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

From: Frederic Weisbecker <frederic@kernel.org>
To: Qais Yousef <qais.yousef@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	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, 29 Dec 2020 15:30:54 +0100	[thread overview]
Message-ID: <20201229143054.GC21613@lothringen> (raw)
In-Reply-To: <20201229141231.c2ppmjucxxbz5j5h@e107158-lin>

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.

> 
> Thanks
> 
> --
> Qais Yousef