linux-kernel - Re: [RFC 4/6] sched: powerpc: create a dedicated topology table

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Date:	Wed, 12 Mar 2014 08:44:59 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <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>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>
Subject: Re: [RFC 4/6] sched: powerpc: create a dedicated topology table

On 12 March 2014 05:42, Preeti U Murthy <preeti@...ux.vnet.ibm.com> wrote:
> On 03/11/2014 06:48 PM, Vincent Guittot wrote:
>> On 11 March 2014 11:08, Preeti U Murthy <preeti@...ux.vnet.ibm.com> wrote:
>>> Hi Vincent,
>>>
>>> On 03/05/2014 12:48 PM, Vincent Guittot wrote:
>>>> Create a dedicated topology table for handling asymetric feature.
>>>> The current proposal creates a new level which describes which groups of CPUs
>>>> take adavantge of SD_ASYM_PACKING. The useless level will be removed during the
>>>> build of the sched_domain topology.
>>>>
>>>> Another solution would be to set SD_ASYM_PACKING in the sd_flags of SMT level
>>>> during the boot sequence and before the build of the sched_domain topology.
>>>
>>> Is the below what you mean as the other solution? If it is so, I would
>>> strongly recommend this approach rather than adding another level to the
>>> topology level to represent the asymmetric behaviour.
>>>
>>> +static struct sched_domain_topology_level powerpc_topology[] = {
>>> +#ifdef CONFIG_SCHED_SMT
>>> +       { cpu_smt_mask, SD_SHARE_CPUPOWER | SD_SHARE_PKG_RESOURCES,
>>> SD_INIT_NAME(SMT) | arch_sd_sibling_asym_packing() },
>>> +#endif
>>> +       { cpu_cpu_mask, SD_INIT_NAME(DIE) },
>>> +       { NULL, },
>>> +};
>>
>> Not exactly like that but something like below
>>
>> +static struct sched_domain_topology_level powerpc_topology[] = {
>> +#ifdef CONFIG_SCHED_SMT
>> + { cpu_smt_mask, SD_SHARE_CPUPOWER | SD_SHARE_PKG_RESOURCES,
>> SD_INIT_NAME(SMT) },
>> +#endif
>> + { cpu_cpu_mask, SD_INIT_NAME(DIE) },
>> + { NULL, },
>> +};
>> +
>> +static void __init set_sched_topology(void)
>> +{
>> +#ifdef CONFIG_SCHED_SMT
>> + powerpc_topology[0].sd_flags |= arch_sd_sibling_asym_packing();
>> +#endif
>> + sched_domain_topology = powerpc_topology;
>> +}
>
> I think we can set it in powerpc_topology[] and not bother about setting
> additional flags outside of this array. It is clearer this way.

IIRC, the arch_sd_sibling_asym_packing of powerpc can be set at
runtime which prevents it from being put directly in the table. Or it
means that we should use a function pointer in the table for setting
flags instead of a static value like the current proposal.

>
> On an additional note, on powerpc we would want to clear the
> SD_SHARE_POWERDOMAIN flag at the CPU domain. On Power8, considering we
> have 8 threads per core, we would want to consolidate tasks atleast upto
> 4 threads without significant performance impact before spilling over to
> the other cores. By doing so, besides making use of the higher power of
> the core we could do cpuidle management at the core level for the
> remaining idle cores as a result of this consolidation.

OK. i will add the SD_SHARE_POWERDOMAIN like below

Thanks
Vincent

>
> So the powerpc_topology[] would be something like the below:
>
> +static struct sched_domain_topology_level powerpc_topology[] = {
> +#ifdef CONFIG_SCHED_SMT
> +       { cpu_smt_mask, SD_SHARE_CPUPOWER | SD_SHARE_PKG_RESOURCES,
> SD_INIT_NAME(SMT) | arch_sd_sibling_asym_packing() | SD_SHARE_POWERDOMAIN },
> +#endif
> +       { cpu_cpu_mask, SD_INIT_NAME(DIE) },
> +       { NULL, },
> +};
>
> The amount of consolidation to the threads in a core, we will probably
> take care of it in cpu capacity or a similar parameter, but the above
> topology level would be required to request the scheduler to try
> consolidating tasks to cores till the cpu capacity(3/4/5 threads) has
> exceeded.
>
> Regards
> Preeti U Murthy
>
>
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