Re: [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree_rcu batching

[Paul E. McKenney]

On Thu, Aug 08, 2019 at 07:26:10PM +0900, Byungchul Park wrote:
> On Wed, Aug 07, 2019 at 05:45:04AM -0400, Joel Fernandes wrote:
> > On Tue, Aug 06, 2019 at 04:56:31PM -0700, Paul E. McKenney wrote:
> 
> [snip]
> 
> > > On Tue, Aug 06, 2019 at 05:20:40PM -0400, Joel Fernandes (Google) wrote:
> > > Of course, I am hoping that a later patch uses an array of pointers built
> > > at kfree_rcu() time, similar to Rao's patch (with or without kfree_bulk)
> > > in order to reduce per-object cache-miss overhead.  This would make it
> > > easier for callback invocation to keep up with multi-CPU kfree_rcu()
> > > floods.
> > 
> > I think Byungchul tried an experiment with array of pointers and wasn't
> > immediately able to see a benefit. Perhaps his patch needs a bit more polish
> > or another test-case needed to show benefit due to cache-misses, and the perf
> > tool could be used to show if cache misses were reduced. For this initial
> > pass, we decided to keep it without the array optimization.
> 
> I'm still seeing no improvement with kfree_bulk().
> 
> I've been thinking I could see improvement with kfree_bulk() because:
> 
>    1. As you guys said, the number of cache misses will be reduced.
>    2. We can save (N - 1) irq-disable instructions while N kfrees.
>    3. As Joel said, saving/restoring CPU status that kfree() does inside
>       is not required.
> 
> But even with the following patch applied, the result was same as just
> batching test. We might need to get kmalloc objects from random
> addresses to maximize the result when using kfree_bulk() and this is
> even closer to real practical world too.
> 
> And the second and third reasons doesn't seem to work as much as I
> expected.
> 
> Do you have any idea? Or what do you think about it?

I would not expect kfree_batch() to help all that much unless the
pre-grace-period kfree_rcu() code segregated the objects on a per-slab
basis.

							Thanx, Paul

> Thanks,
> Byungchul
> 
> -----8<-----
> diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c
> index 988e1ae..6f2ab06 100644
> --- a/kernel/rcu/rcuperf.c
> +++ b/kernel/rcu/rcuperf.c
> @@ -651,10 +651,10 @@ struct kfree_obj {
>  				return -ENOMEM;
>  		}
>  
> -		for (i = 0; i < kfree_alloc_num; i++) {
> -			if (!kfree_no_batch) {
> -				kfree_rcu(alloc_ptrs[i], rh);
> -			} else {
> +		if (!kfree_no_batch) {
> +			kfree_bulk(kfree_alloc_num, (void **)alloc_ptrs);
> +		} else {
> +			for (i = 0; i < kfree_alloc_num; i++) {
>  				rcu_callback_t cb;
>  
>  				cb = (rcu_callback_t)(unsigned long)offsetof(struct kfree_obj, rh);
>