linux-kernel - [PATCH v2 1/6] powerpc/smp: Cache CPU has Asymmetric SMP

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Message-ID: <20231018163751.2423181-2-srikar@linux.vnet.ibm.com>
Date:   Wed, 18 Oct 2023 22:07:41 +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>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Nicholas Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rohan McLure <rmclure@...ux.ibm.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2 1/6] 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>
---
Changelog:
v1->v2: Using static keys instead of a variable.
Using pr_info_once instead of printk

 arch/powerpc/kernel/smp.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 5826f5108a12..37c41297c9ce 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -988,18 +988,16 @@ static int __init init_thread_group_cache_map(int cpu, int cache_property)
 }
 
 static bool shared_caches;
+DEFINE_STATIC_KEY_FALSE(powerpc_asym_packing);
 
 #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 (static_branch_unlikely(&powerpc_asym_packing))
+		return SD_SHARE_CPUCAPACITY | SD_SHARE_PKG_RESOURCES | SD_ASYM_PACKING;
 
-	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;
 }
 #endif
 
@@ -1686,6 +1684,11 @@ static void __init fixup_topology(void)
 {
 	int i;
 
+	if (cpu_has_feature(CPU_FTR_ASYM_SMT)) {
+		pr_info_once("Enabling Asymmetric SMT scheduling\n");
+		static_branch_enable(&powerpc_asym_packing);
+	}
+
 #ifdef CONFIG_SCHED_SMT
 	if (has_big_cores) {
 		pr_info("Big cores detected but using small core scheduling\n");
-- 
2.31.1