[PATCH] show message when exceeded rlimit of pending signals

[Naohiro Ooiwa <nooiwa@miraclelinux.com>]

Hi Andrew,

I was glad to talk to you in Japan Linux Symposium.
I'm writing about it.


I'm working to support kernel.
Recently, I got a inquiry about unexpected system behavior.
I analyzed application of our customer includeing kernel.

Eventually, there was no bug in application or kernel.
I found the cause was the limit of pending signals.
I ran following command. and system behaved expectedly.
   # ulimit -i unlimited

When system behaved unexpectedly, the timer_create() in application
had returned -EAGAIN value.
But we can't imagine the -EAGAIN means that it exceeded limit of
pending signals at all.

Then I thought kernel should at least show some message about it.
And I tried to create a patch.

I'm sure that system engineeres will not have to have the same experience as I did.
How do you think about this idea ?

Thank you
Naohiro Ooiwa.

Signed-off-by: Naohiro Ooiwa <nooiwa@miraclelinux.com>
---
 kernel/signal.c |   13 +++++++++++++
 1 files changed, 13 insertions(+), 0 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 6705320..0bc4934 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -188,6 +188,9 @@ int next_signal(struct sigpending *pending, sigset_t *mask)
 	return sig;
 }

+#define MAX_RLIMIT_CAUTION 5
+static int rlimit_caution_count = 0;
+
 /*
  * allocate a new signal queue record
  * - this may be called without locks if and only if t == current, otherwise an
@@ -211,6 +214,16 @@ static struct sigqueue *__sigqueue_alloc(struct task_struct *t, gfp_t flags,
 	    atomic_read(&user->sigpending) <=
 			t->signal->rlim[RLIMIT_SIGPENDING].rlim_cur)
 		q = kmem_cache_alloc(sigqueue_cachep, flags);
+	else {
+		if (rlimit_caution_count <= MAX_RLIMIT_CAUTION ){
+			printk(KERN_WARNING "reached the limit of pending signalis on pid %d\n", current->pid);
+			/* Last time, show the advice */
+			if (rlimit_caution_count == MAX_RLIMIT_CAUTION)
+				printk(KERN_WARNING "If unexpected your system behavior, you can try ulimit -i unlimited\n");
+			rlimit_caution_count++;
+		}
+	}
+
 	if (unlikely(q == NULL)) {
 		atomic_dec(&user->sigpending);
 		free_uid(user);
-- 
1.5.4.1