Re: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops -5.5% regression

[Eric W. Biederman]

Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:

> [ Adding a few more people that tend to be involved in signal
> handling. Just in case - even if they probably don't care ]
>
> On Mon, Feb 24, 2020 at 12:09 PM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>
>> TOTALLY UNTESTED patch attached. It may be completely buggy garbage,
>> but it _looks_ trivial enough.
>
> I've tested it, and the profiles on the silly microbenchmark look much
> nicer. Now it's just the sigpending update shows up, the refcount case
> clearly still occasionally happens, but it's now in the noise.
>
> I made slight changes to the __sigqueue_alloc() case to generate
> better code: since we now use that atomic_inc_return() anyway, we
> might as well then use the value that is returned for the
> RLIMIT_SIGPENDING check too, instead of reading it again.
>
> That might avoid another potential cacheline bounce, plus the
> generated code just looks better.
>
> Updated (and now slightly tested!) patch attached.
>
> It would be interesting if this is noticeable on your benchmark
> numbers. I didn't actually _time_ anything, I just looked at profiles.
>
> But my setup clearly isn't going to see the horrible contention case
> anyway, so my timing numbers wouldn't be all that interesting.
>
>              Linus

I keep looking at your patch and wondering if there isn't a way
to remove the uid refcount entirely on this path.

Linus I might be wrong but I have this sense that your change will only
help when signal delivery is backed up.  I expect in the common case
there won't be any pending signals outstanding for the user.

Not that I see anything bad jumping out at me from your patch.

Eric



>  kernel/signal.c | 23 ++++++++++++++---------
>  1 file changed, 14 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 9ad8dea93dbb..5b2396350dd1 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -413,27 +413,32 @@ __sigqueue_alloc(int sig, struct task_struct *t, gfp_t flags, int override_rlimi
>  {
>  	struct sigqueue *q = NULL;
>  	struct user_struct *user;
> +	int sigpending;
>  
>  	/*
>  	 * Protect access to @t credentials. This can go away when all
>  	 * callers hold rcu read lock.
> +	 *
> +	 * NOTE! A pending signal will hold on to the user refcount,
> +	 * and we get/put the refcount only when the sigpending count
> +	 * changes from/to zero.
>  	 */
>  	rcu_read_lock();
> -	user = get_uid(__task_cred(t)->user);
> -	atomic_inc(&user->sigpending);
> +	user = __task_cred(t)->user;
> +	sigpending = atomic_inc_return(&user->sigpending);
> +	if (sigpending == 1)
> +		get_uid(user);
>  	rcu_read_unlock();
>  
> -	if (override_rlimit ||
> -	    atomic_read(&user->sigpending) <=
> -			task_rlimit(t, RLIMIT_SIGPENDING)) {
> +	if (override_rlimit || likely(sigpending <= task_rlimit(t, RLIMIT_SIGPENDING))) {
>  		q = kmem_cache_alloc(sigqueue_cachep, flags);
>  	} else {
>  		print_dropped_signal(sig);
>  	}
>  
>  	if (unlikely(q == NULL)) {
> -		atomic_dec(&user->sigpending);
> -		free_uid(user);
> +		if (atomic_dec_and_test(&user->sigpending))
> +			free_uid(user);
>  	} else {
>  		INIT_LIST_HEAD(&q->list);
>  		q->flags = 0;
> @@ -447,8 +452,8 @@ static void __sigqueue_free(struct sigqueue *q)
>  {
>  	if (q->flags & SIGQUEUE_PREALLOC)
>  		return;
> -	atomic_dec(&q->user->sigpending);
> -	free_uid(q->user);
> +	if (atomic_dec_and_test(&q->user->sigpending))
> +		free_uid(q->user);
>  	kmem_cache_free(sigqueue_cachep, q);
>  }
>