[PATCH v3] powerpc/smp: poll cpu_callin_map more aggressively in __cpu_up()

[Nathan Lynch]

At boot time, it is not necessary to delay between polls of
cpu_callin_map when waiting for a kicked CPU to come up. Remove the
delay intervals, but preserve the overall deadline (five seconds).

At run time, the first poll result is usually negative and we incur a
sleeping wait. If we spin on the callin word for a short time first,
we can reduce __cpu_up() from dozens of milliseconds to under 1ms in
the common case on a P9 LPAR:

$ ppc64_cpu --smt=off
$ bpftrace -e 'kprobe:__cpu_up {
                 @start[tid] = nsecs;
               }
               kretprobe:__cpu_up /@start[tid]/ {
                 @us = hist((nsecs - @start[tid]) / 1000);
                 delete(@start[tid]);
               }' -c 'ppc64_cpu --smt=on'

Before:

@us:
[16K, 32K)        85 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[32K, 64K)        13 |@@@@@@@                                             |

After:

@us:
[128, 256)        95 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[256, 512)         3 |@                                                   |

Signed-off-by: Nathan Lynch <nathanl at linux.ibm.com>
---

Notes:
    Changes since v2:
    * Use short optimistic spin for hotplug case and fall back to sleeping
      loop.
    * Preserve original deadline for hotplug case, which was effectively
      100 seconds as coded.
    * Improve benchmark by timing __cpu_up() duration directly.
    
    Changes since v1:
    * Do not poll indefinitely; restore the original 5sec timeout

 arch/powerpc/kernel/smp.c | 38 ++++++++++++++++++++++----------------
 1 file changed, 22 insertions(+), 16 deletions(-)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 169703fead57..b7ce46bbc6f1 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -1257,7 +1257,12 @@ 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;
+	const unsigned long boot_spin_ms = 5 * MSEC_PER_SEC;
+	const bool booting = system_state < SYSTEM_RUNNING;
+	const unsigned long hp_spin_ms = 1;
+	unsigned long deadline;
+	int rc;
+	const unsigned long spin_wait_ms = booting ? boot_spin_ms : hp_spin_ms;
 
 	/*
 	 * Don't allow secondary threads to come online if inhibited
@@ -1302,22 +1307,23 @@ int __cpu_up(unsigned int cpu, struct task_struct *tidle)
 	}
 
 	/*
-	 * wait to see if the cpu made a callin (is actually up).
-	 * use this value that I found through experimentation.
-	 * -- Cort
+	 * At boot time, simply spin on the callin word until the
+	 * deadline passes.
+	 *
+	 * At run time, spin for an optimistic amount of time to avoid
+	 * sleeping in the common case.
 	 */
-	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
+	deadline = jiffies + msecs_to_jiffies(spin_wait_ms);
+	spin_until_cond(cpu_callin_map[cpu] || time_is_before_jiffies(deadline));
+
+	if (!cpu_callin_map[cpu] && system_state >= SYSTEM_RUNNING) {
+		const unsigned long sleep_interval_us = 10 * USEC_PER_MSEC;
+		const unsigned long sleep_wait_ms = 100 * MSEC_PER_SEC;
+
+		deadline = jiffies + msecs_to_jiffies(sleep_wait_ms);
+		while (!cpu_callin_map[cpu] && time_is_after_jiffies(deadline))
+			fsleep(sleep_interval_us);
+	}
 
 	if (!cpu_callin_map[cpu]) {
 		printk(KERN_ERR "Processor %u is stuck.\n", cpu);
-- 
2.37.1