linux-kernel - [PATCH] sched/rt: optimize cpupri

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Message-ID: <20250612031148.455046-1-pan.deng@intel.com>
Date: Thu, 12 Jun 2025 11:11:48 +0800
From: Pan Deng <pan.deng@...el.com>
To: peterz@...radead.org,
	mingo@...nel.org
Cc: linux-kernel@...r.kernel.org,
	tianyou.li@...el.com,
	tim.c.chen@...ux.intel.com,
	pan.deng@...el.com
Subject: [PATCH] sched/rt: optimize cpupri_vec layout

When running a multi-instance ffmpeg transcoding workload which uses rt
thread in a high core count system, cpupri_vec->count contends with the
reading of mask in the same cache line in function cpupri_find_fitness
and cpupri_set.
This change separates each count and mask into different cache lines by
cache aligned attribute to avoid the false sharing.
Tested in a 2 sockets, 240 physical core 480 logical core machine, running
60 ffmpeg transcoding instances. With the change, the kernel cycles% is
reduced from ~20% to ~12%, the fps metric is improved ~11%.
The side effect of this change is that struct cpupri size is increased
from 26 cache lines to 203 cache lines.

Signed-off-by: Pan Deng <pan.deng@...el.com>
Signed-off-by: Tianyou Li <tianyou.li@...el.com>
Reviewed-by: Tim Chen <tim.c.chen@...ux.intel.com>
---
 kernel/sched/cpupri.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/cpupri.h b/kernel/sched/cpupri.h
index d6cba0020064..245b0fa626be 100644
--- a/kernel/sched/cpupri.h
+++ b/kernel/sched/cpupri.h
@@ -9,7 +9,7 @@

 struct cpupri_vec {
 	atomic_t		count;
-	cpumask_var_t		mask;
+	cpumask_var_t		mask	____cacheline_aligned;
 };

 struct cpupri {
-- 
2.43.5