Messages in this thread | | | Date | Fri, 19 Sep 2008 09:04:12 +0800 | From | Lai Jiangshan <> | Subject | Re: [RFC PATCH] rcu: introduce kfree_rcu() |
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Paul E. McKenney wrote: > On Thu, Sep 18, 2008 at 12:18:28PM +0800, Lai Jiangshan wrote: >> 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. > > Interesting! Please see questions and comments below. > >> 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. > > You lost me on this one. Suppose that the following sequence of > events occurred: > > a. The module invokes call_rcu() or kfree_rcu(). The callback > is queued on CPU 0. > > b. Perhaps a grace period completes, and the callback is therefore > moved to CPU 0's donelist. But CPU 0 is busy, so doesn't get > around to invoking the callback. (For example, ksoftirqd.) > > c. The module is unloaded, and uses kfree_rcu() instead of > rcu_barrier(). The callback is queued on CPU 1. > > d. A grace period completes, and CPU 1 is relatively idle, so > invokes its callback quickly. The module is therefore unloaded. > > e. CPU 0 finally gets around to executing its callback, but the > module has been unloaded, so there is nothingness where the > callback function used to be. We get an oops. > > What prevents this sequence of events from happening?
We save a rcu_barrier() only when all rcu callback defined in this module are trivial callback and we use kfree_rcu to instead them.
trivial callbacks are the most common callbacks, so some module may used trivial callback only.
> >> 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. > > Indeed! There was something similar to kfree_rcu() proposed some > years back, but it was rejected because it contained more code than > did the trivial callbacks. :-/ > > But there are more such callbacks these days, so might be worth > revisiting. > >> 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. > > Reducing code footprint would be a good thing. Do you have stats on > the kernel text size, before and after?
I did not have stats on the kernel text size, I think these cache missing are caused by lots of different trivial callbacks in everywhere, not too big kernel text.
> >> 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. > > Ditto. > >> 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) > > OK, so the idea is that structures whose rcu_head is near the front > of the structure have the offset of the rcu_head put into the > ->func field instead of a pointer to the callback function? > > Of course, it doesn't need to be too near the beginning of the > function... > > All arches are guaranteed not to have kernel text in the low 16K > of memory (for 32-bit arches) or low 32K of memory (for 64-bit arches)?
(unsigned long)head->func is always <= 4095, not 14K or 32K. we just guaranteed not to have kernel text in the low 4k of memory.
the real offset is (sizeof(void *) * (unsigned long)head->func), it's 16K or 32K.
> >> +/** >> + * 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); > > OK, consistent with above. > >> 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 *))); > > OK, so we pass in the pointer to the rcu_head structure, followed > by the offset in pointer-sized units, but with the latter cast to > a pointer to a callback function? Hmmm.... Kinky.... > > Then after the grace period completes, the __rcu_reclaim() sorts > things out.
Yes, kernel pointers have redundant information, we use the low 4k as offset. when ->func < 4k, it stand for offset, when ->func >= 4k, it stand for function pointer.
> >> +} >> +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); > > And we do this for preemptable RCU as well. > >> list = next; >> RCU_TRACE_ME(rcupreempt_trace_invoke); >> } >> > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > > >
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