linux-kernel - Re: [PATCH v3 1/6] sched: rework of sched

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Date:	Thu, 20 Mar 2014 17:18:07 +0000
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
CC:	"peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tony.luck@...el.com" <tony.luck@...el.com>,
	"fenghua.yu@...el.com" <fenghua.yu@...el.com>,
	"schwidefsky@...ibm.com" <schwidefsky@...ibm.com>,
	"james.hogan@...tec.com" <james.hogan@...tec.com>,
	"cmetcalf@...era.com" <cmetcalf@...era.com>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>
Subject: Re: [PATCH v3 1/6] sched: rework of sched_domain topology definition

On 20/03/14 17:02, Vincent Guittot wrote:
> On 20 March 2014 13:41, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>> On 19/03/14 16:22, Vincent Guittot wrote:
>>> We replace the old way to configure the scheduler topology with a new method
>>> which enables a platform to declare additionnal level (if needed).
>>>
>>> We still have a default topology table definition that can be used by platform
>>> that don't want more level than the SMT, MC, CPU and NUMA ones. This table can
>>> be overwritten by an arch which either wants to add new level where a load balance
>>> make sense like BOOK or powergating level or wants to change the flags
>>> configuration of some levels.
>>>
>>> For each level, we need a function pointer that returns cpumask for each cpu,
>>> a function pointer that returns the flags for the level and a name. Only flags
>>> that describe topology, can be set by an architecture. The current topology
>>> flags are:
>>>  SD_SHARE_CPUPOWER
>>>  SD_SHARE_PKG_RESOURCES
>>>  SD_NUMA
>>>  SD_ASYM_PACKING
>>>
>>> Then, each level must be a subset on the next one. The build sequence of the
>>> sched_domain will take care of removing useless levels like those with 1 CPU
>>> and those with the same CPU span and relevant information for load balancing
>>> than its child.
>>
>> The paragraph above contains important information to set this up
>> correctly, that's why it might be worth clarifying:
>>
>> - "next one" of sd means "child of sd" ?
> 
> It's the next one in the table so the parent in the sched_domain

Right, it's this way around. DIE is parent of MC is parent of GMC. Maybe
you could be more explicit about the parent of relation here?

> 
>> - "subset" means really "subset" and not "proper subset" ?
> 
> yes, it's really "subset" and not "proper subset"
> 
> Vincent
> 
>>
>> On TC2 w/ the following change in cpu_corepower_mask()
>>
>>  const struct cpumask *cpu_corepower_mask(int cpu)
>>  {
>> -       return &cpu_topology[cpu].thread_sibling;
>> +       return cpu_topology[cpu].socket_id ?
>> &cpu_topology[cpu].thread_sibling :
>> +                       &cpu_topology[cpu].core_sibling;
>>  }
>>
>> I get this e.g. for CPU0,2:
>>
>> CPU0: cpu_corepower_mask=0-1  -> GMC is subset of MC
>> CPU0: cpu_coregroup_mask=0-1
>> CPU0: cpu_cpu_mask=0-4
>>
>> CPU2: cpu_corepower_mask=2    -> GMC is proper sunset of MC
>> CPU2: cpu_coregroup_mask=2-4
>> CPU2: cpu_cpu_mask=0-4
>>
>> I assume here that this is a correct set-up.

So this is a correct setup?

>>
>> The domain degenerate part:
>>
>> "useless levels like those with 1 CPU" ... that's the case for GMC level
>> for CPU2,3,4
>>
>> The GMC level is destroyed because of the following code snippet in
>> sd_degenerate(): if (cpumask_weight(sched_domain_span(sd)) == 1)
>>
>> so that's fine.
>>
>> In case of CPU0,1 since GMC and MC have the same span, the code in
>> build_sched_groups() creates only one group for MC  and that's why
>> pflags is altered in sd_parent_degenerate() to SD_WAKE_AFFINE (0x20) and
>> the if condition 'if (~cflags & pflags)' is not hit and
>> sd_parent_degenerate() finally returns 1 for MC.
>>
>> So the "those with the same CPU span and relevant information for load
>> balancing than its child." is not so easy to understand for me. Because
>> both levels have the same span we actually don't take the flags of the
>> parent into consideration which require at least 2 groups.
>>
>> So the TC2 example covers for me two corner cases: (1) The level I want
>> to get rid of only contains 1 CPU (GMC for CPU2,3,4) and (2) The span of
>> the parent level I want to get rid of (MC for CPU0,1) of is the same as
>> the span of the level which should stay.
>>
>> Are these two corner cases the only one supported here? If yes this has
>> to be stated somewhere, otherwise if somebody will try this approach on
>> a different topology, (s)he might be surprised.

Could you please comment on the paragraph above too?

Thanks,

-- Dietmar

>>
>> If we only consider SD_SHARE_POWERDOMAIN for the socket related level,
>> this works fine.
>>
>> I would like to test this on more platforms but I only have my TC2
>> available :-)
>>
>> -- Dietmar
>>
>> [...]
>>
> 


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