signal: Print warning message when dropping signals

When the system has too many timers or too many aggregate
queued signals, the EAGAIN error is returned to application
from kernel, including timer_create() [POSIX.1b].

It means that the app exceeded the limit of pending signals,
but in general application writers do not expect this
outcome and the current silent failure can cause rare app
failures under very high load.

This patch adds a new message when we reach the limit
and if print_fatal_signals is enabled:

    task/1234: reached RLIMIT_SIGPENDING, dropping signal

If you see this message and your system behaved unexpectedly,
you can run following command to lift the limit:

   # ulimit -i unlimited

With help from Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>.

Signed-off-by: Naohiro Ooiwa <nooiwa@miraclelinux.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: oleg@redhat.com
LKML-Reference: <4AF6E7E2.9080406@miraclelinux.com>
[ Modified a few small details, gave surrounding code some love. ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 9 insertions, 2 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 9107b387e91f..3bbd92f805a6 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2032,8 +2032,15 @@ and is between 256 and 4096 characters. It is defined in the file
 
         print-fatal-signals=
                         [KNL] debug: print fatal signals
-                        print-fatal-signals=1: print segfault info to
-                        the kernel console.
+
+                        If enabled, warn about various signal handling
+                        related application anomalies: too many signals,
+                        too many POSIX.1 timers, fatal signals causing a
+                        coredump - etc.
+
+                        If you hit the warning due to signal overflow,
+                        you might want to try "ulimit -i unlimited".
+
                         default: off.
 
         printk.time=    Show timing data prefixed to each printk message line