Messages in this thread | | | Date | Thu, 18 Sep 2008 20:58:42 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [RFC PATCH] rcu: introduce kfree_rcu() |
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On Fri, Sep 19, 2008 at 09:04:12AM +0800, Lai Jiangshan wrote: > 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.
Understood.
> > > >> 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.
The Tiny Linux guys might be interested in even a small reduction in kernel text size.
> >> 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.
Good point!
Thanx, Paul
> >> +/** > >> + * 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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