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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20201201143545.GC72897@lothringen>
Date:   Tue, 1 Dec 2020 15:35:45 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
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 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();