diff mbox series

[v6,03/10] KVM: selftests: Use flag CLOCK_MONOTONIC_RAW for timing

Message ID 20210330080856.14940-4-wangyanan55@huawei.com (mailing list archive)
State New, archived
Headers show
Series KVM: selftests: some improvement and a new test for kvm page table | expand

Commit Message

Yanan Wang March 30, 2021, 8:08 a.m. UTC 
In addition to function of CLOCK_MONOTONIC, flag CLOCK_MONOTONIC_RAW can
also shield possiable impact of NTP, which can provide more robustness.

Suggested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
Reviewed-by: Ben Gardon <bgardon@google.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
---
 tools/testing/selftests/kvm/demand_paging_test.c  |  8 ++++----
 tools/testing/selftests/kvm/dirty_log_perf_test.c | 14 +++++++-------
 tools/testing/selftests/kvm/lib/test_util.c       |  2 +-
 tools/testing/selftests/kvm/steal_time.c          |  4 ++--
 4 files changed, 14 insertions(+), 14 deletions(-)

Comments

Paolo Bonzini April 17, 2021, 1:23 p.m. UTC | #1 
On 30/03/21 10:08, Yanan Wang wrote:
> In addition to function of CLOCK_MONOTONIC, flag CLOCK_MONOTONIC_RAW can
> also shield possiable impact of NTP, which can provide more robustness.
> 
> Suggested-by: Vitaly Kuznetsov<vkuznets@redhat.com>
> Signed-off-by: Yanan Wang<wangyanan55@huawei.com>
> Reviewed-by: Ben Gardon<bgardon@google.com>
> Reviewed-by: Andrew Jones<drjones@redhat.com>

I'm not sure about this one, is the effect visible?

Paolo
Yanan Wang April 19, 2021, 6:40 a.m. UTC | #2 
Hi Paolo,

On 2021/4/17 21:23, Paolo Bonzini wrote:
> On 30/03/21 10:08, Yanan Wang wrote:
>> In addition to function of CLOCK_MONOTONIC, flag CLOCK_MONOTONIC_RAW can
>> also shield possiable impact of NTP, which can provide more robustness.
>>
>> Suggested-by: Vitaly Kuznetsov<vkuznets@redhat.com>
>> Signed-off-by: Yanan Wang<wangyanan55@huawei.com>
>> Reviewed-by: Ben Gardon<bgardon@google.com>
>> Reviewed-by: Andrew Jones<drjones@redhat.com>
>
> I'm not sure about this one, is the effect visible?
>
In practice, difference between results got with CLOCK_MONOTONIC and 
CLOCK_MONOTONIC_RAW
actually is too little to be visible. But if just in theory, 
CLOCK_MONOTONIC_RAW can ensure time results
of the compared tests are based on the same local oscillator frequency, 
which is not subject to possible
NTP frequency adjustment. Change in this patch seems like a bit of 
optimization.

But either of these two flags is good to me. If this patch is not 
convincing enough to be accepted, I will
post a patch later in fix use of CLOCK_MONOTONIC_RAW in 
kvm_page_table_test.c just to be consistent
with other kvm tests, please queue. :）

Thanks,
Yanan
David Laight April 19, 2021, 8:22 a.m. UTC | #3 
From: wangyanan (Y)
> Sent: 19 April 2021 07:40
> 
> Hi Paolo,
> 
> On 2021/4/17 21:23, Paolo Bonzini wrote:
> > On 30/03/21 10:08, Yanan Wang wrote:
> >> In addition to function of CLOCK_MONOTONIC, flag CLOCK_MONOTONIC_RAW can
> >> also shield possiable impact of NTP, which can provide more robustness.
> >>
> >> Suggested-by: Vitaly Kuznetsov<vkuznets@redhat.com>
> >> Signed-off-by: Yanan Wang<wangyanan55@huawei.com>
> >> Reviewed-by: Ben Gardon<bgardon@google.com>
> >> Reviewed-by: Andrew Jones<drjones@redhat.com>
> >
> > I'm not sure about this one, is the effect visible?
> >
> In practice, difference between results got with CLOCK_MONOTONIC and
> CLOCK_MONOTONIC_RAW
> actually is too little to be visible. But if just in theory,
> CLOCK_MONOTONIC_RAW can ensure time results
> of the compared tests are based on the same local oscillator frequency,
> which is not subject to possible
> NTP frequency adjustment. Change in this patch seems like a bit of
> optimization.

The real annoyance is when NTP is realigning the local clock.
This typically happens after boot - but can take quite a few
minutes (don't think it can quite get to an hour).
(I think something similar is caused by leap seconds.)

During this period CLOCK_MONOTONIC can run at a significantly
different rate from 'real time'.
This may not matter for timing self tests, but is significant
for RTP audio.

The problem there is that you want the NTP corrected time
during 'normal running' because the small correction (for
crystal error) is useful.

But the kernel HR timers are only defined for CLOCK_MONOTONIC
and the userspace requests for CLOCK_MONOTONIC_RAW are likely
to be real system calls.

