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

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Message-ID: <20201229155852.paudsonz6m66x2nz@e107158-lin>
Date:   Tue, 29 Dec 2020 15:58:52 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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 12/29/20 15:30, Frederic Weisbecker wrote:
> On Tue, Dec 29, 2020 at 02:12:31PM +0000, Qais Yousef wrote:
> > On 12/29/20 14:41, Frederic Weisbecker wrote:
> > > > > -void vtime_account_irq(struct task_struct *tsk)
> > > > > +void vtime_account_irq(struct task_struct *tsk, unsigned int offset)
> > > > >  {
> > > > > -	if (hardirq_count()) {
> > > > > +	unsigned int pc = preempt_count() - offset;
> > > > > +
> > > > > +	if (pc & HARDIRQ_OFFSET) {
> > > > 
> > > > Shouldn't this be HARDIRQ_MASK like above?
> > > 
> > > In the rare cases of nested hardirqs happening with broken drivers, Only the outer hardirq
> > > does matter. All the time spent in the inner hardirqs is included in the outer
> > > one.
> > 
> > Ah I see. The original code was doing hardirq_count(), which apparently wasn't
> > right either.
> > 
> > Shouldn't it be pc == HARDIRQ_OFFSET then? All odd nest counts will trigger
> > this otherwise, and IIUC we want this to trigger once on first entry only.
> 
> Right but we must also handle hardirqs interrupting either preempt disabled sections
> or softirq servicing/disabled section.
> 
> 3 stacking hardirqs should be rare enough that we don't really care. In the
> worst case we are going to account the third IRQ seperately. Not a correctness
> issue, just a rare unoptimized case.

I admit I need to wrap my head around some more details to fully comprehend
that, but that's my own confusion to clear out :-)

Thanks for your answer.

Cheers

--
Qais Yousef