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

[Linus Torvalds]

On Mon, Feb 24, 2020 at 11:24 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> I don't know. This does not seem to be a particularly serious load.
> But it does feel like it should be possible to combine the two atomic
> accesses into one, where you don't need to do the refcount thing
> except for the case where sigcount goes from zero to non-zero (and
> back to zero again).

Ok, that looks just as simple as I thought it would be.

TOTALLY UNTESTED patch attached. It may be completely buggy garbage,
but it _looks_ trivial enough. Just make the rule be that "if we have
any user->sigpending cases, we'll get a ref to the user for the first
one, and drop it only when getting rid of the last one".

So it might be worth testing this. But again: I have NOT done so.

There might be some silly reason why this doesn't work because I just
did the tests wrong or missed some case.

Or there might be some subtle reason why it doesn't work because I
didn't think this through properly.

But it _looks_ obvious and simple enough. And it compiles for me. So
maybe it works.

              Linus
 kernel/signal.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 9ad8dea93dbb..00addaa8319f 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -417,10 +417,15 @@ __sigqueue_alloc(int sig, struct task_struct *t, gfp_t flags, int override_rlimi
 	/*
 	 * 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;
+	if (atomic_inc_return(&user->sigpending) == 1)
+		get_uid(user);
 	rcu_read_unlock();
 
 	if (override_rlimit ||
@@ -432,8 +437,8 @@ __sigqueue_alloc(int sig, struct task_struct *t, gfp_t flags, int override_rlimi
 	}
 
 	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);
 }