Re: [RFC PATCH] rcu: introduce kfree_rcu()

[Manfred Spraul]

Andrew Morton wrote:
> On Thu, 18 Sep 2008 12:18:28 +0800 Lai Jiangshan <laijs@xxxxxxxxxxxxxx> 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.
> >
> > 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.
> >
> >     
>
> This is all rather mysterious.
>
>   
> > ---
> > 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)
> >     
>
> All the above could do with some comments explaining what it does.
>   
__rcu_reclaim either treats head->func as an offset for kfree or as a 
function pointer.


> >  #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);
> >     
Here it's used:
the softirq that is called after the grace period calls kfree directly 
instead of calling a wrapper function around kfree.

> >  		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;
> >     
What about offset_of? the computation is known at compile time.
> > +	BUG_ON(offset > KFREE_RCU_MAX_OFFSET);
> > +
> >     
I'd try to make that a compile time error. Is that possible? perhaps 
with some __builtin_constant_p (head-ptr) or something like that. Or 
with offset_of.

> > +	call_rcu(head, (rcu_callback)(offset / sizeof(void *)));
> >     
>
> How can this work?  We take the difference between two pointers, divide
> that by 4 or 8, then treat the resulting number as the address of an
> RCU callback function.
>
> I think I'm missing something here.
>
>   
__rcu_reclaim() knows that function pointers < 4096 are actually offsets 
for kfree.


I like the idea:
- the call to list->func() is probably very difficult to predict for a 
branch target predictor.
- it's just a waste not to call kfree directly.
- I'm not sure about the implementation.

--
   Manfred
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