linux-kernel - Re: [PATCH 4/5] irqtime: Move irqtime entry accounting after irq offset incrementation

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Date:   Tue, 01 Dec 2020 14:34:49 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tony Luck <tony.luck@...el.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Heiko Carstens <hca@...ux.ibm.com>
Subject: Re: [PATCH 4/5] irqtime: Move irqtime entry accounting after irq offset incrementation

On Tue, Dec 01 2020 at 12:40, Frederic Weisbecker wrote:
> On Tue, Dec 01, 2020 at 12:33:26PM +0100, Thomas Gleixner wrote:
>> > 	/*
>> > 	 * We do not account for softirq time from ksoftirqd here.
>> > 	 * We want to continue accounting softirq time to ksoftirqd thread
>> > 	 * in that case, so as not to confuse scheduler with a special task
>> > 	 * that do not consume any time, but still wants to run.
>> > 	 */
>> > 	if (pc & HARDIRQ_MASK)
>> > 		irqtime_account_delta(irqtime, delta, CPUTIME_IRQ);
>> > 	else if ((pc & SOFTIRQ_OFFSET) && curr != this_cpu_ksoftirqd())
>> > 		irqtime_account_delta(irqtime, delta, CPUTIME_SOFTIRQ);
>> > }
>> 
>> Why not making all of this explicit instead of these conditionals?
>
> Hmm, I'm not sure I get what you suggest?

Instead of playing games with preeempt count and offsets and checking
for ksoftirqd, can't you just have:

        account_hardirqtime()
        account_softirqtime()

and call them from the right spots. See the below for illustration (it's
obviously incomplete).

Thanks,

        tglx
---
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -377,6 +377,7 @@ static inline void invoke_softirq(void)
 		return;
 
 	if (!force_irqthreads) {
+		account_softirq_enter_time(current);
 #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK
 		/*
 		 * We can safely execute softirq on the current stack if
@@ -391,6 +392,7 @@ static inline void invoke_softirq(void)
 		 * to prevent from any overrun.
 		 */
 		do_softirq_own_stack();
+		account_softirq_exit_time(current);
 #endif
 	} else {
 		wakeup_softirqd();
@@ -417,7 +419,7 @@ static inline void __irq_exit_rcu(void)
 #else
 	lockdep_assert_irqs_disabled();
 #endif
-	account_irq_exit_time(current);
+	account_hardirq_exit_time(current);
 	preempt_count_sub(HARDIRQ_OFFSET);
 	if (!in_interrupt() && local_softirq_pending())
 		invoke_softirq();