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SubjectRe: [PATCH RFC 3/9] RCU: Preemptible RCU
On Mon, Sep 10, 2007 at 11:34:12AM -0700, Paul E. McKenney wrote:
> +
> +/*
> + * PREEMPT_RCU data structures.
> + */
> +
> +#define GP_STAGES 4
> +struct rcu_data {
> + spinlock_t lock; /* Protect rcu_data fields. */
> + long completed; /* Number of last completed batch. */
> + int waitlistcount;
> + struct tasklet_struct rcu_tasklet;
> + struct rcu_head *nextlist;
> + struct rcu_head **nexttail;
> + struct rcu_head *waitlist[GP_STAGES];
> + struct rcu_head **waittail[GP_STAGES];
> + struct rcu_head *donelist;
> + struct rcu_head **donetail;
> +#ifdef CONFIG_RCU_TRACE
> + struct rcupreempt_trace trace;
> +#endif /* #ifdef CONFIG_RCU_TRACE */
> +};
> +struct rcu_ctrlblk {
> + spinlock_t fliplock; /* Protect state-machine transitions. */
> + long completed; /* Number of last completed batch. */
> +};
> +static DEFINE_PER_CPU(struct rcu_data, rcu_data);
> +static struct rcu_ctrlblk rcu_ctrlblk = {
> + .fliplock = SPIN_LOCK_UNLOCKED,
> + .completed = 0,
> +};
> +static DEFINE_PER_CPU(int [2], rcu_flipctr) = { 0, 0 };
> +
> +/*
> + * States for rcu_try_flip() and friends.
> + */
> +
> +enum rcu_try_flip_states {
> + rcu_try_flip_idle_state, /* "I" */
> + rcu_try_flip_waitack_state, /* "A" */
> + rcu_try_flip_waitzero_state, /* "Z" */
> + rcu_try_flip_waitmb_state /* "M" */
> +};
> +static enum rcu_try_flip_states rcu_try_flip_state = rcu_try_flip_idle_state;
> +#ifdef CONFIG_RCU_TRACE
> +static char *rcu_try_flip_state_names[] =
> + { "idle", "waitack", "waitzero", "waitmb" };
> +#endif /* #ifdef CONFIG_RCU_TRACE */

[snip]

> +/*
> + * If a global counter flip has occurred since the last time that we
> + * advanced callbacks, advance them. Hardware interrupts must be
> + * disabled when calling this function.
> + */
> +static void __rcu_advance_callbacks(struct rcu_data *rdp)
> +{
> + int cpu;
> + int i;
> + int wlc = 0;
> +
> + if (rdp->completed != rcu_ctrlblk.completed) {
> + if (rdp->waitlist[GP_STAGES - 1] != NULL) {
> + *rdp->donetail = rdp->waitlist[GP_STAGES - 1];
> + rdp->donetail = rdp->waittail[GP_STAGES - 1];
> + RCU_TRACE_RDP(rcupreempt_trace_move2done, rdp);
> + }
> + for (i = GP_STAGES - 2; i >= 0; i--) {
> + if (rdp->waitlist[i] != NULL) {
> + rdp->waitlist[i + 1] = rdp->waitlist[i];
> + rdp->waittail[i + 1] = rdp->waittail[i];
> + wlc++;
> + } else {
> + rdp->waitlist[i + 1] = NULL;
> + rdp->waittail[i + 1] =
> + &rdp->waitlist[i + 1];
> + }
> + }
> + if (rdp->nextlist != NULL) {
> + rdp->waitlist[0] = rdp->nextlist;
> + rdp->waittail[0] = rdp->nexttail;
> + wlc++;
> + rdp->nextlist = NULL;
> + rdp->nexttail = &rdp->nextlist;
> + RCU_TRACE_RDP(rcupreempt_trace_move2wait, rdp);
> + } else {
> + rdp->waitlist[0] = NULL;
> + rdp->waittail[0] = &rdp->waitlist[0];
> + }
> + rdp->waitlistcount = wlc;
> + rdp->completed = rcu_ctrlblk.completed;
> + }
> +
> + /*
> + * Check to see if this CPU needs to report that it has seen
> + * the most recent counter flip, thereby declaring that all
> + * subsequent rcu_read_lock() invocations will respect this flip.
> + */
> +
> + cpu = raw_smp_processor_id();
> + if (per_cpu(rcu_flip_flag, cpu) == rcu_flipped) {
> + smp_mb(); /* Subsequent counter accesses must see new value */
> + per_cpu(rcu_flip_flag, cpu) = rcu_flip_seen;
> + smp_mb(); /* Subsequent RCU read-side critical sections */
> + /* seen -after- acknowledgement. */
> + }
> +}

[snip]

> +/*
> + * Attempt a single flip of the counters. Remember, a single flip does
> + * -not- constitute a grace period. Instead, the interval between
> + * at least three consecutive flips is a grace period.
> + *
> + * If anyone is nuts enough to run this CONFIG_PREEMPT_RCU implementation
> + * on a large SMP, they might want to use a hierarchical organization of
> + * the per-CPU-counter pairs.
> + */
> +static void rcu_try_flip(void)
> +{
> + unsigned long oldirq;
> +
> + RCU_TRACE_ME(rcupreempt_trace_try_flip_1);
> + if (unlikely(!spin_trylock_irqsave(&rcu_ctrlblk.fliplock, oldirq))) {
> + RCU_TRACE_ME(rcupreempt_trace_try_flip_e1);
> + return;
> + }
> +
> + /*
> + * Take the next transition(s) through the RCU grace-period
> + * flip-counter state machine.
> + */
> +
> + switch (rcu_try_flip_state) {
> + case rcu_try_flip_idle_state:
> + if (rcu_try_flip_idle())
> + rcu_try_flip_state = rcu_try_flip_waitack_state;
> + break;
> + case rcu_try_flip_waitack_state:
> + if (rcu_try_flip_waitack())
> + rcu_try_flip_state = rcu_try_flip_waitzero_state;
> + break;
> + case rcu_try_flip_waitzero_state:
> + if (rcu_try_flip_waitzero())
> + rcu_try_flip_state = rcu_try_flip_waitmb_state;
> + break;
> + case rcu_try_flip_waitmb_state:
> + if (rcu_try_flip_waitmb())
> + rcu_try_flip_state = rcu_try_flip_idle_state;
> + }
> + spin_unlock_irqrestore(&rcu_ctrlblk.fliplock, oldirq);
> +}

Paul,

Looking further into this, I still think this is a bit of overkill. We
go through 20 states from call_rcu to list->func().

On call_rcu we put our stuff on the next list. Before we move stuff from
next to wait, we need to go through 4 states. So we have

next -> 4 states -> wait[0] -> 4 states -> wait[1] -> 4 states ->
wait[2] -> 4 states -> wait[3] -> 4 states -> done.

That's 20 states that we go through from the time we add our function to
the list to the time it actually gets called. Do we really need the 4
wait lists?

Seems a bit overkill to me.

What am I missing?

-- Steve

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