Re: [PATCH -v4] generic-ipi: remove kmalloc()

Paul E. McKenney
2009-02-17T21:30:47Z

On Tue, Feb 17, 2009 at 08:29:18PM +0100, Peter Zijlstra wrote:
> On Tue, 2009-02-17 at 18:40 +0100, Peter Zijlstra wrote:
> > Let me spin a new patch and build a kernel with it ;-)
>
> On top of Nick's patch.
>
> My quad has been happily building kernels with this the past 30 minutes
> or so.
>
> ---
> Subject: generic-ipi: remove kmalloc()
>
> Remove the use of kmalloc() from the smp_call_function_*() calls.
>
> Steven's generic-ipi patch (d7240b98: generic-ipi: use per cpu data for
> single cpu ipi calls) started the discussion on the use of kmalloc() in
> this code and fixed the smp_call_function_single(.wait=0) fallback case.
>
> In this patch we complete this by also providing means for the _many()
> call, which fully removes the need for kmalloc() in this code.
>
> The problem with the _many() call is that other cpus might still be
> observing our entry when we're done with it. It solved this by
> dynamically allocating data elements and RCU-freeing it.
>
> We solve it by using a single per-cpu entry which provides static
> storage and solves one half of the problem (avoiding referencing freed
> data).
>
> The other half, ensuring the queue iteration it still possible, is done
> by placing re-used entries at the head of the list. This means that if
> someone was still iterating that entry when it got moved will now
> re-visit the entries on the list it had already seen, but avoids
> skipping over entries like would have happened had we placed the new
> entry at the end.
>
> Furthermore, visiting entries twice is not a problem, since we remove
> our cpu from the entry's cpumask once its called.
>
> We also optimize both the _single() and _many() interrupt handler to
> copy the entry to their local stack and freeing it for re-use before we
> call the function.
>
> Many thanks to Oleg for his suggestions and poking him holes in my
> earlier attempts.
>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
> smp.c | 285 +++++++++++++++++++++++++++++++++++++++---------------------------
> 1 file changed, 171 insertions(+), 114 deletions(-)
>
> Index: linux-2.6/kernel/smp.c
> ===================================================================
> --- linux-2.6.orig/kernel/smp.c
> +++ linux-2.6/kernel/smp.c
> @@ -10,23 +10,28 @@
> #include <linux/rcupdate.h>
> #include <linux/rculist.h>
> #include <linux/smp.h>
> +#include <linux/cpu.h>
>
> static DEFINE_PER_CPU(struct call_single_queue, call_single_queue);
> -static LIST_HEAD(call_function_queue);
> -__cacheline_aligned_in_smp DEFINE_SPINLOCK(call_function_lock);
> +
> +static struct {
> + struct list_head queue;
> + spinlock_t lock;
> +} call_function __cacheline_aligned_in_smp = {
> + .queue = LIST_HEAD_INIT(call_function.queue),
> + .lock = __SPIN_LOCK_UNLOCKED(call_function.lock),
> +};
>
> enum {
> CSD_FLAG_WAIT = 0x01,
> - CSD_FLAG_ALLOC = 0x02,
> - CSD_FLAG_LOCK = 0x04,
> + CSD_FLAG_LOCK = 0x02,
> };
>
> struct call_function_data {
> struct call_single_data csd;
> spinlock_t lock;
> unsigned int refs;
> - struct rcu_head rcu_head;
> - unsigned long cpumask_bits[];
> + cpumask_var_t cpumask;
> };
>
> struct call_single_queue {
> @@ -34,8 +39,45 @@ struct call_single_queue {
> spinlock_t lock;
> };
>
> +static DEFINE_PER_CPU(struct call_function_data, cfd_data) = {
> + .lock = __SPIN_LOCK_UNLOCKED(cfd_data.lock),
> +};
> +
> +static int
> +hotplug_cfd(struct notifier_block *nfb, unsigned long action, void *hcpu)
> +{
> + long cpu = (long)hcpu;
> + struct call_function_data *cfd = &per_cpu(cfd_data, cpu);
> +
> + switch (action) {
> + case CPU_UP_PREPARE:
> + case CPU_UP_PREPARE_FROZEN:
> + if (!alloc_cpumask_var_node(&cfd->cpumask, GFP_KERNEL,
> + cpu_to_node(cpu)))
> + return NOTIFY_BAD;
> + break;
> +
> +#ifdef CONFIG_CPU_HOTPLUG
> + case CPU_UP_CANCELED:
> + case CPU_UP_CANCELED_FROZEN:
> +
> + case CPU_DEAD:
> + case CPU_DEAD_FROZEN:
> + free_cpumask_var(cfd->cpumask);
> + break;
> +#endif
> + };
> +
> + return NOTIFY_OK;
> +}
> +
> +static struct notifier_block __cpuinitdata hotplug_cfd_notifier = {
> + .notifier_call = hotplug_cfd,
> +};
> +
> static int __cpuinit init_call_single_data(void)
> {
> + void *cpu = (void *)(long)smp_processor_id();
> int i;
>
> for_each_possible_cpu(i) {
> @@ -44,18 +86,59 @@ static int __cpuinit init_call_single_da
> spin_lock_init(&q->lock);
> INIT_LIST_HEAD(&q->list);
> }
> +
> + hotplug_cfd(&hotplug_cfd_notifier, CPU_UP_PREPARE, cpu);
> + register_cpu_notifier(&hotplug_cfd_notifier);
> +
> return 0;
> }
> early_initcall(init_call_single_data);
>
> -static void csd_flag_wait(struct call_single_data *data)
> +/*
> + * csd_wait/csd_complete are used for synchronous ipi calls
> + */
> +static void csd_wait_prepare(struct call_single_data *data)
> +{
> + data->flags |= CSD_FLAG_WAIT;
> +}
> +
> +static void csd_complete(struct call_single_data *data)
> +{
> + /*
> + * Serialize stores to data with the flag clear and wakeup.
> + */
> + smp_wmb();

