Messages in this thread Patch in this message | | | Date | Thu, 18 Sep 2008 12:18:28 +0800 | From | Lai Jiangshan <> | Subject | [RFC PATCH] rcu: introduce kfree_rcu() |
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sometimes a rcu callback is just calling kfree() to free a struct's memory (we say this callback is a trivial callback.). this patch introduce kfree_rcu() to do these things directly, easily.
There are 4 reasons that we need kfree_rcu():
1) unloadable modules: a module(rcu callback is defined in this module) using rcu must call rcu_barrier() when unload. rcu_barrier() will increase the system's overhead(the more cpus the worse) and rcu_barrier() is very time-consuming. if all rcu callback defined in this module are trivial callback, we can just call kfree_rcu() instead, save a rcu_barrier() when unload.
2) duplicate code: all trivial callback are duplicate code though the structs to be freed are different. it's just a container_of() and a kfree(). There are about 50% callbacks are trivial callbacks for call_rcu() in current kernel code.
3) cache: the instructions of trivial callback is not in the cache supposedly. calling a trivial callback will let to cache missing very likely. the more trivial callback the more cache missing. OK, this is not a problem now or in a few days: Only less than 1% trivial callback are called in running kernel.
4) future: the number of user of rcu is increasing. new code for rcu is trivial callback very likely. it means more modules using rcu and more duplicate code(may come to 90% of callbacks is trivial callbacks) and more cache missing.
Implementation: there were a lot of ideas came out when i implemented kfree_rcu(). I chose the simplest one as this patch shows. but these implementation may cannot be used for to free a struct larger than 16KBytes.
kfree_rcu_bh()? kfree_rcu_sched()? these two are not need current. call_rcu_bh() & call_rcu_sched() are hardly be called(and hardly be called for trivial callback).
vfree_rcu()? No, vfree() is not atomic function, will not be called in softirq.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com> --- diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index e8b4039..04c654f 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -253,4 +253,25 @@ extern void rcu_barrier_sched(void); extern void rcu_init(void); extern int rcu_needs_cpu(int cpu); +#define __KFREE_RCU_MAX_OFFSET 4095 +#define KFREE_RCU_MAX_OFFSET (sizeof(void *) * __KFREE_RCU_MAX_OFFSET) + +#define __rcu_reclaim(head) \ +do { \ + unsigned long __offset = (unsigned long)head->func; \ + if (__offset <= __KFREE_RCU_MAX_OFFSET) \ + kfree((void *)head - sizeof(void *) * __offset); \ + else \ + head->func(head); \ +} while(0) + + +/** + * kfree_rcu - free previously allocated memory after a grace period. + * @ptr: pointer returned by kmalloc. + * @head: structure to be used for queueing the RCU updates. This structure + * is a part of previously allocated memory @ptr. + */ +extern void kfree_rcu(const void *ptr, struct rcu_head *head); + #endif /* __LINUX_RCUPDATE_H */ diff --git a/kernel/rcuclassic.c b/kernel/rcuclassic.c index aad93cd..5a14190 100644 --- a/kernel/rcuclassic.c +++ b/kernel/rcuclassic.c @@ -232,7 +232,7 @@ static void rcu_do_batch(struct rcu_data *rdp) while (list) { next = list->next; prefetch(next); - list->func(list); + __rcu_reclaim(list); list = next; if (++count >= rdp->blimit) break; diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c index 467d594..aa9b56a 100644 --- a/kernel/rcupdate.c +++ b/kernel/rcupdate.c @@ -162,6 +162,18 @@ void rcu_barrier_sched(void) } EXPORT_SYMBOL_GPL(rcu_barrier_sched); +void kfree_rcu(const void *ptr, struct rcu_head *head) +{ + unsigned long offset; + typedef void (*rcu_callback)(struct rcu_head *); + + offset = (void *)head - (void *)ptr; + BUG_ON(offset > KFREE_RCU_MAX_OFFSET); + + call_rcu(head, (rcu_callback)(offset / sizeof(void *))); +} +EXPORT_SYMBOL_GPL(kfree_rcu); + void __init rcu_init(void) { __rcu_init(); diff --git a/kernel/rcupreempt.c b/kernel/rcupreempt.c index 2782793..62a9e54 100644 --- a/kernel/rcupreempt.c +++ b/kernel/rcupreempt.c @@ -1108,7 +1108,7 @@ static void rcu_process_callbacks(struct softirq_action *unused) spin_unlock_irqrestore(&rdp->lock, flags); while (list) { next = list->next; - list->func(list); + __rcu_reclaim(list); list = next; RCU_TRACE_ME(rcupreempt_trace_invoke); }
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