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Subject: Re: [PATCH v3 1/2] cpuidle: Trace IPI based and timer based wakeup
 latency from idle states
To:     "Rafael J. Wysocki" <rafael@kernel.org>
Cc:     "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>, srivatsa@csail.mit.edu,
        Shuah Khan <shuah@kernel.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>,
        Vaidyanathan Srinivasan <svaidy@linux.ibm.com>,
        pratik.r.sampat@gmail.com, Linux PM <linux-pm@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-kselftest@vger.kernel.org
References: <20200721124300.65615-1-psampat@linux.ibm.com>
 <20200721124300.65615-2-psampat@linux.ibm.com>
 <CAJZ5v0j3ip77opkaW3Rtn0cqT7VTL_8goctFBDVehWoZowDY0Q@mail.gmail.com>
From:   Pratik Sampat <psampat@linux.ibm.com>
Message-ID: <9dfff062-9e68-c91f-f36c-8699e8c3fc1b@linux.ibm.com>
Date:   Tue, 28 Jul 2020 19:00:12 +0530
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Hello Rafael,


On 27/07/20 7:12 pm, Rafael J. Wysocki wrote:
> On Tue, Jul 21, 2020 at 2:43 PM Pratik Rajesh Sampat
> <psampat@linux.ibm.com> wrote:
>> Fire directed smp_call_function_single IPIs from a specified source
>> CPU to the specified target CPU to reduce the noise we have to wade
>> through in the trace log.
> And what's the purpose of it?

The idea for this comes from that fact that estimating wake-up
latencies and residencies for stop states is not an easy task.

The purpose is essentially to determine wakeup latencies, that are
caused by either, an IPI or a timer and compare with the advertised
wakeup latencies for each stop state.

This might help in determining the accuracy of our advertised values
and/or if they need any re-calibration.

>> The module is based on the idea written by Srivatsa Bhat and maintained
>> by Vaidyanathan Srinivasan internally.
>>
>> Queue HR timer and measure jitter. Wakeup latency measurement for idle
>> states using hrtimer.  Echo a value in ns to timer_test_function and
>> watch trace. A HRtimer will be queued and when it fires the expected
>> wakeup vs actual wakeup is computes and delay printed in ns.
>>
>> Implemented as a module which utilizes debugfs so that it can be
>> integrated with selftests.
>>
>> To include the module, check option and include as module
>> kernel hacking -> Cpuidle latency selftests
>>
>> [srivatsa.bhat@linux.vnet.ibm.com: Initial implementation in
>>   cpidle/sysfs]
>>
>> [svaidy@linux.vnet.ibm.com: wakeup latency measurements using hrtimer
>>   and fix some of the time calculation]
>>
>> [ego@linux.vnet.ibm.com: Fix some whitespace and tab errors and
>>   increase the resolution of IPI wakeup]
>>
>> Signed-off-by: Pratik Rajesh Sampat <psampat@linux.ibm.com>
>> Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
>> ---
>>   drivers/cpuidle/Makefile               |   1 +
>>   drivers/cpuidle/test-cpuidle_latency.c | 150 +++++++++++++++++++++++++
>>   lib/Kconfig.debug                      |  10 ++
>>   3 files changed, 161 insertions(+)
>>   create mode 100644 drivers/cpuidle/test-cpuidle_latency.c
>>
>> diff --git a/drivers/cpuidle/Makefile b/drivers/cpuidle/Makefile
>> index f07800cbb43f..2ae05968078c 100644
>> --- a/drivers/cpuidle/Makefile
>> +++ b/drivers/cpuidle/Makefile
>> @@ -8,6 +8,7 @@ obj-$(CONFIG_ARCH_NEEDS_CPU_IDLE_COUPLED) += coupled.o
>>   obj-$(CONFIG_DT_IDLE_STATES)             += dt_idle_states.o
>>   obj-$(CONFIG_ARCH_HAS_CPU_RELAX)         += poll_state.o
>>   obj-$(CONFIG_HALTPOLL_CPUIDLE)           += cpuidle-haltpoll.o
>> +obj-$(CONFIG_IDLE_LATENCY_SELFTEST)      += test-cpuidle_latency.o
>>
>>   ##################################################################################
>>   # ARM SoC drivers
>> diff --git a/drivers/cpuidle/test-cpuidle_latency.c b/drivers/cpuidle/test-cpuidle_latency.c
>> new file mode 100644
>> index 000000000000..61574665e972
>> --- /dev/null
>> +++ b/drivers/cpuidle/test-cpuidle_latency.c
>> @@ -0,0 +1,150 @@
>> +// SPDX-License-Identifier: GPL-2.0-or-later
>> +/*
>> + * Module-based API test facility for cpuidle latency using IPIs and timers
> I'd like to see a more detailed description of what it does and how it
> works here.

