linux-kernel - [PATCH v6 11/11] x86/smpboot: reuse timer calibration

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Date:   Thu,  2 Feb 2023 21:56:25 +0000
From:   Usama Arif <usama.arif@...edance.com>
To:     dwmw2@...radead.org, tglx@...utronix.de, arjan@...ux.intel.com
Cc:     mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, x86@...nel.org, pbonzini@...hat.com,
        paulmck@...nel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, rcu@...r.kernel.org, mimoja@...oja.de,
        hewenliang4@...wei.com, thomas.lendacky@....com, seanjc@...gle.com,
        pmenzel@...gen.mpg.de, fam.zheng@...edance.com,
        punit.agrawal@...edance.com, simon.evans@...edance.com,
        liangma@...ngbit.com, Usama Arif <usama.arif@...edance.com>
Subject: [PATCH v6 11/11] x86/smpboot: reuse timer calibration

From: Arjan van de Ven <arjan@...ux.intel.com>

No point recalibrating for known-constant tsc.
When tested on a 128 core, 2 socket server, this reduces
the parallel smpboot time from 100ms to 30ms.

Not-signed-off-by: Arjan van de Ven <arjan@...ux.intel.com>
Signed-off-by: Usama Arif <usama.arif@...edance.com>
---
 arch/x86/kernel/tsc.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index a78e73da4a74..bab8a98080cf 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1569,6 +1569,9 @@ unsigned long calibrate_delay_is_known(void)
 	if (!constant_tsc || !mask)
 		return 0;
 
+	if (cpu != 0)
+		return cpu_data(0).loops_per_jiffy;
+
 	sibling = cpumask_any_but(mask, cpu);
 	if (sibling < nr_cpu_ids)
 		return cpu_data(sibling).loops_per_jiffy;
-- 
2.25.1