Shouldn't the above be an smp_mb()? There are reads preceding the calls
to csd_complete() that look to me like they need to remain ordered
before the flag-clearing below -- just in case of a quick reuse of this
call_single_data structure.

> + data->flags &= ~CSD_FLAG_WAIT;
> +}
> +
> +static void csd_wait(struct call_single_data *data)
> +{
> + while (data->flags & CSD_FLAG_WAIT)
> + cpu_relax();
> +}
> +
> +/*
> + * csd_lock/csd_unlock used to serialize access to per-cpu csd resources
> + *
> + * For non-synchronous ipi calls the csd can still be in use by the previous
> + * function call. For multi-cpu calls its even more interesting as we'll have
> + * to ensure no other cpu is observing our csd.
> + */
> +static void csd_lock(struct call_single_data *data)
> {
> - /* Wait for response */
> - do {
> - if (!(data->flags & CSD_FLAG_WAIT))
> - break;
> + while (data->flags & CSD_FLAG_LOCK)
> cpu_relax();
> - } while (1);
> + data->flags = CSD_FLAG_LOCK;

OK, I'll bite... Why don't we need a memory barrier here?

> +}
> +
> +static void csd_unlock(struct call_single_data *data)
> +{
> + WARN_ON(!(data->flags & CSD_FLAG_LOCK));
> + /*
> + * Serialize stores to data with the flags clear.
> + */
> + smp_wmb();

I am a bit worried about this being smp_wmb() rather than smp_mb(),
but don't have a smoking gun.

