1.) Background.
1.1) In arm64, start a guest with Qemu which is running as a VMM of KVM,
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
multiplexing in hrtimer:
perf_mux_hrtimer_restart() --> perf_rotate_context().
If the hrtimer occurs when the host is running, it's fine.
If the hrtimer occurs when the guest is running,
the perf_rotate_context() will program the PMU with filters for
host context. The KVM does not have a chance to restore
PMU registers with kvm_vcpu_pmu_restore_guest().
The PMU does not work correctly, so we got wrong result.
3.) About this patch.
3.1) Add a week arch_perf_mux() for perf_mux_hrtimer_handler().
3.2) In the arm64, implement the arch_perf_mux().
Make a KVM_REQ_PMU_RESTORE_GUEST request if the
perf multiplexing occurs in the guest 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
1.001006400 3,298,348,656 cycles:G (70.03%)
1.001006400 3,144,532 cycles:H (70.03%)
1.001006400 941,149 L1-dcache-loads (70.03%)
1.001006400 17,937 L1-dcache-load-misses # 1.91% of all L1-dcache accesses (70.03%)
1.001006400 <not supported> LLC-loads
1.001006400 <not supported> LLC-load-misses
1.001006400 1,101,889 L1-icache-loads (70.03%)
1.001006400 121,638 L1-icache-load-misses # 11.04% of all L1-icache accesses (70.03%)
1.001006400 1,031,228 dTLB-loads (70.03%)
1.001006400 26,952 dTLB-load-misses # 2.61% of all dTLB cache accesses (69.93%)
1.001006400 1,030,678 iTLB-loads (59.94%)
1.001006400 338 iTLB-load-misses # 0.03% of all iTLB cache accesses (59.94%)
The result is correct. The "cycle:G" is nearly 3.3G now.
Signed-off-by: Huang Shijie <[email protected]>
---
v1 --> v2:
Do not change perf/core code, only change the ARM64 kvm code.
v1: https://lkml.org/lkml/2023/8/8/1465
v2 --> v3:
Disdurb the perf/core again.
v2:http://lists.infradead.org/pipermail/linux-arm-kernel/2023-August/858427.html
---
arch/arm64/include/asm/kvm_host.h | 1 +
arch/arm64/kvm/arm.c | 18 ++++++++++++++++++
include/linux/perf_event.h | 2 ++
kernel/events/core.c | 5 +++++
4 files changed, 26 insertions(+)
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 8b6096753740..151810445d79 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -49,6 +49,7 @@
#define KVM_REQ_RELOAD_GICv4 KVM_ARCH_REQ(4)
#define KVM_REQ_RELOAD_PMU KVM_ARCH_REQ(5)
#define KVM_REQ_SUSPEND KVM_ARCH_REQ(6)
+#define KVM_REQ_PMU_RESTORE_GUEST KVM_ARCH_REQ(7)
#define KVM_DIRTY_LOG_MANUAL_CAPS (KVM_DIRTY_LOG_MANUAL_PROTECT_ENABLE | \
KVM_DIRTY_LOG_INITIALLY_SET)
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index c2c14059f6a8..53d6555912d1 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -792,6 +792,9 @@ static int check_vcpu_requests(struct kvm_vcpu *vcpu)
preempt_enable();
}
+ if (kvm_check_request(KVM_REQ_PMU_RESTORE_GUEST, vcpu))
+ kvm_vcpu_pmu_restore_guest(vcpu);
+
if (kvm_check_request(KVM_REQ_RELOAD_PMU, vcpu))
kvm_pmu_handle_pmcr(vcpu,
__vcpu_sys_reg(vcpu, PMCR_EL0));
@@ -878,6 +881,21 @@ static int noinstr kvm_arm_vcpu_enter_exit(struct kvm_vcpu *vcpu)
return ret;
}
+void arch_perf_mux(bool rotations)
+{
+ struct kvm_vcpu *vcpu;
+
+ if (!kvm_arm_support_pmu_v3() || !has_vhe())
+ return;
+
+ vcpu = kvm_get_running_vcpu();
+ if (!vcpu)
+ return;
+
+ if (rotations)
+ kvm_make_request(KVM_REQ_PMU_RESTORE_GUEST, vcpu);
+}
+
/**
* kvm_arch_vcpu_ioctl_run - the main VCPU run function to execute guest code
* @vcpu: The VCPU pointer
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 2166a69e3bf2..07ef1aa00226 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1860,6 +1860,8 @@ extern void arch_perf_update_userpage(struct perf_event *event,
struct perf_event_mmap_page *userpg,
u64 now);
+extern void arch_perf_mux(bool rotations);
+
#ifdef CONFIG_MMU
extern __weak u64 arch_perf_get_page_size(struct mm_struct *mm, unsigned long addr);
#endif
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 6fd9272eec6e..eefb28198dd1 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1059,6 +1059,10 @@ static void perf_cgroup_switch(struct task_struct *task)
}
#endif
+void __weak arch_perf_mux(bool rotations)
+{
+}
+
/*
* set default to be dependent on timer tick just
* like original code
@@ -1076,6 +1080,7 @@ static enum hrtimer_restart perf_mux_hrtimer_handler(struct hrtimer *hr)
cpc = container_of(hr, struct perf_cpu_pmu_context, hrtimer);
rotations = perf_rotate_context(cpc);
+ arch_perf_mux(rotations);
raw_spin_lock(&cpc->hrtimer_lock);
if (rotations)
--
2.39.2