linux-kernel - Re: [PATCH 1/3] pm/qos: allow state control of qos class

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Message-ID: <7329787.rQFSuIRyzO@vostro.rjw.lan>
Date:	Wed, 22 Jan 2014 00:15:44 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Linux PM <linux-pm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Len Brown <len.brown@...el.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Zhang Rui <rui.zhang@...el.com>
Subject: Re: [PATCH 1/3] pm/qos: allow state control of qos class

On Tuesday, January 21, 2014 02:10:42 PM Jacob Pan wrote:
> On Thu, 16 Jan 2014 02:17:01 +0100
> "Rafael J. Wysocki" <rjw@...ysocki.net> wrote:
> 
> > On Wednesday, November 27, 2013 12:28:16 AM Rafael J. Wysocki wrote:
> > > On 11/27/2013 12:20 AM, Jacob Pan wrote:
> > > > When power capping or thermal control is needed, CPU QOS latency
> > > > cannot be satisfied. This patch adds a state variable to indicate
> > > > whether a QOS class (including all constraint requests) should be
> > > > ignored.
> > > >
> > > > Signed-off-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> > > 
> > > Honestly, I don't like this.  I know the motivation and what you're 
> > > trying to achieve, but I don't like the approach.
> > > 
> > > I need to think a bit more about that.
> > 
> > So the reason I don't like this patch is mainly because it affects
> > all of the users of struct pm_qos_constraints and
> > pm_qos_read_value(), which include device PM QoS among other things,
> > but it only really needs to affect PM_QOS_CPU_DMA_LATENCY.
> > 
> > I would add a special routine, say pm_qos_cpu_dma_latency(), for
> > reading the current effective PM_QOS_CPU_DMA_LATENCY constraint and
> > checking whether or not it should be ignored.  Then, I'd make cpuidle
> > use that.
> > 
> Agreed, it was a little too broad. I will send an updated patch soon.
> 
> Alternatively, can we add a special check for ignored system wide QOS
> class in:
> int pm_qos_request(int pm_qos_class)
> 
> i.e.
> diff --git a/kernel/power/qos.c b/kernel/power/qos.c
> index 8dff9b4..9342da4 100644
> --- a/kernel/power/qos.c
> +++ b/kernel/power/qos.c
> @@ -286,10 +286,28 @@ bool pm_qos_update_flags(struct pm_qos_flags *pqf,
>   */
>  int pm_qos_request(int pm_qos_class)
>  {
> -       return pm_qos_read_value(pm_qos_array[pm_qos_class]->constraints);
> +       struct pm_qos_constraints *c;
> +
> +       c = pm_qos_array[pm_qos_class]->constraints;
> +       if (c->state == PM_QOS_CONSTRAINT_IGNORED)
> +               return PM_QOS_DEFAULT_VALUE;
> +       return pm_qos_read_value(c);
> 
> 
> Then we don't have to add a special routine just for CPU_DMA_LATENCY
> class. It does not affect other system wide QOS classes unless the
> state is set to be ignored.

Yes, but then the check has to be done regardless which is slightly inefficient
and I'm not sure if we need/want a mechanism to set "ignored" for all classes.

It actually is specific to CPU in practice, so I'd prefer to make it specific
in the code as well.

Thanks,
Rafael

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