linux-kernel - Re: [patch 1/2] tick/broadcast: Prevent deep idle states if no broadcast device available

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Date:	Mon, 06 Jul 2015 16:44:22 +0100
From:	Sudeep Holla <sudeep.holla@....com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Sudeep Holla <sudeep.holla@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Suzuki Poulose <Suzuki.Poulose@....com>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [patch 1/2] tick/broadcast: Prevent deep idle states if no broadcast
 device available



On 06/07/15 16:35, Thomas Gleixner wrote:
> On Mon, 6 Jul 2015, Sudeep Holla wrote:
>> On 05/07/15 21:53, Thomas Gleixner wrote:
>>> If no broadcast device is installed and the cpu local timers stop in
>>> deeper idle states, then there is currently nothing telling the idle
>>> code that it should not go into deep idle states, so the timers stop
>>> and nothing wakes up the cpus.
>>>
>>> Make the broadcast_enter/exit() functions independent of the
>>> configuration options and always check on enter:
>>>
>>> - whether the cpu local device is affected by idle states
>>> - whether a broadcast device is available
>>>
>>> This covers all possible config combinations including
>>> CONFIG_BROADCAST=n.
>>>
>>> Reported-by: Sudeep Holla <sudeep.holla@....com>
>>
>> Sorry for the delay, took a while testing few configuration:
>>
>> +--------------+--------+--------------+--------------------+
>> |   Configs    | PERIOD | HRTimers+NOHz|Cmdline(HR+NoHZ=off)|
>> +--------------+--------+--------------+--------------------+
>> | UP w/o H/W BC|   OK   |      OK      |        OK          |
>> +--------------+--------+--------------+--------------------+
>> | UP w/ H/W BC |   OK   |      OK      |        OK          |
>> +--------------+--------+--------------+--------------------+
>> |SMP w/o H/W BC|   OK*  |      OK      |        Not OK(**)  |
>> +--------------+--------+--------------+--------------------+
>> |SMP w/ H/W BC |   OK   |      OK      |        OK          |
>> +--------------+--------+--------------+--------------------+
>>
>> H/W BC - Hardware Broadcast Timer source
>>
>> (*) None of the CPUs enters deeper idle states losing local timers
>>
>> (**)SMP build without Hardware Broadcast Timer source(i.e. one cpu is
>> the broadcast source) with HRTimers+NOHz configs but disabled in cmdline
>> fails to boot.
>
> That's using the hrtimer broadcast mechanism, right?
>

Correct.

>> On connecting debugger, I found all the cpus are in
>> shallow idle state(i.e. WFI in ARM) but with interrupts disabled.
>
> And that means?
>

All CPUs have entered WFI with interrupts disabled and no way to wake 
them up.

>> I am not really keen on the failing configuration. We have never tested
>> that before, though I found it working with CPUIdle disabled.
>
> Well, we should figure out what happens while we are at it before
> everything gets paged out again.
>

True. I just wanted to mention that this patch works for all the
practical purposes.

> In the case of CONFIG_NOHZ=n and CONFIG_HIGHRES=n the broadcast
> hrtimer is not compiled as it depends on CONFIG_TICK_ONESHOT, so it
> works via the bc.evtdev == NULL check.
>
> With either option enabled CONFIG_TICK_ONESHOT gets set, so the
> broadcast timer gets installed but somehow does not work proper if
> nohz and highres are disabled on the kernel command line.
>

Let me know if you want to test something to help debug this configuration.

Regards,
Sudeep
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