Messages in this thread | | | Date | Tue, 8 Apr 2008 00:34:49 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH,RFC] Add call_rcu_sched() |
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On Sun, 6 Apr 2008 14:37:19 -0700 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> Hello! > > Third cut of patch to provide the call_rcu_sched(). This is again to > synchronize_sched() as call_rcu() is to synchronize_rcu(). > > Should be fine for experimental use, but not ready for inclusion.
Let me know when to come out of hiding ;)
> Passes multi-hour rcutorture sessions with concurrent CPU hotplugging. > > Fixes since the first version include a bug that could result in > indefinite blocking (spotted by Gautham Shenoy), better resiliency > against CPU-hotplug operations, and other minor fixes. > > Fixes since the second version include reworking grace-period detection > to avoid deadlocks that could happen when running concurrently with > CPU hotplug, adding Mathieu's fix to avoid the softlockup messages, > as well as Mathieu's fix to allow use earlier in boot. > > Known/suspected shortcomings: > > o Only moderately tested on x86-64 and POWER -- a few hours of > rcutorture with concurrent CPU hotplugging. In particular, I > still do not trust the sleep/wakeup logic between call_rcu_sched() > and rcu_sched_grace_period(). > > o Need to add call_rcu_sched() testing to rcutorture. > > o Still needs rcu_barrier_sched() -- intending to incorporate > the version Mathieu provided. > > This patch also fixes a long-standing bug in the earlier preemptable-RCU > implementation of synchronize_rcu() that could result in loss of > concurrent external changes to a task's CPU affinity mask. I still cannot > remember who reported this... > > ... > > +#define call_rcu_sched(head, func) call_rcu(head, func) > + > extern void __rcu_init(void); > +#define rcu_init_sched() do { } while (0)
There are lots of creepy macros-which-probably-dont-need-to-be-macros in here.
> + > +static inline int > +rcu_qsctr_inc_needed_dyntick(int cpu)
Unneeded newline.
> +{ > + long curr; > + long snap; > + struct rcu_dyntick_sched *rdssp = &per_cpu(rcu_dyntick_sched, cpu); > + > + curr = rdssp->dynticks; > + snap = rdssp->sched_dynticks_snap; > + smp_mb(); /* force ordering with cpu entering/leaving dynticks. */ > + > + /* > + * If the CPU remained in dynticks mode for the entire time > + * and didn't take any interrupts, NMIs, SMIs, or whatever, > + * then it cannot be in the middle of an rcu_read_lock(), so > + * the next rcu_read_lock() it executes must use the new value > + * of the counter. Therefore, this CPU has been in a quiescent > + * state the entire time, and we don't need to wait for it. > + */ > + > + if ((curr == snap) && ((curr & 0x1) == 0)) > + return 0; > + > + /* > + * If the CPU passed through or entered a dynticks idle phase with > + * no active irq handlers, then, as above, this CPU has already > + * passed through a quiescent state. > + */ > + > + if ((curr - snap) > 2 || (snap & 0x1) == 0) > + return 0; > + > + /* We need this CPU to go through a quiescent state. */ > + > + return 1; > +}
That's a pretty big inline. It only has a single callsite so the compiler should inline it for us. And if it grows a second callsite, the inlining is probably wrong.
> +static inline int > +rcu_qsctr_inc_needed(int cpu)
Unneeded newline.
> /* > * Get here when RCU is idle. Decide whether we need to > * move out of idle state, and return non-zero if so. > @@ -821,6 +924,13 @@ void rcu_check_callbacks(int cpu, int us > unsigned long flags; > struct rcu_data *rdp = RCU_DATA_CPU(cpu); > > + if (user || > + (idle_cpu(cpu) && !in_softirq() && > + hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
I think this test could do with a bigfatcomment explaining what it is doing.
> + smp_mb(); /* Guard against aggressive schedule(). */ > + rcu_qsctr_inc(cpu); > + } > + > rcu_check_mb(cpu); > if (rcu_ctrlblk.completed == rdp->completed) > rcu_try_flip(); > > ... > > + > + /* > + * The rcu_sched grace-period processing might have bypassed > + * this CPU, given that it was not in the rcu_cpu_online_map > + * when the grace-period scan started. This means that the > + * grace-period task might sleep. So make sure that if this > + * should happen, the first callback posted to this CPU will > + * wake up the grace-period task if need be. > + */ > + > + local_irq_save(flags); > + rdp = RCU_DATA_ME(); > + spin_lock(&rdp->lock);
I assume that splitting the irq-disable from the spin_lock is a little latency optimisation?
