linux-kernel - [PATCH 16/17] pseries/smp: Allow users to override steal thresholds

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Message-ID: <20251204175405.1511340-17-srikar@linux.ibm.com>
Date: Thu,  4 Dec 2025 23:24:04 +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 16/17] pseries/smp: Allow users to override steal thresholds

Different shared LPARs will have different entitled cores, different
number of cores in shared pools and different virtual processors. Also
the number and configuration of other shared LPARs sharing the same pool
will differ in each case. Hence a single set of threshold values may not
work. Hence provide a debugfs interface by which a privileged user can set
the high and low threshold values.

Signed-off-by: Srikar Dronamraju <srikar@...ux.ibm.com>
---
 arch/powerpc/platforms/pseries/smp.c | 30 +++++++++++++++++++++-------
 1 file changed, 23 insertions(+), 7 deletions(-)

diff --git a/arch/powerpc/platforms/pseries/smp.c b/arch/powerpc/platforms/pseries/smp.c
index a3daac4c3e1e..909f2d58384a 100644
--- a/arch/powerpc/platforms/pseries/smp.c
+++ b/arch/powerpc/platforms/pseries/smp.c
@@ -21,6 +21,9 @@
 #include <linux/device.h>
 #include <linux/cpu.h>
 #include <linux/pgtable.h>
+#if defined(CONFIG_DEBUG_FS) && defined(CONFIG_PPC_SPLPAR)
+#include <linux/debugfs.h>
+#endif
 
 #include <asm/ptrace.h>
 #include <linux/atomic.h>
@@ -285,9 +288,6 @@ static __init void pSeries_smp_probe(void)
  * lower threshold values below which allow work to spread out to more
  * cores.
  */
-#define STEAL_RATIO_HIGH (10 * STEAL_RATIO)
-#define STEAL_RATIO_LOW (5 * STEAL_RATIO)
-
 static unsigned int max_virtual_cores __read_mostly;
 static unsigned int entitled_cores __read_mostly;
 static unsigned int available_cores;
@@ -323,6 +323,9 @@ unsigned int pseries_num_available_cores(void)
 	return available_cores;
 }
 
+static u8 steal_ratio_high = 10;
+static u8 steal_ratio_low = 5;
+
 void trigger_softoffline(unsigned long steal_ratio)
 {
 	int currcpu = smp_processor_id();
@@ -340,7 +343,7 @@ void trigger_softoffline(unsigned long steal_ratio)
 	 * 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) {
+	if (steal_ratio >= STEAL_RATIO * steal_ratio_high && prev_direction > 0) {
 		/*
 		 * System entitlement was reduced earlier but we continue to
 		 * see steal time. Reduce entitlement further if possible.
@@ -358,7 +361,7 @@ void trigger_softoffline(unsigned long steal_ratio)
 		}
 		if (success)
 			available_cores--;
-	} else if (steal_ratio <= STEAL_RATIO_LOW && prev_direction < 0) {
+	} else if (steal_ratio <= STEAL_RATIO * steal_ratio_low && prev_direction < 0) {
 		/*
 		 * System entitlement was increased but we continue to see
 		 * less steal time. Increase entitlement further if possible.
@@ -381,9 +384,9 @@ void trigger_softoffline(unsigned long steal_ratio)
 		if (success)
 			available_cores++;
 	}
-	if (steal_ratio >= STEAL_RATIO_HIGH)
+	if (steal_ratio >= STEAL_RATIO * steal_ratio_high)
 		prev_direction = 1;
-	else if (steal_ratio <= STEAL_RATIO_LOW)
+	else if (steal_ratio <= STEAL_RATIO * steal_ratio_low)
 		prev_direction = -1;
 	else
 		prev_direction = 0;
@@ -437,3 +440,16 @@ void __init smp_init_pseries(void)
 
 	pr_debug(" <- smp_init_pSeries()\n");
 }
+
+#if defined(CONFIG_DEBUG_FS) && defined(CONFIG_PPC_SPLPAR)
+static int __init steal_ratio_debugfs_init(void)
+{
+	if (!firmware_has_feature(FW_FEATURE_SPLPAR))
+		return 0;
+
+	debugfs_create_u8("steal_high", 0600, arch_debugfs_dir, &steal_ratio_high);
+	debugfs_create_u8("steal_low", 0600, arch_debugfs_dir, &steal_ratio_low);
+	return 0;
+}
+machine_arch_initcall(pseries, steal_ratio_debugfs_init);
+#endif /* CONFIG_DEBUG_FS && CONFIG_PPC_SPLPAR*/
-- 
2.43.7