linux-kernel - [PATCH v2 1/2] sched/topology: Allow archs to override cpu_smt

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Message-Id: <20200807074517.27957-1-srikar@linux.vnet.ibm.com>
Date:   Fri,  7 Aug 2020 13:15:16 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Michael Neuling <mikey@...ling.org>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Vaidyanathan Srinivasan <svaidy@...ux.ibm.com>
Subject: [PATCH v2 1/2] sched/topology: Allow archs to override cpu_smt_mask

cpu_smt_mask tracks topology_sibling_cpumask. This would be good for
most architectures. One of the users of cpu_smt_mask(), would be to
identify idle-cores. On Power9, a pair of SMT4 cores can be presented by
the firmware as a SMT8 core for backward compatibility reasons.

Powerpc allows LPARs to be live migrated from Power8 to Power9. Do note
Power8 had only SMT8 cores. Existing software which has been
developed/configured for Power8 would expect to see SMT8 core.
Maintaining the illusion of SMT8 core is a requirement to make that
work.

In order to maintain above userspace backward compatibility with
previous versions of processor, Power9 onwards there is option to the
firmware to advertise a pair of SMT4 cores as a fused cores aka SMT8
core. On Power9 this pair shares the L2 cache as well. However, from the
scheduler's point of view, a core should be determined by SMT4, since
its a completely independent unit of compute. Hence allow PowerPc
architecture to override the default cpu_smt_mask() to point to the SMT4
cores in a SMT8 mode.

This will ensure the scheduler is always given the right information.

Cc: linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Cc: LKML <linux-kernel@...r.kernel.org>
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: Michael Neuling <mikey@...ling.org>
Cc: Gautham R Shenoy <ego@...ux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Valentin Schneider <valentin.schneider@....com>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Mel Gorman <mgorman@...hsingularity.net>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Vaidyanathan Srinivasan <svaidy@...ux.ibm.com>
Acked-by; Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
---
Changelog v1->v2:
	Update the commit msg based on the discussion in community esp
	with Peter Zijlstra and Michael Ellerman.

 include/linux/topology.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/topology.h b/include/linux/topology.h
index 608fa4aadf0e..ad03df1cc266 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -198,7 +198,7 @@ static inline int cpu_to_mem(int cpu)
 #define topology_die_cpumask(cpu)		cpumask_of(cpu)
 #endif

-#ifdef CONFIG_SCHED_SMT
+#if defined(CONFIG_SCHED_SMT) && !defined(cpu_smt_mask)
 static inline const struct cpumask *cpu_smt_mask(int cpu)
 {
 	return topology_sibling_cpumask(cpu);
-- 
2.18.2