linux-kernel - [PATCH v3] x86/hyper-v: micro-optimize send_ipi

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Message-Id: <20191027151938.7296-1-vkuznets@redhat.com>
Date:   Sun, 27 Oct 2019 16:19:38 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     linux-hyperv@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Sasha Levin <sashal@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Joe Perches <joe@...ches.com>
Subject: [PATCH v3] x86/hyper-v: micro-optimize send_ipi_one case

When sending an IPI to a single CPU there is no need to deal with cpumasks.
With 2 CPU guest on WS2019 I'm seeing a minor (like 3%, 8043 -> 7761 CPU
cycles) improvement with smp_call_function_single() loop benchmark. The
optimization, however, is tiny and straitforward. Also, send_ipi_one() is
important for PV spinlock kick.

I was also wondering if it would make sense to switch to using regular
APIC IPI send for CPU > 64 case but no, it is twice as expesive (12650 CPU
cycles for __send_ipi_mask_ex() call, 26000 for orig_apic.send_IPI(cpu,
vector)).

Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
---
Changes since v2:
 - Check VP number instead of CPU number against >= 64 [Michael]
 - Check for VP_INVAL
---
 arch/x86/hyperv/hv_apic.c           | 16 +++++++++++++---
 arch/x86/include/asm/trace/hyperv.h | 15 +++++++++++++++
 2 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/arch/x86/hyperv/hv_apic.c b/arch/x86/hyperv/hv_apic.c
index e01078e93dd3..40e0e322161d 100644
--- a/arch/x86/hyperv/hv_apic.c
+++ b/arch/x86/hyperv/hv_apic.c
@@ -194,10 +194,20 @@ static bool __send_ipi_mask(const struct cpumask *mask, int vector)
 
 static bool __send_ipi_one(int cpu, int vector)
 {
-	struct cpumask mask = CPU_MASK_NONE;
+	int vp = hv_cpu_number_to_vp_number(cpu);
 
-	cpumask_set_cpu(cpu, &mask);
-	return __send_ipi_mask(&mask, vector);
+	trace_hyperv_send_ipi_one(cpu, vector);
+
+	if (!hv_hypercall_pg || (vp == VP_INVAL))
+		return false;
+
+	if ((vector < HV_IPI_LOW_VECTOR) || (vector > HV_IPI_HIGH_VECTOR))
+		return false;
+
+	if (vp >= 64)
+		return __send_ipi_mask_ex(cpumask_of(cpu), vector);
+
+	return !hv_do_fast_hypercall16(HVCALL_SEND_IPI, vector, BIT_ULL(vp));
 }
 
 static void hv_send_ipi(int cpu, int vector)
diff --git a/arch/x86/include/asm/trace/hyperv.h b/arch/x86/include/asm/trace/hyperv.h
index ace464f09681..4d705cb4d63b 100644
--- a/arch/x86/include/asm/trace/hyperv.h
+++ b/arch/x86/include/asm/trace/hyperv.h
@@ -71,6 +71,21 @@ TRACE_EVENT(hyperv_send_ipi_mask,
 		      __entry->ncpus, __entry->vector)
 	);
 
+TRACE_EVENT(hyperv_send_ipi_one,
+	    TP_PROTO(int cpu,
+		     int vector),
+	    TP_ARGS(cpu, vector),
+	    TP_STRUCT__entry(
+		    __field(int, cpu)
+		    __field(int, vector)
+		    ),
+	    TP_fast_assign(__entry->cpu = cpu;
+			   __entry->vector = vector;
+		    ),
+	    TP_printk("cpu %d vector %x",
+		      __entry->cpu, __entry->vector)
+	);
+
 #endif /* CONFIG_HYPERV */
 
 #undef TRACE_INCLUDE_PATH
-- 
2.20.1