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

[Namhyung Kim]

On Fri, Aug 26, 2022 at 10:48 AM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> On Fri, Aug 26, 2022 at 10:41 AM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
> >
> > On 26/08/22 19:06, Ian Rogers wrote:
> > > On Fri, Aug 26, 2022 at 5:12 AM Adrian Hunter <adrian.hunter@xxxxxxxxx> 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@xxxxxxxxxx>
> > >>> ---
> > >>>  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?

Thanks,
Namhyung