Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 23 Nov 2022 23:25:43 +0100
From:   Frederic Weisbecker <frederic@kernel.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Zhouyi Zhou <zhouzhouyi@gmail.com>, fweisbec@gmail.com,
        tglx@linutronix.de, mingo@kernel.org, dave@stgolabs.net,
        josh@joshtriplett.org, mpe@ellerman.id.au,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH linux-next][RFC]torture: avoid offline tick_do_timer_cpu
Message-ID: <20221123222543.GB1395324@lothringen>
References: <20221121035140.118651-1-zhouzhouyi@gmail.com>
 <20221122013754.GY4001@paulmck-ThinkPad-P17-Gen-1>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20221122013754.GY4001@paulmck-ThinkPad-P17-Gen-1>
Precedence: bulk

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
> >