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

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Message-ID: <CAFTL4hzi-NdsQSEC5emG8ikKSMzuh3S0jfS3NupLrjjjiYR5Ag@mail.gmail.com>
Date:	Fri, 4 Jan 2013 13:51:46 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.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>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 07/27] nohz: Assign timekeeping duty to a non-full-nohz CPU

2013/1/2 Christoph Lameter <cl@...ux.com>:
> 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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