Re: [PATCH v3 16/18] perf sched: Fixes for thread safety analysis

Adrian Hunter
2022-08-26T19:36:18Z

On 26/08/22 21:26, Namhyung Kim wrote:
> On Fri, Aug 26, 2022 at 10:48 AM Ian Rogers <irogers@google.com> wrote:
>>
>> On Fri, Aug 26, 2022 at 10:41 AM Adrian Hunter <adrian.hunter@intel.com> wrote:
>>>
>>> On 26/08/22 19:06, Ian Rogers wrote:
>>>> On Fri, Aug 26, 2022 at 5:12 AM Adrian Hunter <adrian.hunter@intel.com> wrote:
>>>>>
>>>>> On 24/08/22 18:38, Ian Rogers wrote:
>>>>>> Add annotations to describe lock behavior. Add unlocks so that mutexes
>>>>>> aren't conditionally held on exit from perf_sched__replay. Add an exit
>>>>>> variable so that thread_func can terminate, rather than leaving the
>>>>>> threads blocked on mutexes.
>>>>>>
>>>>>> Signed-off-by: Ian Rogers <irogers@google.com>
>>>>>> ---
>>>>>> tools/perf/builtin-sched.c | 46 ++++++++++++++++++++++++--------------
>>>>>> 1 file changed, 29 insertions(+), 17 deletions(-)
>>>>>>
>>>>>> diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
>>>>>> index 7e4006d6b8bc..b483ff0d432e 100644
>>>>>> --- a/tools/perf/builtin-sched.c
>>>>>> +++ b/tools/perf/builtin-sched.c
>>>>>> @@ -246,6 +246,7 @@ struct perf_sched {
>>>>>> const char *time_str;
>>>>>> struct perf_time_interval ptime;
>>>>>> struct perf_time_interval hist_time;
>>>>>> + volatile bool thread_funcs_exit;
>>>>>> };
>>>>>>
>>>>>> /* per thread run time data */
>>>>>> @@ -633,31 +634,34 @@ static void *thread_func(void *ctx)
>>>>>> prctl(PR_SET_NAME, comm2);
>>>>>> if (fd < 0)
>>>>>> return NULL;
>>>>>> -again:
>>>>>> - ret = sem_post(&this_task->ready_for_work);
>>>>>> - BUG_ON(ret);
>>>>>> - mutex_lock(&sched->start_work_mutex);
>>>>>> - mutex_unlock(&sched->start_work_mutex);
>>>>>>
>>>>>> - cpu_usage_0 = get_cpu_usage_nsec_self(fd);
>>>>>> + while (!sched->thread_funcs_exit) {
>>>>>> + ret = sem_post(&this_task->ready_for_work);
>>>>>> + BUG_ON(ret);
>>>>>> + mutex_lock(&sched->start_work_mutex);
>>>>>> + mutex_unlock(&sched->start_work_mutex);
>>>>>>
>>>>>> - for (i = 0; i < this_task->nr_events; i++) {
>>>>>> - this_task->curr_event = i;
>>>>>> - perf_sched__process_event(sched, this_task->atoms[i]);
>>>>>> - }
>>>>>> + cpu_usage_0 = get_cpu_usage_nsec_self(fd);
>>>>>>
>>>>>> - cpu_usage_1 = get_cpu_usage_nsec_self(fd);
>>>>>> - this_task->cpu_usage = cpu_usage_1 - cpu_usage_0;
>>>>>> - ret = sem_post(&this_task->work_done_sem);
>>>>>> - BUG_ON(ret);
>>>>>> + for (i = 0; i < this_task->nr_events; i++) {
>>>>>> + this_task->curr_event = i;
>>>>>> + perf_sched__process_event(sched, this_task->atoms[i]);
>>>>>> + }
>>>>>>
>>>>>> - mutex_lock(&sched->work_done_wait_mutex);
>>>>>> - mutex_unlock(&sched->work_done_wait_mutex);
>>>>>> + cpu_usage_1 = get_cpu_usage_nsec_self(fd);
>>>>>> + this_task->cpu_usage = cpu_usage_1 - cpu_usage_0;
>>>>>> + ret = sem_post(&this_task->work_done_sem);
>>>>>> + BUG_ON(ret);
>>>>>>
>>>>>> - goto again;
>>>>>> + mutex_lock(&sched->work_done_wait_mutex);