> + rdp->rcu_sched_sleeping = 1; > + spin_unlock_irqrestore(&rdp->lock, flags); > } > > #else /* #ifdef CONFIG_HOTPLUG_CPU */ > @@ -993,26 +1129,194 @@ void call_rcu(struct rcu_head *head, voi > } > EXPORT_SYMBOL_GPL(call_rcu); > > +void call_rcu_sched(struct rcu_head *head, void (*func)(struct rcu_head *rcu)) > +{ > + unsigned long flags; > + struct rcu_data *rdp; > + int wake_gp = 0; > + > + head->func = func; > + head->next = NULL; > + local_irq_save(flags); > + rdp = RCU_DATA_ME(); > + spin_lock(&rdp->lock); > + *rdp->nextschedtail = head; > + rdp->nextschedtail = &head->next; > + if (rdp->rcu_sched_sleeping) { > + > + /* Grace-period processing might be sleeping... */ > + > + rdp->rcu_sched_sleeping = 0; > + wake_gp = 1; > + } > + spin_unlock(&rdp->lock); > + local_irq_restore(flags);
spin_unlock_irqrestore() here would be consistent with the above.
> + if (wake_gp) { > + > + /* Wake up grace-period processing, unless someone beat us. */ > + > + spin_lock_irqsave(&rcu_ctrlblk.schedlock, flags);
If wake_gp!=0 is common then we could microoptimise straight-line performance here by retaining the irq-offness from above.
> + if (rcu_ctrlblk.sched_sleep != rcu_sched_sleeping) > + wake_gp = 0; > + rcu_ctrlblk.sched_sleep = rcu_sched_not_sleeping; > + spin_unlock_irqrestore(&rcu_ctrlblk.schedlock, flags); > + if (wake_gp) > + wake_up_interruptible(&rcu_ctrlblk.sched_wq); > + } > +} > +EXPORT_SYMBOL_GPL(call_rcu_sched); > > ... > > +static int > +rcu_sched_grace_period(void *arg)
Unneeded newline.
> { > - cpumask_t oldmask; > + int couldsleep; /* might sleep after current pass. */ > + int couldsleepnext = 0; /* might sleep after next pass. */ > int cpu; > + unsigned long flags; > + struct rcu_data *rdp; > + int ret; > > - if (sched_getaffinity(0, &oldmask) < 0) > - oldmask = cpu_possible_map; > - for_each_online_cpu(cpu) { > - sched_setaffinity(0, cpumask_of_cpu(cpu)); > - schedule(); > - } > - sched_setaffinity(0, oldmask); > + /* > + * Each pass through the following loop handles one > + * rcu_sched grace period cycle. > + */ > + > + do { > + > + /* Save each CPU's current state. */ > + > + for_each_online_cpu(cpu) {
Numerous unneeded newline ;)
> + dyntick_save_progress_counter_sched(cpu); > + save_qsctr_sched(cpu); > + } > + > + /* > + * Sleep for about an RCU grace-period's worth to > + * allow better batching and to consume less CPU. > + */ > + > + schedule_timeout_interruptible(HZ / 20);
eek, a magic number.
> + /* > + * If there was nothing to do last time, prepare to > + * sleep at the end of the current grace period cycle. > + */ > + > + couldsleep = couldsleepnext; > + couldsleepnext = 1; > + if (couldsleep) { > + spin_lock_irqsave(&rcu_ctrlblk.schedlock, flags); > + rcu_ctrlblk.sched_sleep = rcu_sched_sleep_prep; > + spin_unlock_irqrestore(&rcu_ctrlblk.schedlock, flags); > + }
If the above locking actually correct and needed? The write to rcu_ctrlblk.sched_sleep is a single word...
> + /* > + * Wait on each CPU in turn to have either visited > + * a quiescent state or been in dynticks-idle mode. > + */ > + > + for_each_online_cpu(cpu) { > + while (rcu_qsctr_inc_needed(cpu) && > + rcu_qsctr_inc_needed_dyntick(cpu)) { > + /* resched_cpu(cpu); */ > + schedule_timeout_interruptible(1); > + } > + } > + > + /* > + * Advance callbacks for each CPU. > + */ > + > + for_each_online_cpu(cpu) {
It's more conventional to omit the blank line after the above form of comment block.
> + rdp = RCU_DATA_CPU(cpu); > + spin_lock_irqsave(&rdp->lock, flags); > + > + /* > + * We are running on this CPU irq-disabled, so no > + * CPU can go offline until we re-enable irqs.
but, but, but. The cpu at `cpu' could have gone offline just before we disabled local interrupts.
