linux-kernel - [patch V3 2/6] irqtime: Make accounting correct on RT

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Message-Id: <20210309085727.153926793@linutronix.de>
Date:   Tue, 09 Mar 2021 09:55:54 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Paul McKenney <paulmck@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: [patch V3 2/6] 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@...utronix.de>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Tested-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
---
 kernel/sched/cputime.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -60,7 +60,7 @@ void irqtime_account_irq(struct task_str
 	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_struc
 
 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);