linux-kernel - Re: [patch 00/20] timer: Refactor the timer wheel

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Date:	Mon, 13 Jun 2016 09:15:14 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Chris Mason <clm@...com>,
	Arjan van de Ven <arjan@...radead.org>, rt@...utronix.de
Subject: Re: [patch 00/20] timer: Refactor the timer wheel

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 <paulmck@...ux.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 <paulmck@...ux.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
 
 /*