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SubjectRe: [PATCH] sched/fair: vruntime should normalize when switching from fair
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On 08/23/2018 11:54 PM, Juri Lelli wrote:
>> I tried to catch this issue on my Arm64 Juno board using pi_test (and a
>> slightly adapted pip_test (usleep_val = 1500 and keep low as cfs)) from
>> rt-tests but wasn't able to do so.
>>
>> # pi_stress --inversions=1 --duration=1 --groups=1 --sched id=low,policy=cfs
>>
>> Starting PI Stress Test
>> Number of thread groups: 1
>> Duration of test run: 1 seconds
>> Number of inversions per group: 1
>> Admin thread SCHED_FIFO priority 4
>> 1 groups of 3 threads will be created
>> High thread SCHED_FIFO priority 3
>> Med thread SCHED_FIFO priority 2
>> Low thread SCHED_OTHER nice 0
>>
>> # ./pip_stress
>>
>> In both cases, the cfs task entering rt_mutex_setprio() is queued, so
>> dequeue_task_fair()->dequeue_entity(), which subtracts cfs_rq->min_vruntime
>> from se->vruntime, is called on it before it gets the rt prio.
>>
>> Maybe it requires a very specific use of the pthread library to provoke this
>> issue by making sure that the cfs tasks really blocks/sleeps?
>
> Maybe one could play with rt-app to recreate such specific use case?
>
> https://github.com/scheduler-tools/rt-app/blob/master/doc/tutorial.txt#L459

This was reproduced for me on tip of mainline by using the program at
the end of this mail. It was run in a 2 CPU virtualbox VM. Relevant
annotated bits of the trace:

low-prio thread vruntime is 752ms
pi-vruntime-tes-598 [001] d... 520.572459: sched_stat_runtime:
comm=pi-vruntime-tes pid=598 runtime=29953 [ns] vruntime=752888705 [ns]

low-prio thread waits on a_sem
pi-vruntime-tes-598 [001] d... 520.572465: sched_switch:
prev_comm=pi-vruntime-tes prev_pid=598 prev_prio=120 prev_state=D ==>
next_comm=swapper/1 next_pid=0 next_prio=120

high prio thread finishes wakeup, then sleeps for 1ms
<idle>-0 [000] dNh. 520.572483: sched_wakeup:
comm=pi-vruntime-tes pid=597 prio=19 target_cpu=000
<idle>-0 [000] d... 520.572486: sched_switch:
prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=S ==>
next_comm=pi-vruntime-tes next_pid=597 next_prio=19
pi-vruntime-tes-597 [000] d... 520.572498: sched_switch:
prev_comm=pi-vruntime-tes prev_pid=597 prev_prio=19 prev_state=D ==>
next_comm=swapper/0 next_pid=0 next_prio=120

high prio thread wakes up after 1ms sleep, posts a_sem which starts to
wake low-prio thread, then tries to grab pi_mutex, which low-prio thread has
<idle>-0 [000] d.h. 520.573876: sched_waking:
comm=pi-vruntime-tes pid=597 prio=19 target_cpu=000
<idle>-0 [000] dNh. 520.573879: sched_wakeup:
comm=pi-vruntime-tes pid=597 prio=19 target_cpu=000
<idle>-0 [000] d... 520.573887: sched_switch:
prev_comm=swapper/0 prev_pid=0 prev_prio=120 prev_state=S ==>
next_comm=pi-vruntime-tes next_pid=597 next_prio=19
pi-vruntime-tes-597 [000] d... 520.573895: sched_waking:
comm=pi-vruntime-tes pid=598 prio=120 target_cpu=001

low-prio thread pid 598 gets pi_mutex priority inheritance, this happens
while low-prio thread is in waking state
pi-vruntime-tes-597 [000] d... 520.573911: sched_pi_setprio:
comm=pi-vruntime-tes pid=598 oldprio=120 newprio=19

high-prio thread sleeps on pi_mutex
pi-vruntime-tes-597 [000] d... 520.573919: sched_switch:
prev_comm=pi-vruntime-tes prev_pid=597 prev_prio=19 prev_state=D ==>
next_comm=swapper/0 next_pid=0 next_prio=120

low-prio thread finishes wakeup
<idle>-0 [001] dNh. 520.573932: sched_wakeup:
comm=pi-vruntime-tes pid=598 prio=19 target_cpu=001
<idle>-0 [001] d... 520.573936: sched_switch:
prev_comm=swapper/1 prev_pid=0 prev_prio=120 prev_state=S ==>
next_comm=pi-vruntime-tes next_pid=598 next_prio=19

