[PATCH] powerpc: mitigate impact of decrementer reset

[Paul Clarke <pc@us.ibm.com>]

The POWER ISA defines an always-running decrementer which can be used
to schedule interrupts after a certain time interval has elapsed.
The decrementer counts down at the same frequency as the Time Base,
which is 512 MHz.  The maximum value of the decrementer is 0x7fffffff.
This works out to a maximum interval of about 4.19 seconds.

If a larger interval is desired, the kernel will set the decrementer
to its maximum value and reset it after it expires (underflows)
a sufficient number of times until the desired interval has elapsed.

The negative effect of this is that an unwanted latency spike will
impact normal processing at most every 4.19 seconds.  On an IBM
POWER8-based system, this spike was measured at about 25-30
microseconds, much of which was basic, opportunistic housekeeping
tasks that could otherwise have waited.

This patch short-circuits the reset of the decrementer, exiting after
the decrementer reset, but before the housekeeping tasks if the only
need for the interrupt is simply to reset it.  After this patch,
the latency spike was measured at about 150 nanoseconds.

Signed-off-by: Paul A. Clarke <pc@us.ibm.com>
---
  arch/powerpc/kernel/time.c | 13 +++++++++++++
  1 file changed, 13 insertions(+)

diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 368ab37..962a06b 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -528,6 +528,7 @@ void timer_interrupt(struct pt_regs * regs)
  {
  	struct pt_regs *old_regs;
  	u64 *next_tb = &__get_cpu_var(decrementers_next_tb);
+	u64 now;

  	/* Ensure a positive value is written to the decrementer, or else
  	 * some CPUs will continue to take decrementer exceptions.
@@ -550,6 +551,18 @@ void timer_interrupt(struct pt_regs * regs)
  	 */
  	may_hard_irq_enable();

+	/* If this is simply the decrementer expiring (underflow) due to
+	 * the limited size of the decrementer, and not a set timer,
+	 * reset (if needed) and return
+	 */
+	now = get_tb_or_rtc();
+	if (now < *next_tb) {
+		now = *next_tb - now;
+		if (now <= DECREMENTER_MAX)
+			set_dec((int)now);
+		__get_cpu_var(irq_stat).timer_irqs_others++;
+		return;
+	}

  #if defined(CONFIG_PPC32) && defined(CONFIG_PPC_PMAC)
  	if (atomic_read(&ppc_n_lost_interrupts) != 0)
-- 
2.1.2.330.g565301e