> > +/*
> > + * Advance the full-system-idle state. This is invoked when all of
> > + * the non-timekeeping CPUs are idle.
> > + */
> > +static void rcu_sysidle(unsigned long j)
> > +{
> > + /* Check the current state. */
> > + switch (ACCESS_ONCE(full_sysidle_state)) {
> > + case RCU_SYSIDLE_NOT:
> > +
> > + /* First time all are idle, so note a short idle period. */
> > + ACCESS_ONCE(full_sysidle_state) = RCU_SYSIDLE_SHORT;
> > + break;
> > +
> > + case RCU_SYSIDLE_SHORT:
> > +
> > + /*
> > + * Idle for a bit, time to advance to next state?
> > + * cmpxchg failure means race with non-idle, let them win.
> > + */
> > + if (ULONG_CMP_GE(jiffies, j + rcu_sysidle_delay()))
> > + (void)cmpxchg(&full_sysidle_state,
> > + RCU_SYSIDLE_SHORT, RCU_SYSIDLE_LONG);
> > + break;
>
> I don't think it will race with any body.
> I think ACCESS_ONCE(full_sysidle_state) = RCU_SYSIDLE_LONG is enough.
>
> note:
> rcu_sysidle_force_exit() doesn't change full_sysidle_state if it is
> RCU_SYSIDLE_SHORT.
>
> > +
> > + case RCU_SYSIDLE_LONG:
> > +
> > + /*
> > + * Do an additional check pass before advancing to full.
> > + * cmpxchg failure means race with non-idle, let them win.
> > + */
> > + if (ULONG_CMP_GE(jiffies, j + rcu_sysidle_delay()))
> > + (void)cmpxchg(&full_sysidle_state,
> > + RCU_SYSIDLE_LONG, RCU_SYSIDLE_FULL);
> > + break;
> > +
> > + default:
> > + break;
> > + }
> > +}
> > +
> > +/*
> > + * Found a non-idle non-timekeeping CPU, so kick the system-idle state
> > + * back to the beginning.
> > + */
> > +static void rcu_sysidle_cancel(void)
> > +{
> > + smp_mb();
> > + ACCESS_ONCE(full_sysidle_state) = RCU_SYSIDLE_NOT;
> > +}
> > +
> > +/*
> > + * Update the sysidle state based on the results of a force-quiescent-state
> > + * scan of the CPUs' dyntick-idle state.
> > + */
> > +static void rcu_sysidle_report(struct rcu_state *rsp, int isidle,
> > + unsigned long maxj, bool gpkt)
> > +{
> > + if (rsp != rcu_sysidle_state)
> > + return; /* Wrong flavor, ignore. */
> > + if (isidle) {
> > + if (gpkt && nr_cpu_ids > RCU_SYSIDLE_SMALL)
> > + rcu_sysidle(maxj); /* More idle! */
> > + } else {
> > + rcu_sysidle_cancel(); /* Idle is over. */
> > + }
> > +}
>
> "gpkt" is always equal to "nr_cpu_ids > RCU_SYSIDLE_SMALL",
>
> so we can remove "gpkt" argument and rcu_sysidle_report_gp().
>
>
> > +
> > +static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,
> > + unsigned long maxj)
> > +{
> > + rcu_sysidle_report(rsp, isidle, maxj, true);
> > +}
> > +
> > +/* Callback and function for forcing an RCU grace period. */
> > +struct rcu_sysidle_head {
> > + struct rcu_head rh;
> > + int inuse;
> > +};
> > +
> > +static void rcu_sysidle_cb(struct rcu_head *rhp)
> > +{
> > + struct rcu_sysidle_head *rshp;
> > +
> > + smp_mb(); /* grace period precedes setting inuse. */
>
> Why we need this mb()?
>
> > + rshp = container_of(rhp, struct rcu_sysidle_head, rh);
> > + ACCESS_ONCE(rshp->inuse) = 0;
> > +}
> > +
> > +/*
> > + * Check to see if the system is fully idle, other than the timekeeping
> > CPU.
> > + * The caller must have disabled interrupts.
> > + */
> > +bool rcu_sys_is_idle(void)
> > +{
> > + static struct rcu_sysidle_head rsh;
> > + int rss = ACCESS_ONCE(full_sysidle_state);
> > +
> > + if (WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu))
> > + return false;
> > +
> > + /* Handle small-system case by doing a full scan of CPUs. */
> > + if (nr_cpu_ids <= RCU_SYSIDLE_SMALL) {
> > + int oldrss = rss - 1;
> > +
> > + /*
> > + * One pass to advance to each state up to _FULL.
> > + * Give up if any pass fails to advance the state.
> > + */
> > + while (rss < RCU_SYSIDLE_FULL && oldrss < rss) {
> > + int cpu;
> > + bool isidle = true;
> > + unsigned long maxj = jiffies - ULONG_MAX / 4;
> > + struct rcu_data *rdp;
> > +
> > + /* Scan all the CPUs looking for nonidle CPUs. */
> > + for_each_possible_cpu(cpu) {
> > + rdp = per_cpu_ptr(rcu_sysidle_state->rda, cpu);
> > + rcu_sysidle_check_cpu(rdp, &isidle, &maxj);
> > + if (!isidle)
> > + break;
> > + }
> > + rcu_sysidle_report(rcu_sysidle_state,
> > + isidle, maxj, false);
> > + oldrss = rss;
> > + rss = ACCESS_ONCE(full_sysidle_state);
> > + }
> > + }
> > +
> > + /* If this is the first observation of an idle period, record it. */
> > + if (rss == RCU_SYSIDLE_FULL) {
> > + rss = cmpxchg(&full_sysidle_state,
> > + RCU_SYSIDLE_FULL, RCU_SYSIDLE_FULL_NOTED);
> > + return rss == RCU_SYSIDLE_FULL;
> > + }
> > +
> > + smp_mb(); /* ensure rss load happens before later caller actions. */
> > +
> > + /* If already fully idle, tell the caller (in case of races). */
> > + if (rss == RCU_SYSIDLE_FULL_NOTED)
> > + return true;
> > +
> > + /*
> > + * If we aren't there yet, and a grace period is not in flight,
> > + * initiate a grace period. Either way, tell the caller that
> > + * we are not there yet.
> > + */
> > + if (nr_cpu_ids > RCU_SYSIDLE_SMALL &&
> > + !rcu_gp_in_progress(rcu_sysidle_state) &&
> > + !rsh.inuse && xchg(&rsh.inuse, 1) == 0)
>
> why need to use xchg()? Who will it race with?
>
> > + call_rcu(&rsh.rh, rcu_sysidle_cb);
> > + return false;
> > }
> >
> > /*
> > @@ -2496,6 +2757,21 @@ static void rcu_sysidle_exit(struct rcu_dynticks
> > *rdtp, int irq)
> > {
> > }
> >
> > +static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
> > + unsigned long *maxj)
> > +{
> > +}
> > +
> > +static bool is_sysidle_rcu_state(struct rcu_state *rsp)
> > +{
> > + return false;
> > +}
> > +
> > +static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,
> > + unsigned long maxj)
> > +{
> > +}
> > +
> > static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
> > {
> > }
>
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