linux-kernel - Re: nohz problem with idle time on old hardware

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Date:	Wed, 9 Apr 2014 21:04:46 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Matthew Whitehead <tedheadster@...il.com>,
	John Stultz <john.stultz@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	mwhitehe@...hat.com, Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: nohz problem with idle time on old hardware

On 9 April 2014 21:01, Steven Rostedt <rostedt@...dmis.org> wrote:
>> Do we? This is only called by tick_check_oneshot_change() which has the
>> following:
>>
>> int tick_check_oneshot_change(int allow_nohz)
>> {
>>       struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
>>
>>       if (!test_and_clear_bit(0, &ts->check_clocks))
>>               return 0;
>>
>>       if (ts->nohz_mode != NOHZ_MODE_INACTIVE)
>>               return 0;
>>
>>       if (!timekeeping_valid_for_hres() || !tick_is_oneshot_available())
>>               return 0;
>>
>>       if (!allow_nohz)
>>               return 1;
>>
>>       tick_nohz_switch_to_nohz();
>>       return 0;
>> }
>>
>> How often does it make it to that last check?

Probably we will reach here only once per boot per cpu.

> Hmm, looking at the code, I see it probably should still do the check.

But still we need it for that one time. :)
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