low-prio thread releases pi-mutex, loses pi boost, high-prio thread
wakes for pi-mutex
pi-vruntime-tes-598 [001] d... 520.573946: sched_pi_setprio:
comm=pi-vruntime-tes pid=598 oldprio=19 newprio=120
pi-vruntime-tes-598 [001] dN.. 520.573954: sched_waking:
comm=pi-vruntime-tes pid=597 prio=19 target_cpu=000

low-prio thread vruntime is 1505ms
pi-vruntime-tes-598 [001] dN.. 520.573966: sched_stat_runtime:
comm=pi-vruntime-tes pid=598 runtime=20150 [ns] vruntime=1505797560 [ns]

The timing is quite sensitive since the task being boosted has to be
caught in the TASK_WAKING state. The program:

/*
* Test case for vruntime management during rtmutex priority inheritance
* promotion and demotion.
*
* build with -lpthread
*/

#define _GNU_SOURCE
#include <pthread.h>
#include <semaphore.h>
#include <sched.h>
#include <stdio.h>
#include <unistd.h>

#define ERROR_CHECK(x) \
if (x) \
fprintf(stderr, "Error at line %d", __LINE__);

pthread_mutex_t pi_mutex;
sem_t a_sem;
sem_t b_sem;

void *rt_thread_func(void *arg) {
int policy;
int i = 0;
cpu_set_t cpuset;

CPU_ZERO(&cpuset);
CPU_SET(0, &cpuset);
ERROR_CHECK(pthread_setaffinity_np(pthread_self(),
sizeof(cpu_set_t),
&cpuset));

while(i++ < 100) {
sem_wait(&b_sem);
usleep(1000);
sem_post(&a_sem);
pthread_mutex_lock(&pi_mutex);
pthread_mutex_unlock(&pi_mutex);
}
}

void *low_prio_thread_func(void *arg) {
int i = 0;
cpu_set_t cpuset;

CPU_ZERO(&cpuset);
CPU_SET(1, &cpuset);
ERROR_CHECK(pthread_setaffinity_np(pthread_self(),
sizeof(cpu_set_t),
&cpuset));
while(i++ < 100) {
pthread_mutex_lock(&pi_mutex);
sem_post(&b_sem);
sem_wait(&a_sem);
pthread_mutex_unlock(&pi_mutex);
}
}

int main()
{
pthread_t rt_thread;
pthread_t low_prio_thread;
pthread_attr_t rt_thread_attrs;
pthread_attr_t low_prio_thread_attrs;
struct sched_param rt_thread_sched_params;
struct sched_param low_prio_thread_sched_params;

pthread_mutexattr_t mutex_attrs;

sem_init(&a_sem, 0, 0);
sem_init(&b_sem, 0, 0);

ERROR_CHECK(pthread_mutexattr_init(&mutex_attrs));
ERROR_CHECK(pthread_mutexattr_setprotocol(&mutex_attrs,
PTHREAD_PRIO_INHERIT));
ERROR_CHECK(pthread_mutex_init(&pi_mutex, &mutex_attrs));

rt_thread_sched_params.sched_priority = 80;
low_prio_thread_sched_params.sched_priority = 0;

pthread_attr_init(&rt_thread_attrs);
pthread_attr_init(&low_prio_thread_attrs);

ERROR_CHECK(pthread_attr_setinheritsched(&rt_thread_attrs,
PTHREAD_EXPLICIT_SCHED));
ERROR_CHECK(pthread_attr_setinheritsched(&low_prio_thread_attrs,
PTHREAD_EXPLICIT_SCHED));

ERROR_CHECK(pthread_attr_setschedpolicy(&rt_thread_attrs,
SCHED_FIFO));
ERROR_CHECK(pthread_attr_setschedpolicy(&low_prio_thread_attrs,
SCHED_OTHER));

ERROR_CHECK(pthread_attr_setschedparam(&rt_thread_attrs,
&rt_thread_sched_params));
ERROR_CHECK(pthread_attr_setschedparam(&low_prio_thread_attrs,
&low_prio_thread_sched_params));

ERROR_CHECK(pthread_create(&rt_thread, &rt_thread_attrs,
rt_thread_func, NULL));
ERROR_CHECK(pthread_create(&low_prio_thread,
&low_prio_thread_attrs,
low_prio_thread_func, NULL));

ERROR_CHECK(pthread_join(rt_thread, NULL));
ERROR_CHECK(pthread_join(low_prio_thread, NULL));

return 0;
}

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