linux-kernel - [RFC PATCH v4 14/17] powerpc: process steal values at fixed intervals

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Message-ID: <20251119062100.1112520-15-sshegde@linux.ibm.com>
Date: Wed, 19 Nov 2025 11:50:57 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Cc: sshegde@...ux.ibm.com, mingo@...hat.com, peterz@...radead.org,
        juri.lelli@...hat.com, vincent.guittot@...aro.org, tglx@...utronix.de,
        yury.norov@...il.com, maddy@...ux.ibm.com, srikar@...ux.ibm.com,
        gregkh@...uxfoundation.org, pbonzini@...hat.com, seanjc@...gle.com,
        kprateek.nayak@....com, vschneid@...hat.com, iii@...ux.ibm.com,
        huschle@...ux.ibm.com, rostedt@...dmis.org, dietmar.eggemann@....com,
        christophe.leroy@...roup.eu
Subject: [RFC PATCH v4 14/17] powerpc: process steal values at fixed intervals

Process steal time at regular intervals. Sum of steal time across the
vCPUs is computed against the time to get the steal ratio.

Only first online CPU does this work. That reduces the racing issues.
This is done only on SPLPAR (non kvm guest). This assumes PowerVM being
the hypervisor.
 
Originally-by: Srikar Dronamraju <srikar@...ux.ibm.com>
Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
---
 arch/powerpc/platforms/pseries/lpar.c | 59 +++++++++++++++++++++++++++
 1 file changed, 59 insertions(+)

diff --git a/arch/powerpc/platforms/pseries/lpar.c b/arch/powerpc/platforms/pseries/lpar.c
index 825b5b4e2b43..c16d97e1a1fe 100644
--- a/arch/powerpc/platforms/pseries/lpar.c
+++ b/arch/powerpc/platforms/pseries/lpar.c
@@ -660,10 +660,58 @@ static int __init vcpudispatch_stats_procfs_init(void)
 machine_device_initcall(pseries, vcpudispatch_stats_procfs_init);
 
 #ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
+
+#define STEAL_MULTIPLE 10000
+#define PURR_UPDATE_TB NSEC_PER_SEC
+
+static bool should_cpu_process_steal(int cpu)
+{
+	if (cpu == cpumask_first(cpu_online_mask))
+		return true;
+
+	return false;
+}
+
+static void process_steal(int cpu)
+{
+	static unsigned long next_tb_ns, prev_steal;
+	unsigned long steal_ratio, delta_tb;
+	unsigned long tb_ns = tb_to_ns(mftb());
+	unsigned long steal = 0;
+	unsigned int i;
+
+	if (!should_cpu_process_steal(cpu))
+		return;
+
+	if (tb_ns < next_tb_ns)
+		return;
+
+	for_each_online_cpu(i) {
+		struct lppaca *lppaca = &lppaca_of(i);
+
+		steal += be64_to_cpu(READ_ONCE(lppaca->ready_enqueue_tb));
+		steal += be64_to_cpu(READ_ONCE(lppaca->enqueue_dispatch_tb));
+	}
+
+	steal = tb_to_ns(steal);
+
+	if (next_tb_ns && prev_steal) {
+		delta_tb = max(tb_ns - (next_tb_ns - PURR_UPDATE_TB), 1);
+		steal_ratio = (steal - prev_steal) * STEAL_MULTIPLE;
+		steal_ratio /= (delta_tb * num_online_cpus());
+		update_soft_entitlement(steal_ratio);
+	}
+
+	next_tb_ns = tb_ns + PURR_UPDATE_TB;
+	prev_steal = steal;
+}
+
 u64 pseries_paravirt_steal_clock(int cpu)
 {
 	struct lppaca *lppaca = &lppaca_of(cpu);
 
+	if (is_shared_processor() && !is_kvm_guest())
+		process_steal(cpu);
 	/*
 	 * VPA steal time counters are reported at TB frequency. Hence do a
 	 * conversion to ns before returning
@@ -2061,6 +2109,17 @@ void pseries_init_ec_vp_cores(void)
 #define STEAL_RATIO_HIGH 400
 #define STEAL_RATIO_LOW  150
 
+/*
+ * [0]<----------->[EC]---->{AC}-->[VP]
+ * EC == Entitled Cores. Guaranteed number of cores by hypervsior.
+ * VP == Virtual Processors. Total number of cores. When there is overcommit
+ * this will be higher than EC.
+ * AC == Available Cores Varies between EC <-> VP.
+ *
+ * If Steal time is high, then reduce Available Cores.
+ * If steal time is low, increase Available Cores
+ */
+
 void update_soft_entitlement(unsigned long steal_ratio)
 {
 	static int prev_direction;
-- 
2.47.3