Re: [RFC v2 1/5] rcu: Introduce for_each_leaf_node_cpu()

[Mark Rutland]

On Thu, Dec 15, 2016 at 10:42:00AM +0800, Boqun Feng wrote:
> There are some places inside RCU core, where we need to iterate all mask
> (->qsmask, ->expmask, etc) bits in a leaf node, in order to iterate all
> corresponding CPUs. The current code iterates all possible CPUs in this
> leaf node and then checks with the mask to see whether the bit is set.
> 
> However, given the fact that most bits in cpu_possible_mask are set but
> rare bits in an RCU leaf node mask are set(in other words, ->qsmask and
> its friends are usually more sparse than cpu_possible_mask), it's better
> to iterate in the other way, that is iterating mask bits in a leaf node.
> By doing so, we can save several checks in the loop, moreover, that fast
> path checking(e.g. ->qsmask == 0) could then be consolidated into the
> loop logic.
> 
> This patch introduce for_each_leaf_node_cpu() to iterate mask bits in a
> more efficient way.
> 
> By design, The CPUs whose bits are set in the leaf node masks should be
> a subset of possible CPUs, so we don't need extra check with
> cpu_possible(), however, a WARN_ON_ONCE() is put in the loop to check
> whether there are some nasty cases we miss.
> 
> Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> ---
>  kernel/rcu/tree.h | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index c0a4bf8f1ed0..70ef44a082e0 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -295,6 +295,22 @@ struct rcu_node {
>  	     cpu <= rnp->grphi; \
>  	     cpu = cpumask_next((cpu), cpu_possible_mask))
>  
> +
> +#define MASK_BITS(mask)	(BITS_PER_BYTE * sizeof(mask))
> +/*
> + * Iterate over all CPUs a leaf RCU node which are still masked in
> + * @mask.
> + *
> + * Note @rnp has to be a leaf node and @mask has to belong to @rnp.

Not a big deal, but perhaps it's worth enforcing this? If we took just
the name of the mask here, (e.g. qsmask rather than rnp->qsmask), we
could have the macro always use (rnp)->(mask). That would also make the
invocations shorter.

> And we
> + * assume that no CPU is masked in @mask but not set in cpu_possible_mask. IOW,
> + * masks of a leaf node never set a bit for an "impossible" CPU.
> + */
> +#define for_each_leaf_node_cpu(rnp, mask, cpu) \
> +	for ((cpu) = (rnp)->grplo + find_first_bit(&(mask), MASK_BITS(mask)); \
> +	     (cpu) <= (rnp)->grphi && !WARN_ON_ONCE(!cpu_possible(cpu)); \

If this happens, we'll exit the loop. If there are any reamining
possible CPUs, we'll skip them, which would be less than ideal.

I guess this shouldn't happen anyway, but it might be worth continuing.

> +	     (cpu) = (rnp)->grplo + find_next_bit(&(mask), MASK_BITS(mask), \
> +						  (cpu) - (rnp)->grplo + 1))
> +

I was going to ask if that + 1 was correct, but I see that it is!

So FWIW:

Acked-by: Mark Rutland <mark.rutland@xxxxxxx>


I had a go at handling my comments above, but I'm not sure it's any
better:

#define cpu_to_grp(rnp, cpu)	((cpu) - (rnp)->grplo)

#define grp_to_cpu(rnp, cpu)	((cpu) + (rnp)->grplo)

#define node_first_cpu(rnp, mask) \
	grp_to_cpu(find_first_bit(&(rnp)->mask, MASK_BITS((rnp)->mask)))

#define node_next_cpu(rnp, mask, cpu)
	grp_to_cpu(rnp, find_next_bit(&(rnp)->mask, MASK_BITS((rnp)->mask),
				      cpu_to_grp(rnp, cpu) + 1))

#define for_each_leaf_node_cpu(rnp, mask, cpu) \
	for ((cpu) = node_first_cpu(rnp, mask); \
	     (cpu) <= (rnp)->grphi; \
	     (cpu) = node_next_cpu(rnp, mask, cpu)) \
		if (WARN_ON_ONCE(!cpu_possible(cpu))) \
			continue; \
		else

Thanks,
Mark.