linux-kernel - Re: [PATCH v5] nohz: set isolcpus when nohz

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Date:	Fri, 10 Apr 2015 11:33:18 -0400
From:	Chris Metcalf <cmetcalf@...hip.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5] nohz: set isolcpus when nohz_full is set

On 04/09/2015 09:05 PM, Mike Galbraith wrote:
> On Thu, 2015-04-09 at 19:12 +0200, Peter Zijlstra wrote:
>> On Thu, Apr 09, 2015 at 12:59:39PM -0400, Chris Metcalf wrote:
>>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>>> index 6d77432e14ff..18a961b9beba 100644
>>> --- a/include/linux/sched.h
>>> +++ b/include/linux/sched.h
>>> @@ -323,6 +323,7 @@ struct task_struct;
>>>   extern int lockdep_tasklist_lock_is_held(void);
>>>   #endif /* #ifdef CONFIG_PROVE_RCU */
>>>   
>>> +extern void sched_isolated_map_add(const struct cpumask *);
>>>   extern void sched_init(void);
>>>   extern void sched_init_smp(void);
>>>   extern asmlinkage void schedule_tail(struct task_struct *prev);
>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>>> index f0f831e8a345..b055c5e0e65c 100644
>>> --- a/kernel/sched/core.c
>>> +++ b/kernel/sched/core.c
>>> @@ -5824,6 +5824,11 @@ static int __init isolated_cpu_setup(char
>>> *str)
>>>   
>>>   __setup("isolcpus=", isolated_cpu_setup);
>>>   
>>> +void sched_isolated_map_add(const struct cpumask *cpumask)
>>> +{
>>> +   cpumask_or(cpu_isolated_map, cpu_isolated_map, cpumask);
>>> +}
>>> +
>>>   struct s_data {
>>>      struct sched_domain ** __percpu sd;
>>>      struct root_domain      *rd;
>>> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
>>> index a4c4edac4528..b0092d02ca3f 100644
>>> --- a/kernel/time/tick-sched.c
>>> +++ b/kernel/time/tick-sched.c
>>> @@ -385,6 +385,9 @@ void __init tick_nohz_init(void)
>>>      for_each_cpu(cpu, tick_nohz_full_mask)
>>>              context_tracking_cpu_set(cpu);
>>>   
>>> +   /* It's not meaningful to be nohz without disabling the
>>> scheduler. */
>>> +   sched_isolated_map_add(tick_nohz_full_mask);
>>> +
>>>      cpu_notifier(tick_nohz_cpu_down_callback, 0);
>>>      pr_info("NO_HZ: Full dynticks CPUs: %*pbl.\n",
>>>              cpumask_pr_args(tick_nohz_full_mask));
>> Right, this could work. Although I would suggest adding a comment
>> somewhere that we should be careful with init order. I checked, this
>> appears to be ordered right, but...
> I'd embed it in domain construction, that way it'd be ready for the
> day nohz_full becomes dynamic, and people can start using cpusets to
> set up /tear down isolated sets on the fly.

So, move it to the top of build_sched_domains(), and then for
every "const struct cpumask *cpu_map" argument, create a
temporary cpu_map so we can mask out the nohz_full cores?

The problem is, we already allow partition_sched_domains()
to override "isolcpus=", so it seems appropriate that you should
be able to override "nohz_full=" in the same way, which my
current patch (v6) does.

So I think the proposed solution is certainly no worse than what
we have now in terms of a future migration to cpusets.

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com

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