What you really want is a clock whose frequency is adjusted
by NTP but doesn't have the NTP offset adjuctments.
In reality this ought to be CLOCK_MONOTONIC.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
Yanan Wang April 19, 2021, 9:33 a.m. UTC | #4 
On 2021/4/19 16:22, David Laight wrote:
> From: wangyanan (Y)
>> Sent: 19 April 2021 07:40
>>
>> Hi Paolo,
>>
>> On 2021/4/17 21:23, Paolo Bonzini wrote:
>>> On 30/03/21 10:08, Yanan Wang wrote:
>>>> In addition to function of CLOCK_MONOTONIC, flag CLOCK_MONOTONIC_RAW can
>>>> also shield possiable impact of NTP, which can provide more robustness.
>>>>
>>>> Suggested-by: Vitaly Kuznetsov<vkuznets@redhat.com>
>>>> Signed-off-by: Yanan Wang<wangyanan55@huawei.com>
>>>> Reviewed-by: Ben Gardon<bgardon@google.com>
>>>> Reviewed-by: Andrew Jones<drjones@redhat.com>
>>> I'm not sure about this one, is the effect visible?
>>>
>> In practice, difference between results got with CLOCK_MONOTONIC and
>> CLOCK_MONOTONIC_RAW
>> actually is too little to be visible. But if just in theory,
>> CLOCK_MONOTONIC_RAW can ensure time results
>> of the compared tests are based on the same local oscillator frequency,
>> which is not subject to possible
>> NTP frequency adjustment. Change in this patch seems like a bit of
>> optimization.
> The real annoyance is when NTP is realigning the local clock.
> This typically happens after boot - but can take quite a few
> minutes (don't think it can quite get to an hour).
> (I think something similar is caused by leap seconds.)
>
> During this period CLOCK_MONOTONIC can run at a significantly
> different rate from 'real time'.
> This may not matter for timing self tests, but is significant
> for RTP audio.
>
> The problem there is that you want the NTP corrected time
> during 'normal running' because the small correction (for
> crystal error) is useful.
>
> But the kernel HR timers are only defined for CLOCK_MONOTONIC
> and the userspace requests for CLOCK_MONOTONIC_RAW are likely
> to be real system calls.
>
> What you really want is a clock whose frequency is adjusted
> by NTP but doesn't have the NTP offset adjuctments.
> In reality this ought to be CLOCK_MONOTONIC.
Hi David,

I see now, much thanks for the above explanation. :)
Still have a lot to learn about this part.

Thanks,
Yanan
>
> 	David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
diff mbox series

Patch

diff --git a/tools/testing/selftests/kvm/demand_paging_test.c b/tools/testing/selftests/kvm/demand_paging_test.c
index 5f7a229c3af1..efbf0c1e9130 100644
--- a/tools/testing/selftests/kvm/demand_paging_test.c
+++ b/tools/testing/selftests/kvm/demand_paging_test.c
@@ -53,7 +53,7 @@  static void *vcpu_worker(void *data)
 	vcpu_args_set(vm, vcpu_id, 1, vcpu_id);
 	run = vcpu_state(vm, vcpu_id);
 
-	clock_gettime(CLOCK_MONOTONIC, &start);
+	clock_gettime(CLOCK_MONOTONIC_RAW, &start);
 
 	/* Let the guest access its memory */
 	ret = _vcpu_run(vm, vcpu_id);
@@ -86,7 +86,7 @@  static int handle_uffd_page_request(int uffd, uint64_t addr)
 	copy.len = perf_test_args.host_page_size;
 	copy.mode = 0;
 
-	clock_gettime(CLOCK_MONOTONIC, &start);
+	clock_gettime(CLOCK_MONOTONIC_RAW, &start);
 
 	r = ioctl(uffd, UFFDIO_COPY, &copy);
 	if (r == -1) {
@@ -123,7 +123,7 @@  static void *uffd_handler_thread_fn(void *arg)
 	struct timespec start;
 	struct timespec ts_diff;
 
-	clock_gettime(CLOCK_MONOTONIC, &start);
+	clock_gettime(CLOCK_MONOTONIC_RAW, &start);
 	while (!quit_uffd_thread) {
 		struct uffd_msg msg;
 		struct pollfd pollfd[2];
@@ -336,7 +336,7 @@  static void run_test(enum vm_guest_mode mode, void *arg)
 
 	pr_info("Finished creating vCPUs and starting uffd threads\n");
 
-	clock_gettime(CLOCK_MONOTONIC, &start);
+	clock_gettime(CLOCK_MONOTONIC_RAW, &start);
 
 	for (vcpu_id = 0; vcpu_id < nr_vcpus; vcpu_id++) {
 		pthread_create(&vcpu_threads[vcpu_id], NULL, vcpu_worker,
diff --git a/tools/testing/selftests/kvm/dirty_log_perf_test.c b/tools/testing/selftests/kvm/dirty_log_perf_test.c
index 04a2641261be..6cff4ccf9525 100644
--- a/tools/testing/selftests/kvm/dirty_log_perf_test.c
+++ b/tools/testing/selftests/kvm/dirty_log_perf_test.c
@@ -50,7 +50,7 @@  static void *vcpu_worker(void *data)
 	while (!READ_ONCE(host_quit)) {
 		int current_iteration = READ_ONCE(iteration);
 
