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

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Message-ID: <20201202123648.GI3021@hirez.programming.kicks-ass.net>
Date:   Wed, 2 Dec 2020 13:36:48 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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 Wed, Dec 02, 2020 at 12:57:31PM +0100, Frederic Weisbecker wrote:
> IRQ time entry is currently accounted before HARDIRQ_OFFSET or
> SOFTIRQ_OFFSET are incremented. This is convenient to decide to which
> index the cputime to account is dispatched.
> 
> Unfortunately it prevents tick_irq_enter() from being called under
> HARDIRQ_OFFSET because tick_irq_enter() has to be called before the IRQ
> entry accounting due to the necessary clock catch up. As a result we
> don't benefit from appropriate lockdep coverage on tick_irq_enter().
> 
> To prepare for fixing this, move the IRQ entry cputime accounting after
> the preempt offset is incremented. This requires the cputime dispatch
> code to handle the extra offset.
> 
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>