linux-kernel - [PATCH 13/17] pseries/smp: Account cores when triggering softoffline

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Message-ID: <20251204175405.1511340-14-srikar@linux.ibm.com>
Date: Thu,  4 Dec 2025 23:24:01 +0530
From: Srikar Dronamraju <srikar@...ux.ibm.com>
To: linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Peter Zijlstra <peterz@...radead.org>
Cc: Ben Segall <bsegall@...gle.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...nel.org>, Juri Lelli <juri.lelli@...hat.com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        Madhavan Srinivasan <maddy@...ux.ibm.com>,
        Mel Gorman <mgorman@...e.de>, Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Shrikanth Hegde <sshegde@...ux.ibm.com>,
        Srikar Dronamraju <srikar@...ux.ibm.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Swapnil Sapkal <swapnil.sapkal@....com>,
        Thomas Huth <thuth@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        virtualization@...ts.linux.dev, Yicong Yang <yangyicong@...ilicon.com>,
        Ilya Leoshkevich <iii@...ux.ibm.com>
Subject: [PATCH 13/17] pseries/smp: Account cores when triggering softoffline

On a shared LPAR is entitled to certain number of cores, i.e the number
of cores that PowerVM hypervisor is committed to provide at any point of
time. Hence based on steal metrics, soft offline such that at least
soft-offline cores are available.

Also when soft-onlining cores, unless DLPAR, ensure system can only
online up to max virtual cores.

Signed-off-by: Srikar Dronamraju <srikar@...ux.ibm.com>
---
 arch/powerpc/platforms/pseries/smp.c | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/platforms/pseries/smp.c b/arch/powerpc/platforms/pseries/smp.c
index 4c83749018d0..69e209880b6f 100644
--- a/arch/powerpc/platforms/pseries/smp.c
+++ b/arch/powerpc/platforms/pseries/smp.c
@@ -327,25 +327,45 @@ void trigger_softoffline(unsigned long steal_ratio)
 {
 	int currcpu = smp_processor_id();
 	static int prev_direction;
+	int success = 0;
 	int cpu, i;
 
+	/*
+	 * Compare delta runtime versus delta steal time.
+	 *  [0]<----------->[EC]--------->[VP]
+	 *  [0]<------------------>{AC}-->[VP]
+	 *  EC == Entitled Cores
+	 *  VP == Virtual Processors
+	 *  AC == Available Cores Varies between 0 to EC/VP.
+	 * If Steal time is high, then reduce Available Cores.
+	 * If steal time is low, increase Available Cores
+	 */
 	if (steal_ratio >= STEAL_RATIO_HIGH && prev_direction > 0) {
 		/*
 		 * System entitlement was reduced earlier but we continue to
-		 * see steal time. Reduce entitlement further.
+		 * see steal time. Reduce entitlement further if possible.
 		 */
+		if (available_cores <= entitled_cores)
+			return;
+
 		cpu = cpumask_last(cpu_active_mask);
 		for_each_cpu_andnot(i, cpu_sibling_mask(cpu), cpu_sibling_mask(currcpu)) {
 			struct offline_worker *worker = &per_cpu(offline_workers, i);
 
 			worker->offline = 1;
 			schedule_work_on(i, &worker->work);
+			success = 1;
 		}
+		if (success)
+			available_cores--;
 	} else if (steal_ratio <= STEAL_RATIO_LOW && prev_direction < 0) {
 		/*
 		 * System entitlement was increased but we continue to see
-		 * less steal time. Increase entitlement further.
+		 * less steal time. Increase entitlement further if possible.
 		 */
+		if (available_cores >= max_virtual_cores)
+			return;
+
 		cpumask_andnot(cpus, cpu_online_mask, cpu_active_mask);
 		if (cpumask_empty(cpus))
 			return;
@@ -356,7 +376,10 @@ void trigger_softoffline(unsigned long steal_ratio)
 
 			worker->offline = 0;
 			schedule_work_on(i, &worker->work);
+			success = 1;
 		}
+		if (success)
+			available_cores++;
 	}
 	if (steal_ratio >= STEAL_RATIO_HIGH)
 		prev_direction = 1;
-- 
2.43.7