On Mon, Jun 13, 2016 at 08:40:50AM -0000, Thomas Gleixner wrote:
> The current timer wheel has some drawbacks:
>
> 1) Cascading
>
> Cascading can be an unbound operation and is completely pointless in most
> cases because the vast majority of the timer wheel timers are canceled or
> rearmed before expiration.
>
> 2) No fast lookup of the next expiring timer
>
> In NOHZ scenarios the first timer soft interrupt after a long NOHZ period
> must fast forward the base time to current jiffies. As we have no way to
> find the next expiring timer fast, the code loops and increments the base
> time by one and checks for expired timers in each step. I've observed loops
> lasting 1 ms!
>
> There are some other issues caused by the above, but they are minor compare to
> those.
For SMP configurations, this passes light rcutorture testing. For UP
builds, it complains about undefined symbols. Builds succeed with
the following kneejerk patch. Am retesting rcutorture.
Thanx, Paul
------------------------------------------------------------------------
commit 87dbd35cf4034f2b664b6e9d60decd2bdbfc416f
Author: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Date: Mon Jun 13 09:11:41 2016 -0700
Fix undefined get_nohz_timer_target() in non-SMP builds
Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 6e42ada26345..322e52415a29 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -348,6 +348,10 @@ extern int get_nohz_timer_target(void);
#else
static inline void nohz_balance_enter_idle(int cpu) { }
static inline void set_cpu_sd_state_idle(void) { }
+static inline int get_nohz_timer_target(void)
+{
+ return raw_smp_processor_id();
+}
#endif
/*