> + data->flags &= ~CSD_FLAG_LOCK;
> }
>
> /*
> @@ -89,16 +172,7 @@ static void generic_exec_single(int cpu,
> arch_send_call_function_single_ipi(cpu);
>
> if (wait)
> - csd_flag_wait(data);
> -}
> -
> -static void rcu_free_call_data(struct rcu_head *head)
> -{
> - struct call_function_data *data;
> -
> - data = container_of(head, struct call_function_data, rcu_head);
> -
> - kfree(data);
> + csd_wait(data);
> }
>
> /*
> @@ -122,41 +196,36 @@ void generic_smp_call_function_interrupt
> * It's ok to use list_for_each_rcu() here even though we may delete
> * 'pos', since list_del_rcu() doesn't clear ->next
> */
> - rcu_read_lock();
> - list_for_each_entry_rcu(data, &call_function_queue, csd.list) {
> - int refs;
> -
> - if (!cpumask_test_cpu(cpu, to_cpumask(data->cpumask_bits)))
> - continue;
> -
> - data->csd.func(data->csd.info);
> + list_for_each_entry_rcu(data, &call_function.queue, csd.list) {
> + void (*func)(void *);
> + void *info;
> + int refs, wait;
>
> spin_lock(&data->lock);
> - cpumask_clear_cpu(cpu, to_cpumask(data->cpumask_bits));
> + if (!cpumask_test_cpu(cpu, data->cpumask)) {
> + spin_unlock(&data->lock);
> + continue;
> + }
> + cpumask_clear_cpu(cpu, data->cpumask);
> WARN_ON(data->refs == 0);
> - data->refs--;
> - refs = data->refs;
> + refs = --data->refs;
> + func = data->csd.func;
> + info = data->csd.info;
> + wait = (data->csd.flags & CSD_FLAG_WAIT);
> spin_unlock(&data->lock);
>
> - if (refs)
> - continue;
> + if (!refs) {
> + spin_lock(&call_function.lock);
> + list_del_rcu(&data->csd.list);
> + spin_unlock(&call_function.lock);
> + csd_unlock(&data->csd);
> + }
>
> - spin_lock(&call_function_lock);
> - list_del_rcu(&data->csd.list);
> - spin_unlock(&call_function_lock);
> + func(info);
>
> - if (data->csd.flags & CSD_FLAG_WAIT) {
> - /*
> - * serialize stores to data with the flag clear
> - * and wakeup
> - */
> - smp_wmb();
> - data->csd.flags &= ~CSD_FLAG_WAIT;
> - }
> - if (data->csd.flags & CSD_FLAG_ALLOC)
> - call_rcu(&data->rcu_head, rcu_free_call_data);
> + if (!refs && wait)
> + csd_complete(&data->csd);
> }
> - rcu_read_unlock();
>
> put_cpu();
> }
> @@ -169,7 +238,6 @@ void generic_smp_call_function_single_in
> {
> struct call_single_queue *q = &__get_cpu_var(call_single_queue);
> LIST_HEAD(list);
> - unsigned int data_flags;
>
> spin_lock(&q->lock);
> list_replace_init(&q->list, &list);
> @@ -177,29 +245,28 @@ void generic_smp_call_function_single_in
>
> while (!list_empty(&list)) {
> struct call_single_data *data;
> + void (*func)(void *);
> + void *info;
> + int wait;
>
> - data = list_entry(list.next, struct call_single_data,
> - list);
> + data = list_first_entry(&list, struct call_single_data, list);
> list_del(&data->list);
> + func = data->func;
> + info = data->info;
>
> /*
> - * 'data' can be invalid after this call if
> - * flags == 0 (when called through
> - * generic_exec_single(), so save them away before
> - * making the call.
> + * 'data' can be invalid after this if flags == 0
> + * when called through generic_exec_single()
> */
> - data_flags = data->flags;
> + wait = (data->flags & CSD_FLAG_WAIT);
>
> - data->func(data->info);
> + if (data->flags & CSD_FLAG_LOCK)
> + csd_unlock(data);
> +
> + func(info);
>
> - if (data_flags & CSD_FLAG_WAIT) {
> - smp_wmb();
> - data->flags &= ~CSD_FLAG_WAIT;
> - } else if (data_flags & CSD_FLAG_LOCK) {
> - smp_wmb();
> - data->flags &= ~CSD_FLAG_LOCK;
> - } else if (data_flags & CSD_FLAG_ALLOC)
> - kfree(data);
> + if (wait)
> + csd_complete(data);
> }
> }
>
> @@ -218,7 +285,9 @@ static DEFINE_PER_CPU(struct call_single
> int smp_call_function_single(int cpu, void (*func) (void *info), void *info,
> int wait)
> {
> - struct call_single_data d;
> + struct call_single_data d = {
> + .flags = 0,
> + };

And about here I get lost -- trying to find what the heck this patch
applies to... :-/

