[PATCH 14/15] sched/cputime: Handle idle irqtime gracefully

[Frederic Weisbecker <frederic@kernel.org>]

The dyntick-idle cputime accounting always assumes that IRQ time
accounting is enabled and consequently stops elapsing the idle time
during dyntick-idle IRQs.

This doesn't mix up well with disabled IRQ time accounting because then
idle IRQs become a cputime blind-spot. Also this feature is disabled
on most configurations and the overhead of pausing dyntick-idle
accounting while in idle IRQs could then be avoided.

Fix the situation with conditionally pausing dyntick-idle accounting
during idle IRQs only iff either native vtime (which does IRQ time
accounting) or generic IRQ time accounting are enabled.

Also make sure that the accumulated IRQ time is not accidentally
substracted from later accounting.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Tested-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
 kernel/sched/cputime.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 335d2c127763..94be22aa5cb6 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -47,7 +47,8 @@ static void irqtime_account_delta(struct irqtime *irqtime, u64 delta,
 	u64_stats_update_begin(&irqtime->sync);
 	cpustat[idx] += delta;
 	irqtime->total += delta;
-	irqtime->tick_delta += delta;
+	if (!kcpustat_idle_dyntick())
+		irqtime->tick_delta += delta;
 	u64_stats_update_end(&irqtime->sync);
 }
 
@@ -444,6 +445,10 @@ static void kcpustat_idle_stop(struct kernel_cpustat *kc, u64 now)
 
 static void kcpustat_idle_start(struct kernel_cpustat *kc, u64 now)
 {
+	/* Irqtime accounting might have been enabled in the middle of the IRQ */
+	if (kc->idle_elapse)
+		return;
+
 	write_seqcount_begin(&kc->idle_sleeptime_seq);
 	kc->idle_entrytime = now;
 	kc->idle_elapse = true;
@@ -478,7 +483,8 @@ void kcpustat_irq_enter(u64 now)
 {
 	struct kernel_cpustat *kc = kcpustat_this_cpu;
 
-	if (!vtime_generic_enabled_this_cpu())
+	if (!vtime_generic_enabled_this_cpu() &&
+	    (irqtime_enabled() || vtime_accounting_enabled_this_cpu()))
 		kcpustat_idle_stop(kc, now);
 }
 
@@ -486,7 +492,15 @@ void kcpustat_irq_exit(u64 now)
 {
 	struct kernel_cpustat *kc = kcpustat_this_cpu;
 
-	if (!vtime_generic_enabled_this_cpu())
+	/*
+	 * Generic vtime already does its own idle accounting.
+	 * But irqtime accounting or arch vtime which also accounts IRQs
+	 * need to pause nohz accounting. Resume nohz accounting as long
+	 * as the irqtime config is enabled to handle case where irqtime
+	 * accounting got runtime disabled in the middle of an IRQ.
+	 */
+	if (!vtime_generic_enabled_this_cpu() &&
+	    (IS_ENABLED(CONFIG_IRQ_TIME_ACCOUNTING) || vtime_accounting_enabled_this_cpu()))
 		kcpustat_idle_start(kc, now);
 }
 
-- 
2.53.0