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

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Message-ID: <20221123230031.GL4001@paulmck-ThinkPad-P17-Gen-1>
Date:   Wed, 23 Nov 2022 15:00:31 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...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 Wed, Nov 23, 2022 at 11:25:43PM +0100, Frederic Weisbecker wrote:
> 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.

Ah, getting ahead of myself, thank you for the info!

So the thing to do would be to generate only maxcpu-1 choices.

							Thanx, Paul

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