Right, I'll add that.
Based on comments from Daniel I have also been working on a
user-space only variant of this test as that does seem like
a better way to go.

The only downside is that the latency will be higher, but as we are
taking baseline measurements the diff of that from our observed reading
should still remain the same. Just that the test will take longer to run.
I'm yet to accurately confirm this.

I would appreciate your thoughts on that.

>> + */
>> +
>> +#include <linux/debugfs.h>
>> +#include <linux/kernel.h>
>> +#include <linux/module.h>
>> +
>> +/* IPI based wakeup latencies */
>> +struct latency {
>> +       unsigned int src_cpu;
>> +       unsigned int dest_cpu;
>> +       ktime_t time_start;
>> +       ktime_t time_end;
>> +       u64 latency_ns;
>> +} ipi_wakeup;
>> +
>> +static void measure_latency(void *info)
>> +{
>> +       struct latency *v;
>> +       ktime_t time_diff;
>> +
>> +       v = (struct latency *)info;
>> +       v->time_end = ktime_get();
>> +       time_diff = ktime_sub(v->time_end, v->time_start);
>> +       v->latency_ns = ktime_to_ns(time_diff);
>> +}
>> +
>> +void run_smp_call_function_test(unsigned int cpu)
>> +{
>> +       ipi_wakeup.src_cpu = smp_processor_id();
>> +       ipi_wakeup.dest_cpu = cpu;
>> +       ipi_wakeup.time_start = ktime_get();
>> +       smp_call_function_single(cpu, measure_latency, &ipi_wakeup, 1);
>> +}
>> +
>> +/* Timer based wakeup latencies */
>> +struct timer_data {
>> +       unsigned int src_cpu;
>> +       u64 timeout;
>> +       ktime_t time_start;
>> +       ktime_t time_end;
>> +       struct hrtimer timer;
>> +       u64 timeout_diff_ns;
>> +} timer_wakeup;
>> +
>> +static enum hrtimer_restart timer_called(struct hrtimer *hrtimer)
>> +{
>> +       struct timer_data *w;
>> +       ktime_t time_diff;
>> +
>> +       w = container_of(hrtimer, struct timer_data, timer);
>> +       w->time_end = ktime_get();
>> +
>> +       time_diff = ktime_sub(w->time_end, w->time_start);
>> +       time_diff = ktime_sub(time_diff, ns_to_ktime(w->timeout));
>> +       w->timeout_diff_ns = ktime_to_ns(time_diff);
>> +       return HRTIMER_NORESTART;
>> +}
>> +
>> +static void run_timer_test(unsigned int ns)
>> +{
>> +       hrtimer_init(&timer_wakeup.timer, CLOCK_MONOTONIC,
>> +                    HRTIMER_MODE_REL);
>> +       timer_wakeup.timer.function = timer_called;
>> +       timer_wakeup.time_start = ktime_get();
>> +       timer_wakeup.src_cpu = smp_processor_id();
>> +       timer_wakeup.timeout = ns;
>> +
>> +       hrtimer_start(&timer_wakeup.timer, ns_to_ktime(ns),
>> +                     HRTIMER_MODE_REL_PINNED);
>> +}
>> +
>> +static struct dentry *dir;
>> +
>> +static int cpu_read_op(void *data, u64 *value)
>> +{
>> +       *value = ipi_wakeup.dest_cpu;
>> +       return 0;
>> +}
>> +
>> +static int cpu_write_op(void *data, u64 value)
>> +{
>> +       run_smp_call_function_test(value);
>> +       return 0;
>> +}
>> +DEFINE_SIMPLE_ATTRIBUTE(ipi_ops, cpu_read_op, cpu_write_op, "%llu\n");
>> +
>> +static int timeout_read_op(void *data, u64 *value)
>> +{
>> +       *value = timer_wakeup.timeout;
