linux-kernel - Re: [PATCH linux-next][RFC]torture: avoid offline tick_do_timer

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Date:   Wed, 23 Nov 2022 23:25:43 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Zhouyi Zhou <zhouzhouyi@...il.com>, fweisbec@...il.com,
        tglx@...utronix.de, mingo@...nel.org, dave@...olabs.net,
        josh@...htriplett.org, mpe@...erman.id.au,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH linux-next][RFC]torture: avoid offline tick_do_timer_cpu

On Mon, Nov 21, 2022 at 05:37:54PM -0800, Paul E. McKenney wrote:
> On Mon, Nov 21, 2022 at 11:51:40AM +0800, Zhouyi Zhou wrote:
> > @@ -358,7 +359,16 @@ torture_onoff(void *arg)
> >  			schedule_timeout_interruptible(HZ / 10);
> >  			continue;
> >  		}
> > +#ifdef CONFIG_NO_HZ_FULL
> > +		/* do not offline tick do timer cpu */
> > +		if (tick_nohz_full_running) {
> > +			cpu = (torture_random(&rand) >> 4) % maxcpu;
> > +			if (cpu >= tick_do_timer_cpu)
> 
> Why is this ">=" instead of "=="?
> 
> > +				cpu = (cpu + 1) % (maxcpu + 1);
> > +		} else
> > +#else
> >  		cpu = (torture_random(&rand) >> 4) % (maxcpu + 1);
> > +#endif
> 
> What happens if the value of tick_do_timer_cpu changes between the time of
> the check above and the call to torture_offline() below?  Alternatively,
> how is such a change in value prevented?

It can't, currently tick_do_timer_cpu is fixed when nohz_full is running.
It can however have special values at early boot such as TICK_DO_TIMER_NONE.
But if rcutorture is initialized after smp, it should be ok.

Thanks.

> 
> 							Thanx, Paul
> 
> >  		if (!torture_offline(cpu,
> >  				     &n_offline_attempts, &n_offline_successes,
> >  				     &sum_offline, &min_offline, &max_offline))
> > -- 
> > 2.34.1
> >