linux-kernel - [PATCH 1/4] powerpc/smp: Cache CPU has Asymmetric SMP

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Message-ID: <20230830105244.62477-2-srikar@linux.vnet.ibm.com>
Date:   Wed, 30 Aug 2023 16:22:41 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Juergen Gross <jgross@...e.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul E McKenney <paulmck@...nel.org>,
        Valentin Schneider <vschneid@...hat.com>,
        Nathan Lynch <nathanl@...ux.ibm.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/4] powerpc/smp: Cache CPU has Asymmetric SMP

Currently cpu feature flag is checked whenever powerpc_smt_flags gets
called. This is an unnecessary overhead. CPU_FTR_ASYM_SMT is set based
on the processor and all processors will either have this set or will
have it unset.

Hence only check for the feature flag once and cache it to be used
subsequently. This commit will help avoid a branch in powerpc_smt_flags

Signed-off-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
---
 arch/powerpc/kernel/smp.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index fbbb695bae3d..c7d1484ed230 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -987,18 +987,13 @@ static int __init init_thread_group_cache_map(int cpu, int cache_property)
 }
 
 static bool shared_caches;
+static int asym_pack_flag;
 
 #ifdef CONFIG_SCHED_SMT
 /* cpumask of CPUs with asymmetric SMT dependency */
 static int powerpc_smt_flags(void)
 {
-	int flags = SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES;
-
-	if (cpu_has_feature(CPU_FTR_ASYM_SMT)) {
-		printk_once(KERN_INFO "Enabling Asymmetric SMT scheduling\n");
-		flags |= SD_ASYM_PACKING;
-	}
-	return flags;
+	return SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES | asym_pack_flag;
 }
 #endif
 
@@ -1676,6 +1671,11 @@ static void __init fixup_topology(void)
 {
 	int i;
 
+	if (cpu_has_feature(CPU_FTR_ASYM_SMT)) {
+		printk_once(KERN_INFO "Enabling Asymmetric SMT scheduling\n");
+		asym_pack_flag = SD_ASYM_PACKING;
+	}
+
 #ifdef CONFIG_SCHED_SMT
 	if (has_big_cores) {
 		pr_info("Big cores detected but using small core scheduling\n");
-- 
2.41.0