hrtimer: create a "timer_slack" field in the task struct

We want to be able to control the default "rounding" that is used by
select() and poll() and friends. This is a per process property
(so that we can have a "nice" like program to start certain programs with
a looser or stricter rounding) that can be set/get via a prctl().

For this purpose, a field called "timer_slack_ns" is added to the task
struct. In addition, a field called "default_timer_slack"ns" is added
so that tasks easily can temporarily to a more/less accurate slack and then
back to the default.

The default value of the slack is set to 50 usec; this is significantly less
than 2.6.27's average select() and poll() timing error but still allows
the kernel to group timers somewhat to preserve power behavior. Applications
and admins can override this via the prctl()

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/kernel/sys.c b/kernel/sys.c
index 038a7bc0901d..1b96401a0576 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1727,6 +1727,16 @@ asmlinkage long sys_prctl(int option, unsigned long arg2, unsigned long arg3,
                 case PR_SET_TSC:
                         error = SET_TSC_CTL(arg2);
                         break;
+                case PR_GET_TIMERSLACK:
+                        error = current->timer_slack_ns;
+                        break;
+                case PR_SET_TIMERSLACK:
+                        if (arg2 <= 0)
+                                current->timer_slack_ns =
+                                        current->default_timer_slack_ns;
+                        else
+                                current->timer_slack_ns = arg2;
+                        break;
                 default:
                         error = -EINVAL;
                         break;