Thanx, Paul

> unsigned long flags;
> /* prevent preemption and reschedule on another processor,
> as well as CPU removal */
> @@ -239,13 +308,11 @@ int smp_call_function_single(int cpu, vo
> /*
> * We are calling a function on a single CPU
> * and we are not going to wait for it to finish.
> - * We first try to allocate the data, but if we
> - * fail, we fall back to use a per cpu data to pass
> - * the information to that CPU. Since all callers
> - * of this code will use the same data, we must
> - * synchronize the callers to prevent a new caller
> - * from corrupting the data before the callee
> - * can access it.
> + * We use a per cpu data to pass the information to
> + * that CPU. Since all callers of this code will
> + * use the same data, we must synchronize the
> + * callers to prevent a new caller from corrupting
> + * the data before the callee can access it.
> *
> * The CSD_FLAG_LOCK is used to let us know when
> * the IPI handler is done with the data.
> @@ -255,18 +322,11 @@ int smp_call_function_single(int cpu, vo
> * will make sure the callee is done with the
> * data before a new caller will use it.
> */
> - data = kmalloc(sizeof(*data), GFP_ATOMIC);
> - if (data)
> - data->flags = CSD_FLAG_ALLOC;
> - else {
> - data = &per_cpu(csd_data, me);
> - while (data->flags & CSD_FLAG_LOCK)
> - cpu_relax();
> - data->flags = CSD_FLAG_LOCK;
> - }
> + data = &per_cpu(csd_data, me);
> + csd_lock(data);
> } else {
> data = &d;
> - data->flags = CSD_FLAG_WAIT;
> + csd_wait_prepare(data);
> }
>
> data->func = func;
> @@ -326,14 +386,14 @@ void smp_call_function_many(const struct
> {
> struct call_function_data *data;
> unsigned long flags;
> - int cpu, next_cpu;
> + int cpu, next_cpu, me = smp_processor_id();
>
> /* Can deadlock when called with interrupts disabled */
> WARN_ON(irqs_disabled());
>
> /* So, what's a CPU they want? Ignoring this one. */
> cpu = cpumask_first_and(mask, cpu_online_mask);
> - if (cpu == smp_processor_id())
> + if (cpu == me)
> cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
> /* No online cpus? We're done. */
> if (cpu >= nr_cpu_ids)
> @@ -341,7 +401,7 @@ void smp_call_function_many(const struct
>
> /* Do we have another CPU which isn't us? */
> next_cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
> - if (next_cpu == smp_processor_id())
> + if (next_cpu == me)
> next_cpu = cpumask_next_and(next_cpu, mask, cpu_online_mask);
>
> /* Fastpath: do that cpu by itself. */
> @@ -350,31 +410,28 @@ void smp_call_function_many(const struct
> return;
> }
>
> - data = kmalloc(sizeof(*data) + cpumask_size(), GFP_ATOMIC);
> - if (unlikely(!data)) {
> - /* Slow path. */
> - for_each_online_cpu(cpu) {
> - if (cpu == smp_processor_id())
> - continue;
> - if (cpumask_test_cpu(cpu, mask))
> - smp_call_function_single(cpu, func, info, wait);
> - }
> - return;
> - }
> + data = &per_cpu(cfd_data, me);
> + csd_lock(&data->csd);
>
> - spin_lock_init(&data->lock);
> - data->csd.flags = CSD_FLAG_ALLOC;
> + spin_lock_irqsave(&data->lock, flags);
> if (wait)
> - data->csd.flags |= CSD_FLAG_WAIT;
> + csd_wait_prepare(&data->csd);
> +
> data->csd.func = func;
> data->csd.info = info;
> - cpumask_and(to_cpumask(data->cpumask_bits), mask, cpu_online_mask);
> - cpumask_clear_cpu(smp_processor_id(), to_cpumask(data->cpumask_bits));
> - data->refs = cpumask_weight(to_cpumask(data->cpumask_bits));
> -
> - spin_lock_irqsave(&call_function_lock, flags);
> - list_add_tail_rcu(&data->csd.list, &call_function_queue);
> - spin_unlock_irqrestore(&call_function_lock, flags);
> + cpumask_and(data->cpumask, mask, cpu_online_mask);
> + cpumask_clear_cpu(me, data->cpumask);
> + data->refs = cpumask_weight(data->cpumask);
> +
> + spin_lock(&call_function.lock);
> + /*
> + * Place then entry at the _HEAD_ of the list, so that any cpu still
> + * observing the entry in generic_smp_call_function_interrupt() will
> + * not miss any other list entries (instead it can see some twice).
> + */
> + list_add_rcu(&data->csd.list, &call_function.queue);
> + spin_unlock(&call_function.lock);
> + spin_unlock_irqrestore(&data->lock, flags);
>
> /*
> * Make the list addition visible before sending the ipi.
> @@ -384,11 +441,11 @@ void smp_call_function_many(const struct
> smp_mb();
>
> /* Send a message to all CPUs in the map */
> - arch_send_call_function_ipi_mask(to_cpumask(data->cpumask_bits));
> + arch_send_call_function_ipi_mask(data->cpumask);
>
> /* optionally wait for the CPUs to complete */
> if (wait)
> - csd_flag_wait(&data->csd);
> + csd_wait(&data->csd);
> }
> EXPORT_SYMBOL(smp_call_function_many);
>
> @@ -418,20 +475,20 @@ EXPORT_SYMBOL(smp_call_function);
>
> void ipi_call_lock(void)
> {
> - spin_lock(&call_function_lock);
> + spin_lock(&call_function.lock);
> }
>
> void ipi_call_unlock(void)
> {
> - spin_unlock(&call_function_lock);
> + spin_unlock(&call_function.lock);
> }
>
> void ipi_call_lock_irq(void)
> {
> - spin_lock_irq(&call_function_lock);
> + spin_lock_irq(&call_function.lock);
> }
>
> void ipi_call_unlock_irq(void)
> {
> - spin_unlock_irq(&call_function_lock);
> + spin_unlock_irq(&call_function.lock);
> }
>
>