Re: [PATCH v4] posix-timers: Prefer delivery of signals to the current thread

[Marco Elver]

On Thu, 26 Jan 2023 at 16:41, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>
> Prefer to deliver signals to the current thread if SIGEV_THREAD_ID
> is not set. We used to prefer the main thread, but delivering
> to the current thread is both faster, and allows to sample actual thread
> activity for CLOCK_PROCESS_CPUTIME_ID timers, and does not change
> the semantics (since we queue into shared_pending, all thread may
> receive the signal in both cases).
>
> Signed-off-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Suggested-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> Reviewed-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Marco Elver <elver@xxxxxxxxxx>
>
> ---
>
> Changes in v4:
>  - restructured checks in send_sigqueue() as suggested
>
> Changes in v3:
>  - switched to the completely different implementation (much simpler)
>    based on the Oleg's idea
>
> Changes in RFC v2:
>  - added additional Cc as Thomas asked
> ---
>  kernel/signal.c                               | 23 +++++-
>  tools/testing/selftests/timers/posix_timers.c | 73 +++++++++++++++++++
>  2 files changed, 93 insertions(+), 3 deletions(-)

Reviewed-by: Marco Elver <elver@xxxxxxxxxx>

Nice - and and given the test, hopefully this behaviour won't regress in future.


> diff --git a/kernel/signal.c b/kernel/signal.c
> index ae26da61c4d9f..706ad3a21933d 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1003,8 +1003,7 @@ static void complete_signal(int sig, struct task_struct *p, enum pid_type type)
>         /*
>          * Now find a thread we can wake up to take the signal off the queue.
>          *
> -        * If the main thread wants the signal, it gets first crack.
> -        * Probably the least surprising to the average bear.
> +        * Try the suggested task first (may or may not be the main thread).
>          */
>         if (wants_signal(sig, p))
>                 t = p;
> @@ -1970,8 +1969,26 @@ int send_sigqueue(struct sigqueue *q, struct pid *pid, enum pid_type type)
>
>         ret = -1;
>         rcu_read_lock();
> +       /*
> +        * This is called from posix timers. SIGEV_THREAD_ID signals
> +        * (type == PIDTYPE_PID) must be delivered to the user-specified
> +        * thread, so we use that and queue into t->pending.
> +        * Non-SIGEV_THREAD_ID signals must be delivered to "the process",
> +        * so we queue into shared_pending, but prefer to deliver to the
> +        * current thread. We used to prefer the main thread, but delivering
> +        * to the current thread is both faster, and allows user-space to
> +        * sample actual thread activity for CLOCK_PROCESS_CPUTIME_ID timers,
> +        * and does not change the semantics (since we queue into
> +        * shared_pending, all thread may receive the signal in both cases).
> +        * Note: current may be from a completely unrelated process for
> +        * non-cpu-time-based timers, we must be careful to not kick it.
> +        */
>         t = pid_task(pid, type);
> -       if (!t || !likely(lock_task_sighand(t, &flags)))
> +       if (!t)
> +               goto ret;
> +       if (type != PIDTYPE_PID && same_thread_group(t, current))
> +               t = current;
> +       if (!likely(lock_task_sighand(t, &flags)))
>                 goto ret;
>
>         ret = 1; /* the signal is ignored */
> diff --git a/tools/testing/selftests/timers/posix_timers.c b/tools/testing/selftests/timers/posix_timers.c
> index 0ba500056e635..fd3b933a191fe 100644
> --- a/tools/testing/selftests/timers/posix_timers.c
> +++ b/tools/testing/selftests/timers/posix_timers.c
> @@ -188,6 +188,76 @@ static int check_timer_create(int which)
>         return 0;
>  }
>
> +int remain;
> +__thread int got_signal;
> +
> +static void *distribution_thr(void *arg) {
> +       while (__atomic_load_n(&remain, __ATOMIC_RELAXED));
> +       return NULL;
> +}
> +
> +static void distribution_handler(int nr)
> +{
> +       if (!__atomic_exchange_n(&got_signal, 1, __ATOMIC_RELAXED))
> +               __atomic_fetch_sub(&remain, 1, __ATOMIC_RELAXED);
> +}
> +
> +/* Test that all running threads receive CLOCK_PROCESS_CPUTIME_ID signals. */
> +static int check_timer_distribution(void)
> +{
> +       int err, i;
> +       timer_t id;
> +       const int nthreads = 10;
> +       pthread_t threads[nthreads];
> +       struct itimerspec val = {
> +               .it_value.tv_sec = 0,
> +               .it_value.tv_nsec = 1000 * 1000,
> +               .it_interval.tv_sec = 0,
> +               .it_interval.tv_nsec = 1000 * 1000,
> +       };
> +
> +       printf("Check timer_create() per process signal distribution... ");
> +       fflush(stdout);
> +
> +       remain = nthreads + 1;  /* worker threads + this thread */
> +       signal(SIGALRM, distribution_handler);
> +       err = timer_create(CLOCK_PROCESS_CPUTIME_ID, NULL, &id);
> +       if (err < 0) {
> +               perror("Can't create timer\n");
> +               return -1;
> +       }
> +       err = timer_settime(id, 0, &val, NULL);
> +       if (err < 0) {
> +               perror("Can't set timer\n");
> +               return -1;
> +       }
> +
> +       for (i = 0; i < nthreads; i++) {
> +               if (pthread_create(&threads[i], NULL, distribution_thr, NULL)) {
> +                       perror("Can't create thread\n");
> +                       return -1;
> +               }
> +       }
> +
> +       /* Wait for all threads to receive the signal. */
> +       while (__atomic_load_n(&remain, __ATOMIC_RELAXED));
> +
> +       for (i = 0; i < nthreads; i++) {
> +               if (pthread_join(threads[i], NULL)) {
> +                       perror("Can't join thread\n");
> +                       return -1;
> +               }
> +       }
> +
> +       if (timer_delete(id)) {
> +               perror("Can't delete timer\n");
> +               return -1;
> +       }
> +
> +       printf("[OK]\n");
> +       return 0;
> +}
> +
>  int main(int argc, char **argv)
>  {
>         printf("Testing posix timers. False negative may happen on CPU execution \n");
> @@ -217,5 +287,8 @@ int main(int argc, char **argv)
>         if (check_timer_create(CLOCK_PROCESS_CPUTIME_ID) < 0)
>                 return ksft_exit_fail();
>
> +       if (check_timer_distribution() < 0)
> +               return ksft_exit_fail();
> +
>         return ksft_exit_pass();
>  }
>
> base-commit: 7c46948a6e9cf47ed03b0d489fde894ad46f1437
> --
> 2.39.1.456.gfc5497dd1b-goog
>