>> +       return 0;
>> +}
>> +
>> +static int timeout_write_op(void *data, u64 value)
>> +{
>> +       run_timer_test(value);
>> +       return 0;
>> +}
>> +DEFINE_SIMPLE_ATTRIBUTE(timeout_ops, timeout_read_op, timeout_write_op, "%llu\n");
>> +
>> +static int __init latency_init(void)
>> +{
>> +       struct dentry *temp;
>> +
>> +       dir = debugfs_create_dir("latency_test", 0);
>> +       if (!dir) {
>> +               pr_alert("latency_test: failed to create /sys/kernel/debug/latency_test\n");
>> +               return -1;
>> +       }
>> +       temp = debugfs_create_file("ipi_cpu_dest",
>> +                                  0666,
>> +                                  dir,
>> +                                  NULL,
>> +                                  &ipi_ops);
>> +       if (!temp) {
>> +               pr_alert("latency_test: failed to create /sys/kernel/debug/ipi_cpu_dest\n");
>> +               return -1;
>> +       }
>> +       debugfs_create_u64("ipi_latency_ns", 0444, dir, &ipi_wakeup.latency_ns);
>> +       debugfs_create_u32("ipi_cpu_src", 0444, dir, &ipi_wakeup.src_cpu);
>> +
>> +       temp = debugfs_create_file("timeout_expected_ns",
>> +                                  0666,
>> +                                  dir,
>> +                                  NULL,
>> +                                  &timeout_ops);
>> +       if (!temp) {
>> +               pr_alert("latency_test: failed to create /sys/kernel/debug/timeout_expected_ns\n");
>> +               return -1;
>> +       }
>> +       debugfs_create_u64("timeout_diff_ns", 0444, dir, &timer_wakeup.timeout_diff_ns);
>> +       debugfs_create_u32("timeout_cpu_src", 0444, dir, &timer_wakeup.src_cpu);
>> +       pr_info("Latency Test module loaded\n");
>> +       return 0;
>> +}
>> +
>> +static void __exit latency_cleanup(void)
>> +{
>> +       pr_info("Cleaning up Latency Test module.\n");
>> +       debugfs_remove_recursive(dir);
>> +}
>> +
>> +module_init(latency_init);
>> +module_exit(latency_cleanup);
>> +
>> +MODULE_LICENSE("GPL");
>> +MODULE_AUTHOR("IBM Corporation");
>> +MODULE_DESCRIPTION("Measuring idle latency for IPIs and Timers");
>> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
>> index d74ac0fd6b2d..e2283790245a 100644
>> --- a/lib/Kconfig.debug
>> +++ b/lib/Kconfig.debug
>> @@ -1375,6 +1375,16 @@ config DEBUG_KOBJECT
>>            If you say Y here, some extra kobject debugging messages will be sent
>>            to the syslog.
>>
>> +config IDLE_LATENCY_SELFTEST
>> +       tristate "Cpuidle latency selftests"
>> +       depends on CPU_IDLE
>> +       help
>> +         This option provides a kernel module that runs tests using the IPI and
>> +         timers to measure latency.
> What latency does it measure?

It measures latencies incurred on wakeup after an IPI and a timer interrupt.

>> +
>> +         Say M if you want these self tests to build as a module.
>> +         Say N if you are unsure.
>> +
>>   config DEBUG_KOBJECT_RELEASE
>>          bool "kobject release debugging"
>>          depends on DEBUG_OBJECTS_TIMERS
>> --
>> 2.25.4
>>
Thanks,
Pratik