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

Marco Elver
2023-01-26T17:52:18Z

On Thu, 26 Jan 2023 at 16:41, Dmitry Vyukov <dvyukov@google.com> 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@google.com>
> Suggested-by: Oleg Nesterov <oleg@redhat.com>
> Reviewed-by: Oleg Nesterov <oleg@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Eric W. Biederman <ebiederm@xmission.com>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Marco Elver <elver@google.com>
>
> ---
>
> 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@google.com>

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
>