linux-kernel - Re: [PATCH] clockevents: return error from tick_broadcast_oneshot_control if !GENERIC_CLOCKEVENTS

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Date:	Fri, 26 Jun 2015 18:16:46 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Sudeep Holla <sudeep.holla@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	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] clockevents: return error from tick_broadcast_oneshot_control
 if !GENERIC_CLOCKEVENTS_BROADCAST

On 06/26/2015 06:08 PM, Thomas Gleixner wrote:
> On Fri, 26 Jun 2015, Preeti U Murthy wrote:
>> On 06/26/2015 01:17 PM, Thomas Gleixner wrote:
>>>  	if (state == TICK_BROADCAST_ENTER) {
>>> +		/*
>>> +		 * If the current CPU owns the hrtimer broadcast
>>> +		 * mechanism, it cannot go deep idle and we do not add
>>> +		 * the CPU to the broadcast mask. We don't have to go
>>> +		 * through the EXIT path as the local timer is not
>>> +		 * shutdown.
>>> +		 */
>>> +		ret = broadcast_needs_cpu(bc, cpu);
>>> +
>>> +		/*
>>> +		 * If the hrtimer broadcast check tells us that the
>>> +		 * cpu cannot go deep idle, or if the broadcast device
>>> +		 * is in periodic mode, we just return.
>>> +		 */
>>> +		if (ret || tick_broadcast_device.mode == TICKDEV_MODE_PERIODIC)
>>> +			goto out;
>>
>> The check on PERIODIC mode is good, but I don't see the point of moving
>> broadcast_needs_cpu() up above. broadcast_shutdown_local() calls
>> broadcast_needs_cpu() internally.
>>
>> Besides, by jumping to 'out', we will miss programming the broadcast
>> hrtimer in tick_broadcast_set_event() below, if the cpu happen to be the
>> broadcast cpu(which is why it was not allowed to go to deep idle).
> 
> Hmm. Need to think a bit more about this convoluted maze ...

I think you cover all cases just by having that check on periodic mode.
This covers the nohz_full=n,highres=n, TICK_ONESHOT=y and
GENERIC_CLOCKEVENTS_BROADCAST=y. broadcast_needs_cpu() should remain
where it was though.

And of course, the additional patch on tick_broadcast_device.evtdev ==
NULL in BROADCAST_ON.

Regards
Preeti U Murthy
> 

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