> + * Advance the callbacks! We share normal RCU's > + * donelist, since callbacks are invoked the > + * same way in either case. > + */ > + > + if (rdp->waitschedlist != NULL) { > + *rdp->donetail = rdp->waitschedlist; > + rdp->donetail = rdp->waitschedtail; > + > + /* > + * Next rcu_check_callbacks() will > + * do the required raise_softirq(). > + */ > + } > + if (rdp->nextschedlist != NULL) { > + rdp->waitschedlist = rdp->nextschedlist; > + rdp->waitschedtail = rdp->nextschedtail; > + couldsleep = 0; > + couldsleepnext = 0; > + } else { > + rdp->waitschedlist = NULL; > + rdp->waitschedtail = &rdp->waitschedlist; > + } > + rdp->nextschedlist = NULL; > + rdp->nextschedtail = &rdp->nextschedlist; > + > + /* Mark sleep intention. */ > + > + rdp->rcu_sched_sleeping = couldsleep; > + > + spin_unlock_irqrestore(&rdp->lock, flags); > + } > + > + /* If we saw callbacks on the last scan, go deal with them. */ > + > + if (!couldsleep) > + continue; > + > + /* Attempt to block... */ > + > + spin_lock_irqsave(&rcu_ctrlblk.schedlock, flags); > + if (rcu_ctrlblk.sched_sleep != rcu_sched_sleep_prep) { > + > + /* > + * Someone posted a callback after we scanned. > + * Go take care of it. > + */ > + > + spin_unlock_irqrestore(&rcu_ctrlblk.schedlock, flags); > + couldsleepnext = 0; > + continue; > + } > + > + /* Block until the next person posts a callback. */ > + > + rcu_ctrlblk.sched_sleep = rcu_sched_sleeping; > + spin_unlock_irqrestore(&rcu_ctrlblk.schedlock, flags); > + ret = 0; > + __wait_event_interruptible(rcu_ctrlblk.sched_wq, > + rcu_ctrlblk.sched_sleep != rcu_sched_sleeping, > + ret); > + if (ret) > + flush_signals(current);
That flush_signals() was a surprise. A desurprising comment would be nice.
> + couldsleepnext = 0; > + > + } while (!kthread_should_stop()); > + > + return (0); > } > -EXPORT_SYMBOL_GPL(__synchronize_sched); > > /* > * Check to see if any future RCU-related work will need to be done > @@ -1029,7 +1333,9 @@ int rcu_needs_cpu(int cpu) > > return (rdp->donelist != NULL || > !!rdp->waitlistcount || > - rdp->nextlist != NULL); > + rdp->nextlist != NULL || > + rdp->nextschedlist != NULL || > + rdp->waitschedlist != NULL); > } > > int rcu_pending(int cpu) > @@ -1040,7 +1346,9 @@ int rcu_pending(int cpu) > > if (rdp->donelist != NULL || > !!rdp->waitlistcount || > - rdp->nextlist != NULL) > + rdp->nextlist != NULL || > + rdp->nextschedlist != NULL || > + rdp->waitschedlist != NULL) > return 1; > > /* The RCU core needs an acknowledgement from this CPU. */ > @@ -1107,6 +1415,11 @@ void __init __rcu_init(void) > rdp->donetail = &rdp->donelist; > rdp->rcu_flipctr[0] = 0; > rdp->rcu_flipctr[1] = 0; > + rdp->nextschedlist = NULL; > + rdp->nextschedtail = &rdp->nextschedlist; > + rdp->waitschedlist = NULL; > + rdp->waitschedtail = &rdp->waitschedlist; > + rdp->rcu_sched_sleeping = 0; > } > register_cpu_notifier(&rcu_nb); > > @@ -1129,6 +1442,18 @@ void __init __rcu_init(void) > } > > /* > + * Late-boot-time RCU initialization that must wait until after scheduler > + * has been initialized. > + */ > +void __init rcu_init_sched(void) > +{ > + rcu_sched_grace_period_task = kthread_run(rcu_sched_grace_period, > + NULL, > + "rcu_sched_grace_period"); > + WARN_ON(IS_ERR(rcu_sched_grace_period_task)); > +} > + > +/* > * Deprecated, use synchronize_rcu() or synchronize_sched() instead. > */ > void synchronize_kernel(void)
I suspect I don't understand any of the RCU code any more.
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