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 16:09:31 +0100
From:	Sudeep Holla <sudeep.holla@....com>
To:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>
CC:	Sudeep Holla <sudeep.holla@....com>,
	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

Hi Thomas,

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

I am not really keen on the failing configuration. We have never tested
that before, though I found it working with CPUIdle disabled.

So please feel free to add:
Tested-by: Sudeep Holla <sudeep.holla@....com>

Regards,
Sudeep
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