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

From: Frederic Weisbecker <frederic@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Tony Luck <tony.luck@intel.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Fenghua Yu <fenghua.yu@intel.com>,
	Heiko Carstens <hca@linux.ibm.com>
Subject: Re: [PATCH 4/5] irqtime: Move irqtime entry accounting after irq offset incrementation
Date: Tue, 1 Dec 2020 15:35:45 +0100	[thread overview]
Message-ID: <20201201143545.GC72897@lothringen> (raw)
In-Reply-To: <87blfdhcp2.fsf@nanos.tec.linutronix.de>

On Tue, Dec 01, 2020 at 02:34:49PM +0100, Thomas Gleixner wrote:
> 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);

Indeed for the softirq part it simplifies things.

>  #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);

And that one too makes things simple. But note that

    account_hardirq_enter_time()

will still need some preempt count checks to see if
this is a nested hardirq, a hardirq interrupting a softirq
or a hardirq interrupting a task.

But I think it's a win in the end. I'll try that.

Thanks.

>  	preempt_count_sub(HARDIRQ_OFFSET);
>  	if (!in_interrupt() && local_softirq_pending())
>  		invoke_softirq();