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Date: Mon, 13 Aug 2018 17:05:07 -0700
From: Steve Muckle <smuckle@...gle.com>
To: Todd Kjos <tkjos@...gle.com>, LKML <linux-kernel@...r.kernel.org>,
linux-pm@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Paul Turner <pjt@...gle.com>
Cc: John Dias <joaodias@...gle.com>,
Quentin Perret <quentin.perret@....com>,
Patrick Bellasi <Patrick.Bellasi@....com>,
Chris Redpath <Chris.Redpath@....com>,
Morten Rasmussen <Morten.Rasmussen@....com>,
Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [RFC] vruntime updated incorrectly when rt_mutex boots prio?
On 08/07/2018 10:40 AM, 'Todd Kjos' via kernel-team wrote:
> This issue was discovered on a 4.9-based android device, but the
> relevant mainline code appears to be the same. The symptom is that
> over time the some workloads become sluggish resulting in missed
> frames or sluggishness. It appears to be the same issue described in
> http://lists.infradead.org/pipermail/linux-arm-kernel/2018-March/567836.html.
>
> Here is the scenario: A task is deactivated while still in the fair
> class. The task is then boosted to RT, so rt_mutex_setprio() is
> called. This changes the task to RT and calls check_class_changed(),
> which eventually calls detach_task_cfs_rq(), which is where
> vruntime_normalized() sees that the task's state is TASK_WAKING, which
> results in skipping the subtraction of the rq's min_vruntime from the
> task's vruntime. Later, when the prio is deboosted and the task is
> moved back to the fair class, the fair rq's min_vruntime is added to
> the task's vruntime, resulting in vruntime inflation.
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 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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