>>>>>> + mutex_unlock(&sched->work_done_wait_mutex);
>>>>>> + }
>>>>>> + return NULL;
>>>>>> }
>>>>>>
>>>>>> static void create_tasks(struct perf_sched *sched)
>>>>>> + EXCLUSIVE_LOCK_FUNCTION(sched->start_work_mutex)
>>>>>> + EXCLUSIVE_LOCK_FUNCTION(sched->work_done_wait_mutex)
>>>>>> {
>>>>>> struct task_desc *task;
>>>>>> pthread_attr_t attr;
>>>>>> @@ -687,6 +691,8 @@ static void create_tasks(struct perf_sched *sched)
>>>>>> }
>>>>>>
>>>>>> static void wait_for_tasks(struct perf_sched *sched)
>>>>>> + EXCLUSIVE_LOCKS_REQUIRED(sched->work_done_wait_mutex)
>>>>>> + EXCLUSIVE_LOCKS_REQUIRED(sched->start_work_mutex)
>>>>>> {
>>>>>> u64 cpu_usage_0, cpu_usage_1;
>>>>>> struct task_desc *task;
>>>>>> @@ -738,6 +744,8 @@ static void wait_for_tasks(struct perf_sched *sched)
>>>>>> }
>>>>>>
>>>>>> static void run_one_test(struct perf_sched *sched)
>>>>>> + EXCLUSIVE_LOCKS_REQUIRED(sched->work_done_wait_mutex)
>>>>>> + EXCLUSIVE_LOCKS_REQUIRED(sched->start_work_mutex)
>>>>>> {
>>>>>> u64 T0, T1, delta, avg_delta, fluct;
>>>>>>
>>>>>> @@ -3309,11 +3317,15 @@ static int perf_sched__replay(struct perf_sched *sched)
>>>>>> print_task_traces(sched);
>>>>>> add_cross_task_wakeups(sched);
>>>>>>
>>>>>> + sched->thread_funcs_exit = false;
>>>>>> create_tasks(sched);
>>>>>> printf("------------------------------------------------------------\n");
>>>>>> for (i = 0; i < sched->replay_repeat; i++)
>>>>>> run_one_test(sched);
>>>>>>
>>>>>> + sched->thread_funcs_exit = true;
>>>>>> + mutex_unlock(&sched->start_work_mutex);
>>>>>> + mutex_unlock(&sched->work_done_wait_mutex);
>>>>>
>>>>> I think you still need to wait for the threads to exit before
>>>>> destroying the mutexes.
>>>>
>>>> This is a pre-existing issue and beyond the scope of this patch set.
>>>
>>> You added the mutex_destroy functions in patch 8, so it is still
>>> fallout from that.
>>
>> In the previous code the threads were blocked on mutexes that were
>> stack allocated and the stack memory went away. You are correct to say
>> that to those locks I added an init and destroy call. The lifetime of
>> the mutex was wrong previously and remains wrong in this change.
>
> I think you fixed the lifetime issue with sched->thread_funcs_exit here.
> All you need to do is calling pthread_join() after the mutex_unlock, no?

Like this maybe:

diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
index b483ff0d432e..8090c1218855 100644
--- a/tools/perf/builtin-sched.c
+++ b/tools/perf/builtin-sched.c
@@ -3326,6 +3326,13 @@ static int perf_sched__replay(struct perf_sched *sched)
sched->thread_funcs_exit = true;
mutex_unlock(&sched->start_work_mutex);
mutex_unlock(&sched->work_done_wait_mutex);
+ /* Get rid of threads so they won't be upset by mutex destruction */
+ for (i = 0; i < sched->nr_tasks; i++) {
+ int err = pthread_join(sched->tasks[i]->thread, NULL);
+
+ if (err)
+ pr_err("pthread_join() failed for task nr %lu, error %d\n", i, err);
+ }
return 0;
}