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![Jens Axboe](/assets/img/avatar_default.png)
I got a bug report yesterday from Laszlo Ersek in which he states that
his kvm instance fails to suspend. Laszlo bisected it down to this
commit 1cf7e9c68f
("virtio_blk: blk-mq support") where virtio-blk is
converted to use the blk-mq infrastructure.
After digging a bit, it became clear that the issue was with the queue
drain. blk-mq tracks queue usage in a percpu counter, which is
incremented on request alloc and decremented when the request is freed.
The initial hunt was for an inconsistency in blk-mq, but everything
seemed fine. In fact, the counter only returned crazy values when
suspend was in progress.
When a CPU is unplugged, the percpu counters merges that CPU state with
the general state. blk-mq takes care to register a hotcpu notifier with
the appropriate priority, so we know it runs after the percpu counter
notifier. However, the percpu counter notifier only merges the state
when the CPU is fully gone. This leaves a state transition where the
CPU going away is no longer in the online mask, yet it still holds
private values. This means that in this state, percpu_counter_sum()
returns invalid results, and the suspend then hangs waiting for
abs(dead-cpu-value) requests to complete which of course will never
happen.
Fix this by clearing the state earlier, so we never have a case where
the CPU isn't in online mask but still holds private state. This bug
has been there since forever, I guess we don't have a lot of users where
percpu counters needs to be reliable during the suspend cycle.
Signed-off-by: Jens Axboe <axboe@fb.com>
Reported-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
225 lines
5.3 KiB
C
225 lines
5.3 KiB
C
/*
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* Fast batching percpu counters.
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*/
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#include <linux/percpu_counter.h>
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#include <linux/notifier.h>
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#include <linux/mutex.h>
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#include <linux/init.h>
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#include <linux/cpu.h>
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#include <linux/module.h>
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#include <linux/debugobjects.h>
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#ifdef CONFIG_HOTPLUG_CPU
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static LIST_HEAD(percpu_counters);
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static DEFINE_SPINLOCK(percpu_counters_lock);
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#endif
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#ifdef CONFIG_DEBUG_OBJECTS_PERCPU_COUNTER
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static struct debug_obj_descr percpu_counter_debug_descr;
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static int percpu_counter_fixup_free(void *addr, enum debug_obj_state state)
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{
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struct percpu_counter *fbc = addr;
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switch (state) {
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case ODEBUG_STATE_ACTIVE:
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percpu_counter_destroy(fbc);
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debug_object_free(fbc, &percpu_counter_debug_descr);
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return 1;
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default:
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return 0;
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}
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}
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static struct debug_obj_descr percpu_counter_debug_descr = {
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.name = "percpu_counter",
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.fixup_free = percpu_counter_fixup_free,
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};
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static inline void debug_percpu_counter_activate(struct percpu_counter *fbc)
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{
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debug_object_init(fbc, &percpu_counter_debug_descr);
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debug_object_activate(fbc, &percpu_counter_debug_descr);
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}
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static inline void debug_percpu_counter_deactivate(struct percpu_counter *fbc)
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{
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debug_object_deactivate(fbc, &percpu_counter_debug_descr);
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debug_object_free(fbc, &percpu_counter_debug_descr);
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}
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#else /* CONFIG_DEBUG_OBJECTS_PERCPU_COUNTER */
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static inline void debug_percpu_counter_activate(struct percpu_counter *fbc)
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{ }
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static inline void debug_percpu_counter_deactivate(struct percpu_counter *fbc)
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{ }
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#endif /* CONFIG_DEBUG_OBJECTS_PERCPU_COUNTER */
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void percpu_counter_set(struct percpu_counter *fbc, s64 amount)
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{
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int cpu;
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unsigned long flags;
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raw_spin_lock_irqsave(&fbc->lock, flags);
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for_each_possible_cpu(cpu) {
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s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
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*pcount = 0;
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}
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fbc->count = amount;
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raw_spin_unlock_irqrestore(&fbc->lock, flags);
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}
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EXPORT_SYMBOL(percpu_counter_set);
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void __percpu_counter_add(struct percpu_counter *fbc, s64 amount, s32 batch)
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{
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s64 count;
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preempt_disable();
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count = __this_cpu_read(*fbc->counters) + amount;
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if (count >= batch || count <= -batch) {
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unsigned long flags;
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raw_spin_lock_irqsave(&fbc->lock, flags);
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fbc->count += count;
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__this_cpu_sub(*fbc->counters, count - amount);
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raw_spin_unlock_irqrestore(&fbc->lock, flags);
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} else {
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this_cpu_add(*fbc->counters, amount);
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}
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preempt_enable();
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}
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EXPORT_SYMBOL(__percpu_counter_add);
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/*
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* Add up all the per-cpu counts, return the result. This is a more accurate
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* but much slower version of percpu_counter_read_positive()
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*/
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s64 __percpu_counter_sum(struct percpu_counter *fbc)
