[PATCH] perf/core: fix the bug in the event multiplexing
Huang Shijie
shijie at os.amperecomputing.com
Tue Aug 8 18:39:53 PDT 2023
1.) Background.
1.1) In arm64, run a virtual guest with Qemu, and bind the guest
to core 33 and run program "a" in guest.
The code of "a" shows below:
----------------------------------------------------------
#include <stdio.h>
int main()
{
unsigned long i = 0;
for (;;) {
i++;
}
printf("i:%ld\n", i);
return 0;
}
----------------------------------------------------------
1.2) Use the following perf command in host:
#perf stat -e cycles:G,cycles:H -C 33 -I 1000 sleep 1
# time counts unit events
1.000817400 3,299,471,572 cycles:G
1.000817400 3,240,586 cycles:H
This result is correct, my cpu's frequency is 3.3G.
1.3) Use the following perf command in host:
#perf stat -e cycles:G,cycles:H -C 33 -d -d -I 1000 sleep 1
time counts unit events
1.000831480 153,634,097 cycles:G (70.03%)
1.000831480 3,147,940,599 cycles:H (70.03%)
1.000831480 1,143,598,527 L1-dcache-loads (70.03%)
1.000831480 9,986 L1-dcache-load-misses # 0.00% of all L1-dcache accesses (70.03%)
1.000831480 <not supported> LLC-loads
1.000831480 <not supported> LLC-load-misses
1.000831480 580,887,696 L1-icache-loads (70.03%)
1.000831480 77,855 L1-icache-load-misses # 0.01% of all L1-icache accesses (70.03%)
1.000831480 6,112,224,612 dTLB-loads (70.03%)
1.000831480 16,222 dTLB-load-misses # 0.00% of all dTLB cache accesses (69.94%)
1.000831480 590,015,996 iTLB-loads (59.95%)
1.000831480 505 iTLB-load-misses # 0.00% of all iTLB cache accesses (59.95%)
This result is wrong. The "cycle:G" should be nearly 3.3G.
2.) Root cause.
There is only 7 counters in my arm64 platform:
(one cycle counter) + (6 normal counters)
In 1.3 above, we will use 10 event counters.
Since we only have 7 counters, the perf core will trigger
event multiplexing in hrtimer:
merge_sched_in() -->perf_mux_hrtimer_restart() -->
perf_rotate_context().
In the perf_rotate_context(), it does not restore some PMU registers
as context_switch() does. In context_switch():
kvm_sched_in() --> kvm_vcpu_pmu_restore_guest()
kvm_sched_out() --> kvm_vcpu_pmu_restore_host()
So we got wrong result.
3.) About this patch.
3.1) Add arch_perf_rotate_pmu_set()
3.2) Add is_guest().
Check the context for hrtimer.
3.3) In arm64's arch_perf_rotate_pmu_set(),
set the PMU registers by the context.
4.) Test result of this patch:
#perf stat -e cycles:G,cycles:H -C 33 -d -d -I 1000 sleep 1
time counts unit events
1.000817360 3,297,898,244 cycles:G (70.03%)
1.000817360 2,719,941 cycles:H (70.03%)
1.000817360 883,764 L1-dcache-loads (70.03%)
1.000817360 17,517 L1-dcache-load-misses # 1.98% of all L1-dcache accesses (70.03%)
1.000817360 <not supported> LLC-loads
1.000817360 <not supported> LLC-load-misses
1.000817360 1,033,816 L1-icache-loads (70.03%)
1.000817360 103,839 L1-icache-load-misses # 10.04% of all L1-icache accesses (70.03%)
1.000817360 982,401 dTLB-loads (70.03%)
1.000817360 28,272 dTLB-load-misses # 2.88% of all dTLB cache accesses (69.94%)
1.000817360 972,072 iTLB-loads (59.95%)
1.000817360 772 iTLB-load-misses # 0.08% of all iTLB cache accesses (59.95%)
The result is correct. The "cycle:G" is nearly 3.3G now.
Signed-off-by: Huang Shijie <shijie at os.amperecomputing.com>
---
arch/arm64/kvm/pmu.c | 8 ++++++++
include/linux/kvm_host.h | 1 +
kernel/events/core.c | 5 +++++
virt/kvm/kvm_main.c | 9 +++++++++
4 files changed, 23 insertions(+)
diff --git a/arch/arm64/kvm/pmu.c b/arch/arm64/kvm/pmu.c
index 121f1a14c829..a6815c3f0c4e 100644
--- a/arch/arm64/kvm/pmu.c
+++ b/arch/arm64/kvm/pmu.c
@@ -210,6 +210,14 @@ void kvm_vcpu_pmu_restore_host(struct kvm_vcpu *vcpu)
kvm_vcpu_pmu_disable_el0(events_guest);
}
+void arch_perf_rotate_pmu_set(void)
+{
+ if (is_guest())
+ kvm_vcpu_pmu_restore_guest(NULL);
+ else
+ kvm_vcpu_pmu_restore_host(NULL);
+}
+
/*
* With VHE, keep track of the PMUSERENR_EL0 value for the host EL0 on the pCPU
* where PMUSERENR_EL0 for the guest is loaded, since PMUSERENR_EL0 is switched
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 9d3ac7720da9..e350cbc8190f 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -931,6 +931,7 @@ void kvm_destroy_vcpus(struct kvm *kvm);
void vcpu_load(struct kvm_vcpu *vcpu);
void vcpu_put(struct kvm_vcpu *vcpu);
+bool is_guest(void);
#ifdef __KVM_HAVE_IOAPIC
void kvm_arch_post_irq_ack_notifier_list_update(struct kvm *kvm);
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 6fd9272eec6e..fe78f9d17eba 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4229,6 +4229,10 @@ ctx_event_to_rotate(struct perf_event_pmu_context *pmu_ctx)
return event;
}
+void __weak arch_perf_rotate_pmu_set(void)
+{
+}
+
static bool perf_rotate_context(struct perf_cpu_pmu_context *cpc)
{
struct perf_cpu_context *cpuctx = this_cpu_ptr(&perf_cpu_context);
@@ -4282,6 +4286,7 @@ static bool perf_rotate_context(struct perf_cpu_pmu_context *cpc)
if (task_event || (task_epc && cpu_event))
__pmu_ctx_sched_in(task_epc->ctx, pmu);
+ arch_perf_rotate_pmu_set();
perf_pmu_enable(pmu);
perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index dfbaafbe3a00..a77d336552be 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -218,6 +218,15 @@ void vcpu_load(struct kvm_vcpu *vcpu)
}
EXPORT_SYMBOL_GPL(vcpu_load);
+/* Do we in the guest? */
+bool is_guest(void)
+{
+ struct kvm_vcpu *vcpu;
+
+ vcpu = __this_cpu_read(kvm_running_vcpu);
+ return !!vcpu;
+}
+
void vcpu_put(struct kvm_vcpu *vcpu)
{
preempt_disable();
--
2.39.2
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