linux-kernel - Re: [PATCH 20/24] nohz: Full dynticks mode

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Date:	Sat, 29 Dec 2012 14:21:06 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 20/24] nohz: Full dynticks mode

2012/12/26 Namhyung Kim <namhyung@...nel.org>:
> Hi Frederic,
>
> On Thu, 20 Dec 2012 19:33:07 +0100, Frederic Weisbecker wrote:
>> When a CPU is in full dynticks mode, try to switch
>> it to nohz mode from the interrupt exit path if it is
>> running a single non-idle task.
>>
>> Then restart the tick if necessary if we are enqueuing a
>> second task while the timer is stopped, so that the scheduler
>> tick is rearmed.
>>
>> [TODO: Check remaining things to be done from scheduler_tick()]
>>
>> [ Included build fix from Geoff Levand ]
> [snip]
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index db3d4df..f3d8f4a 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -943,6 +943,14 @@ static inline u64 steal_ticks(u64 steal)
>>  static inline void inc_nr_running(struct rq *rq)
>>  {
>>       rq->nr_running++;
>> +
>> +     if (rq->nr_running == 2) {
>> +             if (tick_nohz_full_cpu(rq->cpu)) {
>> +                     /* Order rq->nr_running write against the IPI */
>> +                     smp_wmb();
>> +                     smp_send_reschedule(rq->cpu);
>> +             }
>> +     }
>
> This block should be guarded with #ifdef CONFIG_SMP, otherwise:
>
>   CC      kernel/sched/core.o
> In file included from /home/namhyung/project/linux/kernel/sched/core.c:85:0:
> /home/namhyung/project/linux/kernel/sched/sched.h: In function ‘inc_nr_running’:
> /home/namhyung/project/linux/kernel/sched/sched.h:960:28: error: ‘struct rq’ has no member named ‘cpu’
> /home/namhyung/project/linux/kernel/sched/sched.h:963:26: error: ‘struct rq’ has no member named ‘cpu’

Right, to fix this, I'm making it depend on CONFIG_NO_HZ_FULL and make
CONFIG_NO_HZ_FULL depend on SMP for the next version.

Thanks!
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