Re: [PATCH 4/9] lib/group_cpus: optimize outer loop in grp_spread_init_one()

[Ming Lei]

On Fri, Jan 19, 2024 at 06:50:48PM -0800, Yury Norov wrote:
> Similarly to the inner loop, in the outer loop we can use for_each_cpu()
> macro, and skip CPUs that have been moved.
> 
> With this patch, the function becomes O(1), despite that it's a
> double-loop.
> 
> While here, add a comment why we can't merge outer logic into the inner
> loop.
> 
> Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>
> ---
>  lib/group_cpus.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/lib/group_cpus.c b/lib/group_cpus.c
> index 0a8ac7cb1a5d..952aac9eaa81 100644
> --- a/lib/group_cpus.c
> +++ b/lib/group_cpus.c
> @@ -17,16 +17,17 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
>  	const struct cpumask *siblmsk;
>  	int cpu, sibl;
>  
> -	for ( ; cpus_per_grp > 0; ) {
> -		cpu = cpumask_first(nmsk);
> -
> -		/* Should not happen, but I'm too lazy to think about it */
> -		if (cpu >= nr_cpu_ids)
> +	for_each_cpu(cpu, nmsk) {
> +		if (cpus_per_grp-- == 0)
>  			return;
>  
> +		/*
> +		 * If a caller wants to spread IRQa on offline CPUs, we need to
> +		 * take care of it explicitly because those offline CPUS are not
> +		 * included in siblings cpumask.
> +		 */
>  		__cpumask_clear_cpu(cpu, nmsk);
>  		__cpumask_set_cpu(cpu, irqmsk);
> -		cpus_per_grp--;
>  
>  		/* If the cpu has siblings, use them first */
>  		siblmsk = topology_sibling_cpumask(cpu);
> @@ -38,6 +39,7 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
>  
>  			__cpumask_clear_cpu(sibl, nmsk);
>  			__cpumask_set_cpu(sibl, irqmsk);
> +			cpu = sibl + 1;

It has been tricky enough to update condition variable of for_each_cpu()
(such kind of pattern can't build in Rust at all), and the above line could
be more tricky actually.

You can get O(1)(not sure it matters here) by using cpumask_next(), which
is more readable, isn't it?

diff --git a/lib/group_cpus.c b/lib/group_cpus.c
index 564d8e817f65..e0ce878ac4c4 100644
--- a/lib/group_cpus.c
+++ b/lib/group_cpus.c
@@ -15,10 +15,10 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
 				unsigned int cpus_per_grp)
 {
 	const struct cpumask *siblmsk;
-	int cpu, sibl;
+	int cpu = -1;
 
-	for ( ; cpus_per_grp > 0; ) {
-		cpu = cpumask_first(nmsk);
+	while (cpus_per_grp > 0) {
+		cpu = cpumask_next(cpu, nmsk);
 
 		/* Should not happen, but I'm too lazy to think about it */
 		if (cpu >= nr_cpu_ids)
@@ -30,9 +30,9 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
 
 		/* If the cpu has siblings, use them first */
 		siblmsk = topology_sibling_cpumask(cpu);
-		for_each_cpu_and(sibl, siblmsk, nmsk) {
-			cpumask_clear_cpu(sibl, nmsk);
-			cpumask_set_cpu(sibl, irqmsk);
+		for_each_cpu_and(cpu, siblmsk, nmsk) {
+			cpumask_clear_cpu(cpu, nmsk);
+			cpumask_set_cpu(cpu, irqmsk);
 			if (--cpus_per_grp == 0)
 				return;
 		}




Thanks,
Ming