Re: [PATCH v6 1/2] posix-timers: Prefer delivery of signals to the current thread

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Mar 16, 2023 at 01:30:27PM +0100, Marco Elver wrote:
> From: Dmitry Vyukov <dvyukov@google.com>
> 
> POSIX timers using the CLOCK_PROCESS_CPUTIME_ID clock prefer the main
> thread of a thread group for signal delivery.     However, this has a
> significant downside: it requires waking up a potentially idle thread.
> 
> Instead, prefer to deliver signals to the current thread (in the same
> thread group) if SIGEV_THREAD_ID is not set by the user. This does not
> change guaranteed semantics, since POSIX process CPU time timers have
> never guaranteed that signal delivery is to a specific thread (without
> SIGEV_THREAD_ID set).
> 
> The effect is that we no longer wake up potentially idle threads, and
> the kernel is no longer biased towards delivering the timer signal to
> any particular thread (which better distributes the timer signals esp.
> when multiple timers fire concurrently).
> 
> Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
> Suggested-by: Oleg Nesterov <oleg@redhat.com>
> Reviewed-by: Oleg Nesterov <oleg@redhat.com>
> Signed-off-by: Marco Elver <elver@google.com>

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

> ---
>  kernel/signal.c | 25 ++++++++++++++++++++++---
>  1 file changed, 22 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 8cb28f1df294..605445fa27d4 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 +1969,23 @@ int send_sigqueue(struct sigqueue *q, struct pid *pid, enum pid_type type)
>  
>  	ret = -1;
>  	rcu_read_lock();
> +	/*
> +	 * This function is used by POSIX timers to deliver a timer signal.
> +	 * Where type is PIDTYPE_PID (such as for timers with SIGEV_THREAD_ID
> +	 * set), the signal must be delivered to the specific thread (queues
> +	 * into t->pending).
> +	 *
> +	 * Where type is not PIDTYPE_PID, signals must just be delivered to the
> +	 * current process. In this case, prefer to deliver to current if it is
> +	 * in the same thread group as the target, as it avoids unnecessarily
> +	 * waking up a potentially idle task.
> +	 */
>  	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 */
> @@ -1993,6 +2007,11 @@ int send_sigqueue(struct sigqueue *q, struct pid *pid, enum pid_type type)
>  	q->info.si_overrun = 0;
>  
>  	signalfd_notify(t, sig);
> +	/*
> +	 * If the type is not PIDTYPE_PID, we just use shared_pending, which
> +	 * won't guarantee that the specified task will receive the signal, but
> +	 * is sufficient if t==current in the common case.
> +	 */
>  	pending = (type != PIDTYPE_PID) ? &t->signal->shared_pending : &t->pending;
>  	list_add_tail(&q->list, &pending->list);
>  	sigaddset(&pending->signal, sig);
> -- 
> 2.40.0.rc1.284.g88254d51c5-goog
>