Subject: [PATCH] powerpc/smp: poll cpu_callin_map more aggressively in __cpu_up()

From: Nathan Lynch <[email-masked]>
Date: Tue, 25 Jan 2022 01:21:03 -0600

Replace the outdated iteration and timeout calculations here with
indefinite spin_until_cond()-wrapped poll of cpu_callin_map. __cpu_up()
already does this when waiting for the cpu to set its online bit before
returning, so this change is not really making the function more brittle.

Removing the msleep(1) in the hotplug path here reduces the time it takes
to online a CPU on a P9 PowerVM LPAR from about 30ms to 1ms when exercised
via thaw_secondary_cpus().

Signed-off-by: Nathan Lynch <[email-masked]>
---
 arch/powerpc/kernel/smp.c | 25 ++-----------------------
 1 file changed, 2 insertions(+), 23 deletions(-)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index b7fd6a72aa76..990893365fe0 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -1270,7 +1270,7 @@ static void cpu_idle_thread_init(unsigned int cpu, struct task_struct *idle)
 
 int __cpu_up(unsigned int cpu, struct task_struct *tidle)
 {
-	int rc, c;
+	int rc;
 
 	/*
 	 * Don't allow secondary threads to come online if inhibited
@@ -1314,28 +1314,7 @@ int __cpu_up(unsigned int cpu, struct task_struct *tidle)
 		return rc;
 	}
 
-	/*
-	 * wait to see if the cpu made a callin (is actually up).
-	 * use this value that I found through experimentation.
-	 * -- Cort
-	 */
-	if (system_state < SYSTEM_RUNNING)
-		for (c = 50000; c && !cpu_callin_map[cpu]; c--)
-			udelay(100);
-#ifdef CONFIG_HOTPLUG_CPU
-	else
-		/*
-		 * CPUs can take much longer to come up in the
-		 * hotplug case.  Wait five seconds.
-		 */
-		for (c = 5000; c && !cpu_callin_map[cpu]; c--)
-			msleep(1);
-#endif
-
-	if (!cpu_callin_map[cpu]) {
-		printk(KERN_ERR "Processor %u is stuck.\n", cpu);
-		return -ENOENT;
-	}
+	spin_until_cond(cpu_callin_map[cpu] != 0);
 
 	DBG("Processor %u found.\n", cpu);
 
-- 
2.34.1