Paul E. McKenney wrote:
quoted text
> 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.

quoted text
> 
>> 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.

quoted 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.

quoted text
> 
>> +/**
>> + * 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.

quoted text
> 
>> +}
>> +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);
>>  	}
>>
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> 
> 


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