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{
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s64 ret;
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int cpu;
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unsigned long flags;
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raw_spin_lock_irqsave(&fbc->lock, flags);
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ret = fbc->count;
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for_each_online_cpu(cpu) {
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s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
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ret += *pcount;
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}
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raw_spin_unlock_irqrestore(&fbc->lock, flags);
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return ret;
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}
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EXPORT_SYMBOL(__percpu_counter_sum);
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int __percpu_counter_init(struct percpu_counter *fbc, s64 amount,
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struct lock_class_key *key)
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{
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raw_spin_lock_init(&fbc->lock);
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lockdep_set_class(&fbc->lock, key);
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fbc->count = amount;
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fbc->counters = alloc_percpu(s32);
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if (!fbc->counters)
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return -ENOMEM;
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debug_percpu_counter_activate(fbc);
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#ifdef CONFIG_HOTPLUG_CPU
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INIT_LIST_HEAD(&fbc->list);
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spin_lock(&percpu_counters_lock);
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list_add(&fbc->list, &percpu_counters);
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spin_unlock(&percpu_counters_lock);
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#endif
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return 0;
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}
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EXPORT_SYMBOL(__percpu_counter_init);
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void percpu_counter_destroy(struct percpu_counter *fbc)
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{
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if (!fbc->counters)
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return;
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debug_percpu_counter_deactivate(fbc);
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#ifdef CONFIG_HOTPLUG_CPU
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spin_lock(&percpu_counters_lock);
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list_del(&fbc->list);
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spin_unlock(&percpu_counters_lock);
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#endif
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free_percpu(fbc->counters);
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fbc->counters = NULL;
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}
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EXPORT_SYMBOL(percpu_counter_destroy);
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int percpu_counter_batch __read_mostly = 32;
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EXPORT_SYMBOL(percpu_counter_batch);
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static void compute_batch_value(void)
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{
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int nr = num_online_cpus();
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percpu_counter_batch = max(32, nr*2);
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}
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static int percpu_counter_hotcpu_callback(struct notifier_block *nb,
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unsigned long action, void *hcpu)
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{
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#ifdef CONFIG_HOTPLUG_CPU
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unsigned int cpu;
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struct percpu_counter *fbc;
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compute_batch_value();
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if (action != CPU_DEAD && action != CPU_DEAD_FROZEN)
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return NOTIFY_OK;
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cpu = (unsigned long)hcpu;
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spin_lock(&percpu_counters_lock);
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list_for_each_entry(fbc, &percpu_counters, list) {
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s32 *pcount;
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unsigned long flags;
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raw_spin_lock_irqsave(&fbc->lock, flags);
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pcount = per_cpu_ptr(fbc->counters, cpu);
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fbc->count += *pcount;
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*pcount = 0;
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raw_spin_unlock_irqrestore(&fbc->lock, flags);
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}
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spin_unlock(&percpu_counters_lock);
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#endif
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return NOTIFY_OK;
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}
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/*
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* Compare counter against given value.
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* Return 1 if greater, 0 if equal and -1 if less
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*/
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int percpu_counter_compare(struct percpu_counter *fbc, s64 rhs)
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{
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s64 count;
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count = percpu_counter_read(fbc);
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/* Check to see if rough count will be sufficient for comparison */
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if (abs(count - rhs) > (percpu_counter_batch*num_online_cpus())) {
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if (count > rhs)
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return 1;
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else
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return -1;
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}
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/* Need to use precise count */
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count = percpu_counter_sum(fbc);
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if (count > rhs)
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return 1;
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else if (count < rhs)
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return -1;
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else
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return 0;
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}
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EXPORT_SYMBOL(percpu_counter_compare);
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static int __init percpu_counter_startup(void)
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{
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compute_batch_value();
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hotcpu_notifier(percpu_counter_hotcpu_callback, 0);
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return 0;
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}
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module_init(percpu_counter_startup);
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