[tip: irq/core] irqtime: Make accounting correct on RT

["tip-bot2 for Thomas Gleixner" <tip-bot2@linutronix.de>]

The following commit has been merged into the irq/core branch of tip:

Commit-ID:     6516b386d8a07102aac353daf9c0fe0045faeb74
Gitweb:        https://git.kernel.org/tip/6516b386d8a07102aac353daf9c0fe0045faeb74
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Tue, 09 Mar 2021 09:55:54 +01:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Wed, 17 Mar 2021 16:34:09 +01:00

irqtime: Make accounting correct on RT

vtime_account_irq and irqtime_account_irq() base checks on preempt_count()
which fails on RT because preempt_count() does not contain the softirq
accounting which is seperate on RT.

These checks do not need the full preempt count as they only operate on the
hard and softirq sections.

Use irq_count() instead which provides the correct value on both RT and non
RT kernels. The compiler is clever enough to fold the masking for !RT:

       99b:	65 8b 05 00 00 00 00 	mov    %gs:0x0(%rip),%eax
 -     9a2:	25 ff ff ff 7f       	and    $0x7fffffff,%eax
 +     9a2:	25 00 ff ff 00       	and    $0xffff00,%eax

Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210309085727.153926793@linutronix.de

---
 kernel/sched/cputime.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 5f61165..2c36a5f 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -60,7 +60,7 @@ void irqtime_account_irq(struct task_struct *curr, unsigned int offset)
 	cpu = smp_processor_id();
 	delta = sched_clock_cpu(cpu) - irqtime->irq_start_time;
 	irqtime->irq_start_time += delta;
-	pc = preempt_count() - offset;
+	pc = irq_count() - offset;
 
 	/*
 	 * We do not account for softirq time from ksoftirqd here.
@@ -421,7 +421,7 @@ void vtime_task_switch(struct task_struct *prev)
 
 void vtime_account_irq(struct task_struct *tsk, unsigned int offset)
 {
-	unsigned int pc = preempt_count() - offset;
+	unsigned int pc = irq_count() - offset;
 
 	if (pc & HARDIRQ_OFFSET) {
 		vtime_account_hardirq(tsk);