-		clock_gettime(CLOCK_MONOTONIC, &start);
+		clock_gettime(CLOCK_MONOTONIC_RAW, &start);
 		ret = _vcpu_run(vm, vcpu_id);
 		ts_diff = timespec_elapsed(start);
 
@@ -141,7 +141,7 @@  static void run_test(enum vm_guest_mode mode, void *arg)
 	iteration = 0;
 	host_quit = false;
 
-	clock_gettime(CLOCK_MONOTONIC, &start);
+	clock_gettime(CLOCK_MONOTONIC_RAW, &start);
 	for (vcpu_id = 0; vcpu_id < nr_vcpus; vcpu_id++) {
 		vcpu_last_completed_iteration[vcpu_id] = -1;
 
@@ -162,7 +162,7 @@  static void run_test(enum vm_guest_mode mode, void *arg)
 		ts_diff.tv_sec, ts_diff.tv_nsec);
 
 	/* Enable dirty logging */
-	clock_gettime(CLOCK_MONOTONIC, &start);
+	clock_gettime(CLOCK_MONOTONIC_RAW, &start);
 	vm_mem_region_set_flags(vm, PERF_TEST_MEM_SLOT_INDEX,
 				KVM_MEM_LOG_DIRTY_PAGES);
 	ts_diff = timespec_elapsed(start);
@@ -174,7 +174,7 @@  static void run_test(enum vm_guest_mode mode, void *arg)
 		 * Incrementing the iteration number will start the vCPUs
 		 * dirtying memory again.
 		 */
-		clock_gettime(CLOCK_MONOTONIC, &start);
+		clock_gettime(CLOCK_MONOTONIC_RAW, &start);
 		iteration++;
 
 		pr_debug("Starting iteration %d\n", iteration);
@@ -189,7 +189,7 @@  static void run_test(enum vm_guest_mode mode, void *arg)
 		pr_info("Iteration %d dirty memory time: %ld.%.9lds\n",
 			iteration, ts_diff.tv_sec, ts_diff.tv_nsec);
 
-		clock_gettime(CLOCK_MONOTONIC, &start);
+		clock_gettime(CLOCK_MONOTONIC_RAW, &start);
 		kvm_vm_get_dirty_log(vm, PERF_TEST_MEM_SLOT_INDEX, bmap);
 
 		ts_diff = timespec_elapsed(start);
@@ -199,7 +199,7 @@  static void run_test(enum vm_guest_mode mode, void *arg)
 			iteration, ts_diff.tv_sec, ts_diff.tv_nsec);
 
 		if (dirty_log_manual_caps) {
-			clock_gettime(CLOCK_MONOTONIC, &start);
+			clock_gettime(CLOCK_MONOTONIC_RAW, &start);
 			kvm_vm_clear_dirty_log(vm, PERF_TEST_MEM_SLOT_INDEX, bmap, 0,
 					       host_num_pages);
 
@@ -212,7 +212,7 @@  static void run_test(enum vm_guest_mode mode, void *arg)
 	}
 
 	/* Disable dirty logging */
-	clock_gettime(CLOCK_MONOTONIC, &start);
+	clock_gettime(CLOCK_MONOTONIC_RAW, &start);
 	vm_mem_region_set_flags(vm, PERF_TEST_MEM_SLOT_INDEX, 0);
 	ts_diff = timespec_elapsed(start);
 	pr_info("Disabling dirty logging time: %ld.%.9lds\n",
diff --git a/tools/testing/selftests/kvm/lib/test_util.c b/tools/testing/selftests/kvm/lib/test_util.c
index 906c955384e2..c7c0627c6842 100644
--- a/tools/testing/selftests/kvm/lib/test_util.c
+++ b/tools/testing/selftests/kvm/lib/test_util.c
@@ -89,7 +89,7 @@  struct timespec timespec_elapsed(struct timespec start)
 {
 	struct timespec end;
 
-	clock_gettime(CLOCK_MONOTONIC, &end);
+	clock_gettime(CLOCK_MONOTONIC_RAW, &end);
 	return timespec_sub(end, start);
 }
 
diff --git a/tools/testing/selftests/kvm/steal_time.c b/tools/testing/selftests/kvm/steal_time.c
index fcc840088c91..5bc582d3f2a2 100644
--- a/tools/testing/selftests/kvm/steal_time.c
+++ b/tools/testing/selftests/kvm/steal_time.c
@@ -237,11 +237,11 @@  static void *do_steal_time(void *arg)
 {
 	struct timespec ts, stop;
 
-	clock_gettime(CLOCK_MONOTONIC, &ts);
+	clock_gettime(CLOCK_MONOTONIC_RAW, &ts);
 	stop = timespec_add_ns(ts, MIN_RUN_DELAY_NS);
 
 	while (1) {
-		clock_gettime(CLOCK_MONOTONIC, &ts);
+		clock_gettime(CLOCK_MONOTONIC_RAW, &ts);
 		if (timespec_to_ns(timespec_sub(ts, stop)) >= 0)
 			break;
 	}