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From: Yury Norov <yury.norov@gmail.com>
To: Andrew Morton <akpm@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ming Lei <ming.lei@redhat.com>,
	linux-kernel@vger.kernel.org
Cc: Yury Norov <yury.norov@gmail.com>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Breno Leitao <leitao@debian.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Zi Yan <ziy@nvidia.com>
Subject: [PATCH 4/9] lib/group_cpus: optimize outer loop in grp_spread_init_one()
Date: Fri, 19 Jan 2024 18:50:48 -0800
Message-Id: <20240120025053.684838-5-yury.norov@gmail.com>
In-Reply-To: <20240120025053.684838-1-yury.norov@gmail.com>
References: <20240120025053.684838-1-yury.norov@gmail.com>
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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@gmail.com>
---
 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;
 		}
 	}
 }
-- 
2.40.1