From: Valentin Schneider <valentin.schneider@arm.com>
To: peterz@infradead.org
Cc: Gautham R Shenoy <ego@linux.vnet.ibm.com>,
Michael Neuling <mikey@neuling.org>,
Vincent Guittot <vincent.guittot@linaro.org>,
Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
Rik van Riel <riel@surriel.com>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
LKML <linux-kernel@vger.kernel.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Thomas Gleixner <tglx@linutronix.de>,
Mel Gorman <mgorman@techsingularity.net>,
Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH 2/2] powerpc/topology: Override cpu_smt_mask
Date: Tue, 04 Aug 2020 12:02:49 +0100 [thread overview]
Message-ID: <jhjeeomish2.mognet@arm.com> (raw)
In-Reply-To: <20200804104642.GC2657@hirez.programming.kicks-ass.net>
On 04/08/20 11:46, peterz@infradead.org wrote:
> On Tue, Aug 04, 2020 at 09:03:07AM +0530, Srikar Dronamraju wrote:
>> On Power9 a pair of cores can be presented by the firmware as a big-core
>> for backward compatibility reasons, with 4 threads per (small) core and 8
>> threads per big-core. cpu_smt_mask() should generally point to the cpu mask
>> of the (small)core.
>>
>> In order to maintain userspace backward compatibility (with Power8 chips in
>> case of Power9) in enterprise Linux systems, the topology_sibling_cpumask
>> has to be set to big-core. Hence override the default cpu_smt_mask() to be
>> powerpc specific allowing for better scheduling behaviour on Power.
>
> Why does Linux userspace care about this?
Ditto; from [1], a core contains CPUs that all share the same L1 (and capacity,
as per SD_SHARE_CPUCAPACITY). So IMO it makes perfect sense to have a first
domain spanning L1, and its parent spanning L2 - that means
topology_sibling_cpumask *itself* should span a single core rather than a
pair.
[1]: https://lkml.kernel.org/r/jhjr1sviswg.mognet@arm.com
WARNING: multiple messages have this Message-ID (diff)
From: Valentin Schneider <valentin.schneider@arm.com>
To: peterz@infradead.org
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
Ingo Molnar <mingo@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Mel Gorman <mgorman@techsingularity.net>,
Rik van Riel <riel@surriel.com>,
Thomas Gleixner <tglx@linutronix.de>,
Vincent Guittot <vincent.guittot@linaro.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
Michael Ellerman <mpe@ellerman.id.au>,
Michael Neuling <mikey@neuling.org>,
Gautham R Shenoy <ego@linux.vnet.ibm.com>,
Vaidyanathan Srinivasan <svaidy@linux.ibm.com>
Subject: Re: [PATCH 2/2] powerpc/topology: Override cpu_smt_mask
Date: Tue, 04 Aug 2020 12:02:49 +0100 [thread overview]
Message-ID: <jhjeeomish2.mognet@arm.com> (raw)
In-Reply-To: <20200804104642.GC2657@hirez.programming.kicks-ass.net>
On 04/08/20 11:46, peterz@infradead.org wrote:
> On Tue, Aug 04, 2020 at 09:03:07AM +0530, Srikar Dronamraju wrote:
>> On Power9 a pair of cores can be presented by the firmware as a big-core
>> for backward compatibility reasons, with 4 threads per (small) core and 8
>> threads per big-core. cpu_smt_mask() should generally point to the cpu mask
>> of the (small)core.
>>
>> In order to maintain userspace backward compatibility (with Power8 chips in
>> case of Power9) in enterprise Linux systems, the topology_sibling_cpumask
>> has to be set to big-core. Hence override the default cpu_smt_mask() to be
>> powerpc specific allowing for better scheduling behaviour on Power.
>
> Why does Linux userspace care about this?
Ditto; from [1], a core contains CPUs that all share the same L1 (and capacity,
as per SD_SHARE_CPUCAPACITY). So IMO it makes perfect sense to have a first
domain spanning L1, and its parent spanning L2 - that means
topology_sibling_cpumask *itself* should span a single core rather than a
pair.
[1]: https://lkml.kernel.org/r/jhjr1sviswg.mognet@arm.com
next prev parent reply other threads:[~2020-08-04 11:05 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-04 3:33 [PATCH 1/2] sched/topology: Allow archs to override cpu_smt_mask Srikar Dronamraju
2020-08-04 3:33 ` Srikar Dronamraju
2020-08-04 3:33 ` [PATCH 2/2] powerpc/topology: Override cpu_smt_mask Srikar Dronamraju
2020-08-04 3:33 ` Srikar Dronamraju
2020-08-04 10:46 ` peterz
2020-08-04 10:46 ` peterz
2020-08-04 11:02 ` Valentin Schneider [this message]
2020-08-04 11:02 ` Valentin Schneider
2020-08-04 10:45 ` [PATCH 1/2] sched/topology: Allow archs to override cpu_smt_mask peterz
2020-08-04 10:45 ` peterz
2020-08-04 12:10 ` Srikar Dronamraju
2020-08-04 12:10 ` Srikar Dronamraju
2020-08-04 12:47 ` peterz
2020-08-04 12:47 ` peterz
2020-08-06 5:32 ` Michael Ellerman
2020-08-06 5:32 ` Michael Ellerman
2020-08-06 8:54 ` peterz
2020-08-06 8:54 ` peterz
2020-08-06 12:25 ` Michael Ellerman
2020-08-06 12:25 ` Michael Ellerman
2020-08-06 13:15 ` peterz
2020-08-06 13:15 ` peterz
2020-08-06 14:09 ` Srikar Dronamraju
2020-08-06 14:09 ` Srikar Dronamraju
2020-08-06 12:53 ` Srikar Dronamraju
2020-08-06 12:53 ` Srikar Dronamraju
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