Re: [PATCH 07/27] nohz: Assign timekeeping duty to a non-full-nohz CPU

[Frederic Weisbecker]

2013/1/2 Christoph Lameter :
> On Sat, 29 Dec 2012, Frederic Weisbecker wrote:
>
>> @@ -163,6 +164,8 @@ static int __init tick_nohz_full_setup(char *str)
>>   return 1;
>>  }
>>  __setup("full_nohz=", tick_nohz_full_setup);
>> +#else
>> +#define have_full_nohz_mask (0)
>>  #endif
>>
>>  /*
>> @@ -512,6 +515,10 @@ static bool can_stop_idle_tick(int cpu, struct 
>> tick_sched *ts)
>>   return false;
>>   }
>>
>> + /* If there are full nohz CPUs around, we need to keep the timekeeping 
>> duty */
>> + if (have_full_nohz_mask && tick_do_timer_cpu == cpu)
>> + return false;
>> +
>>   return true;
>>  }
>>
>>
>
> Ok so I guess this means that if I setup all cpus as nohz then a random
> one will continue to do timekeeping?

In fact, although the code doesn't check that yet, you're supposed to
have at least one online CPU outside the full_nohz mask to handle
that.
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Re: [PATCH 07/27] nohz: Assign timekeeping duty to a non-full-nohz CPU

[Christoph Lameter]

On Sat, 29 Dec 2012, Frederic Weisbecker wrote:

> @@ -163,6 +164,8 @@ static int __init tick_nohz_full_setup(char *str)
>   return 1;
>  }
>  __setup("full_nohz=", tick_nohz_full_setup);
> +#else
> +#define have_full_nohz_mask (0)
>  #endif
>
>  /*
> @@ -512,6 +515,10 @@ static bool can_stop_idle_tick(int cpu, struct 
> tick_sched *ts)
>   return false;
>   }
>
> + /* If there are full nohz CPUs around, we need to keep the timekeeping 
> duty */
> + if (have_full_nohz_mask && tick_do_timer_cpu == cpu)
> + return false;
> +
>   return true;
>  }
>
>

Ok so I guess this means that if I setup all cpus as nohz then a random
one will continue to do timekeeping?
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[PATCH 07/27] nohz: Assign timekeeping duty to a non-full-nohz CPU

[Frederic Weisbecker]

This way the full nohz CPUs can safely run with the tick
stopped with a guarantee that somebody else is taking
care of the jiffies and gtod progression.

NOTE: this doesn't handle CPU hotplug. Also we could use something
more elaborated wrt. powersaving if we have more than one non full-nohz
CPU running. But let's use this KISS solution for now.

Signed-off-by: Frederic Weisbecker 
Cc: Alessio Igor Bogani 
Cc: Andrew Morton 
Cc: Chris Metcalf 
Cc: Christoph Lameter 
Cc: Geoff Levand 
Cc: Gilad Ben Yossef 
Cc: Hakan Akkan 
Cc: Ingo Molnar 
Cc: Paul E. McKenney 
Cc: Paul Gortmaker 
Cc: Peter Zijlstra 
Cc: Thomas Gleixner 
[fix have_nohz_full_mask offcase]
Signed-off-by: Steven Rostedt 
---
 kernel/time/tick-broadcast.c |3 ++-
 kernel/time/tick-common.c|5 -
 kernel/time/tick-sched.c |9 -
 3 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
index f113755..596c547 100644
--- a/kernel/time/tick-broadcast.c
+++ b/kernel/time/tick-broadcast.c
@@ -537,7 +537,8 @@ void tick_broadcast_setup_oneshot(struct clock_event_device 
*bc)
bc->event_handler = tick_handle_oneshot_broadcast;
 
/* Take the do_timer update */
-   tick_do_timer_cpu = cpu;
+   if (!tick_nohz_full_cpu(cpu))
+   tick_do_timer_cpu = cpu;
 
/*
 * We must be careful here. There might be other CPUs
diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index b1600a6..83f2bd9 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -163,7 +163,10 @@ static void tick_setup_device(struct tick_device *td,
 * this cpu:
 */
if (tick_do_timer_cpu == TICK_DO_TIMER_BOOT) {
-   tick_do_timer_cpu = cpu;
+   if (!tick_nohz_full_cpu(cpu))
+   tick_do_timer_cpu = cpu;
+   else
+   tick_do_timer_cpu = TICK_DO_TIMER_NONE;
tick_next_period = ktime_get();
tick_period = ktime_set(0, NSEC_PER_SEC / HZ);
}
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index fac9ba4..f19e8bf 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -112,7 +112,8 @@ static void tick_sched_do_timer(ktime_t now)
 * this duty, then the jiffies update is still serialized by
 * jiffies_lock.
 */
-   if (unlikely(tick_do_timer_cpu == TICK_DO_TIMER_NONE))
+   if (unlikely(tick_do_timer_cpu == TICK_DO_TIMER_NONE)
+   && !tick_nohz_full_cpu(cpu))
tick_do_timer_cpu = cpu;
 #endif
 
@@ -163,6 +164,8 @@ static int __init tick_nohz_full_setup(char *str)
return 1;
 }
 __setup("full_nohz=", tick_nohz_full_setup);
+#else
+#define have_full_nohz_mask (0)
 #endif
 
 /*
@@ -512,6 +515,10 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched 
*ts)
return false;
}
 
+   /* If there are full nohz CPUs around, we need to keep the timekeeping 
duty */
+   if (have_full_nohz_mask && tick_do_timer_cpu == cpu)
+   return false;
+
return true;
 }
 
-- 
1.7.5.4

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