linux-kernel - Re: [PATCH] rcutorture: Fix ksoftirqd boosting timing and iteration

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Date:   Fri, 10 Jun 2022 10:33:53 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, rcu@...r.kernel.org
Subject: Re: [PATCH] rcutorture: Fix ksoftirqd boosting timing and iteration

On Fri, Jun 10, 2022 at 03:03:57PM +0200, Frederic Weisbecker wrote:
> The RCU priority boosting can fail on two situations:
> 
> 1) If (nr_cpus= > maxcpus=), which means if the total number of CPUs
> if higher than those brought online on boot, then torture_onoff() may
> later bring up CPUs that weren't online on boot. Now since rcutorture
> initialization only boosts the ksoftirqds of the CPUs that have been
> set online on boot, the CPUs later set online by torture_onoff won't
> benefit from the boost, making RCU priority boosting fail.
> 
> 2) Ksoftirqds kthreads are boosted after the creation of
> rcu_torture_boost() kthreads, which opens a window large enough for them
> to stutter in low FIFO mode while waiting for ksoftirqds that are still
> in SCHED_NORMAL mode.
> 
> The issues can trigger for example with:
> 
> 	./kvm.sh --configs TREE01 --kconfig "CONFIG_RCU_BOOST=y"
> 
> 	[   34.968561] rcu-torture: !!!
> 	[   34.968627] ------------[ cut here ]------------
> 	[   35.014054] WARNING: CPU: 4 PID: 114 at kernel/rcu/rcutorture.c:1979 rcu_torture_stats_print+0x5ad/0x610
> 	[   35.052043] Modules linked in:
> 	[   35.069138] CPU: 4 PID: 114 Comm: rcu_torture_sta Not tainted 5.18.0-rc1 #1
> 	[   35.096424] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.14.0-0-g155821a-rebuilt.opensuse.org 04/01/2014
> 	[   35.154570] RIP: 0010:rcu_torture_stats_print+0x5ad/0x610
> 	[   35.198527] Code: 63 1b 02 00 74 02 0f 0b 48 83 3d 35 63 1b 02 00 74 02 0f 0b 48 83 3d 21 63 1b 02 00 74 02 0f 0b 48 83 3d 0d 63 1b 02 00 74 02 <0f> 0b 83 eb 01 0f 8e ba fc ff ff 0f 0b e9 b3 fc ff f82
> 	[   37.251049] RSP: 0000:ffffa92a0050bdf8 EFLAGS: 00010202
> 	[   37.277320] rcu: De-offloading 8
> 	[   37.290367] RAX: 0000000000000000 RBX: 0000000000000001 RCX: 0000000000000001
> 	[   37.290387] RDX: 0000000000000000 RSI: 00000000ffffbfff RDI: 00000000ffffffff
> 	[   37.290398] RBP: 000000000000007b R08: 0000000000000000 R09: c0000000ffffbfff
> 	[   37.290407] R10: 000000000000002a R11: ffffa92a0050bc18 R12: ffffa92a0050be20
> 	[   37.290417] R13: ffffa92a0050be78 R14: 0000000000000000 R15: 000000000001bea0
> 	[   37.290427] FS:  0000000000000000(0000) GS:ffff96045eb00000(0000) knlGS:0000000000000000
> 	[   37.290448] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> 	[   37.290460] CR2: 0000000000000000 CR3: 000000001dc0c000 CR4: 00000000000006e0
> 	[   37.290470] Call Trace:
> 	[   37.295049]  <TASK>
> 	[   37.295065]  ? preempt_count_add+0x63/0x90
> 	[   37.295095]  ? _raw_spin_lock_irqsave+0x12/0x40
> 	[   37.295125]  ? rcu_torture_stats_print+0x610/0x610
> 	[   37.295143]  rcu_torture_stats+0x29/0x70
> 	[   37.295160]  kthread+0xe3/0x110
> 	[   37.295176]  ? kthread_complete_and_exit+0x20/0x20
> 	[   37.295193]  ret_from_fork+0x22/0x30
> 	[   37.295218]  </TASK>
> 
> Fix this with boosting the ksoftirqds kthreads from the boosting
> hotplug callback itself and before the boosting kthreads are created.
> 
> Fixes: ea6d962e80b6 ("rcutorture: Judge RCU priority boosting on grace periods, not callbacks")
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>

Good catch!  Queued for testing and review, thank you!

							Thanx, Paul

> ---
>  kernel/rcu/rcutorture.c | 28 +++++++++++++---------------
>  1 file changed, 13 insertions(+), 15 deletions(-)
> 
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index abb3f6d720f1..21470ebb15eb 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -2136,6 +2136,19 @@ static int rcutorture_booster_init(unsigned int cpu)
>  	if (boost_tasks[cpu] != NULL)
>  		return 0;  /* Already created, nothing more to do. */
>  
> +	// Testing RCU priority boosting requires rcutorture do
> +	// some serious abuse.  Counter this by running ksoftirqd
> +	// at higher priority.
> +	if (IS_BUILTIN(CONFIG_RCU_TORTURE_TEST)) {
> +		struct sched_param sp;
> +		struct task_struct *t;
> +
> +		t = per_cpu(ksoftirqd, cpu);
> +		WARN_ON_ONCE(!t);
> +		sp.sched_priority = 2;
> +		sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
> +	}
> +
>  	/* Don't allow time recalculation while creating a new task. */
>  	mutex_lock(&boost_mutex);
>  	rcu_torture_disable_rt_throttle();
> @@ -3384,21 +3397,6 @@ rcu_torture_init(void)
>  		rcutor_hp = firsterr;
>  		if (torture_init_error(firsterr))
>  			goto unwind;
> -
> -		// Testing RCU priority boosting requires rcutorture do
> -		// some serious abuse.  Counter this by running ksoftirqd
> -		// at higher priority.
> -		if (IS_BUILTIN(CONFIG_RCU_TORTURE_TEST)) {
> -			for_each_online_cpu(cpu) {
> -				struct sched_param sp;
> -				struct task_struct *t;
> -
> -				t = per_cpu(ksoftirqd, cpu);
> -				WARN_ON_ONCE(!t);
> -				sp.sched_priority = 2;
> -				sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
> -			}
> -		}
>  	}
>  	shutdown_jiffies = jiffies + shutdown_secs * HZ;
>  	firsterr = torture_shutdown_init(shutdown_secs, rcu_torture_cleanup);
> -- 
> 2.25.1
>