Re: [PATCH RT v3 4/5] rcu: Disable use_softirq on PREEMPT_RT

Re: [PATCH RT v3 4/5] rcu: Disable use_softirq on PREEMPT_RT

[Joel Fernandes]

Hi Scott,

Would you mind CC'ing rcu@vger.kernel.org on RCU related patches? I added it
for this time.

On Wed, Sep 11, 2019 at 05:57:28PM +0100, Scott Wood wrote:
> Besides restoring behavior that used to be default on RT, this avoids
> a deadlock on scheduler locks:
> 
> [  136.894657] 039: ============================================
> [  136.900401] 039: WARNING: possible recursive locking detected
> [  136.906146] 039: 5.2.9-rt3.dbg+ #174 Tainted: G            E
> [  136.912152] 039: --------------------------------------------
> [  136.917896] 039: rcu_torture_rea/13474 is trying to acquire lock:
> [  136.923990] 039: 000000005f25146d
> [  136.927310] 039:  (
> [  136.929414] 039: &p->pi_lock
> [  136.932303] 039: ){-...}
> [  136.934840] 039: , at: try_to_wake_up+0x39/0x920
> [  136.939461] 039:
> but task is already holding lock:
> [  136.944425] 039: 000000005f25146d
> [  136.947745] 039:  (
> [  136.949852] 039: &p->pi_lock
> [  136.952738] 039: ){-...}
> [  136.955274] 039: , at: try_to_wake_up+0x39/0x920
> [  136.959895] 039:
> other info that might help us debug this:
> [  136.965555] 039:  Possible unsafe locking scenario:
> 
> [  136.970608] 039:        CPU0
> [  136.973493] 039:        ----
> [  136.976378] 039:   lock(
> [  136.978918] 039: &p->pi_lock
> [  136.981806] 039: );
> [  136.983911] 039:   lock(
> [  136.986451] 039: &p->pi_lock
> [  136.989336] 039: );
> [  136.991444] 039:
>  *** DEADLOCK ***
> 
> [  136.995194] 039:  May be due to missing lock nesting notation
> 
> [  137.001115] 039: 3 locks held by rcu_torture_rea/13474:
> [  137.006341] 039:  #0:
> [  137.008707] 039: 000000005f25146d
> [  137.012024] 039:  (
> [  137.014131] 039: &p->pi_lock
> [  137.017015] 039: ){-...}
> [  137.019558] 039: , at: try_to_wake_up+0x39/0x920
> [  137.024175] 039:  #1:
> [  137.026540] 039: 0000000011c8e51d
> [  137.029859] 039:  (
> [  137.031966] 039: &rq->lock
> [  137.034679] 039: ){-...}
> [  137.037217] 039: , at: try_to_wake_up+0x241/0x920
> [  137.041924] 039:  #2:
> [  137.044291] 039: 00000000098649b9
> [  137.047610] 039:  (
> [  137.049714] 039: rcu_read_lock
> [  137.052774] 039: ){....}
> [  137.055314] 039: , at: cpuacct_charge+0x33/0x1e0
> [  137.059934] 039:
> stack backtrace:
> [  137.063425] 039: CPU: 39 PID: 13474 Comm: rcu_torture_rea Kdump: loaded Tainted: G            E     5.2.9-rt3.dbg+ #174
> [  137.074197] 039: Hardware name: Intel Corporation S2600BT/S2600BT, BIOS SE5C620.86B.01.00.0763.022420181017 02/24/2018
> [  137.084886] 039: Call Trace:
> [  137.087773] 039:  <IRQ>
> [  137.090226] 039:  dump_stack+0x5e/0x8b
> [  137.093997] 039:  __lock_acquire+0x725/0x1100
> [  137.098358] 039:  lock_acquire+0xc0/0x240
> [  137.102374] 039:  ? try_to_wake_up+0x39/0x920
> [  137.106737] 039:  _raw_spin_lock_irqsave+0x47/0x90
> [  137.111534] 039:  ? try_to_wake_up+0x39/0x920
> [  137.115910] 039:  try_to_wake_up+0x39/0x920
> [  137.120098] 039:  rcu_read_unlock_special+0x65/0xb0
> [  137.124977] 039:  __rcu_read_unlock+0x5d/0x70
> [  137.129337] 039:  cpuacct_charge+0xd9/0x1e0
> [  137.133522] 039:  ? cpuacct_charge+0x33/0x1e0
> [  137.137880] 039:  update_curr+0x14b/0x420
> [  137.141894] 039:  enqueue_entity+0x42/0x370
> [  137.146080] 039:  enqueue_task_fair+0xa9/0x490
> [  137.150528] 039:  activate_task+0x5a/0xf0
> [  137.154539] 039:  ttwu_do_activate+0x4e/0x90
> [  137.158813] 039:  try_to_wake_up+0x277/0x920
> [  137.163086] 039:  irq_exit+0xb6/0xf0
> [  137.166661] 039:  smp_apic_timer_interrupt+0xe3/0x3a0
> [  137.171714] 039:  apic_timer_interrupt+0xf/0x20
> [  137.176249] 039:  </IRQ>
> [  137.178785] 039: RIP: 0010:__schedule+0x0/0x8e0
> [  137.183319] 039: Code: 00 02 48 89 43 20 e8 0f 5a 00 00 48 8d 7b 28 e8 86 f2 fd ff 31 c0 5b 5d 41 5c c3 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 <55> 48 89 e5 41 57 41 56 49 c7 c6 c0 ca 1e 00 41 55 41 89 fd 41 54
> [  137.202498] 039: RSP: 0018:ffffc9005835fbc0 EFLAGS: 00000246
> [  137.208158] 039:  ORIG_RAX: ffffffffffffff13
> [  137.212428] 039: RAX: 0000000000000000 RBX: ffff8897c3e1bb00 RCX: 0000000000000001
> [  137.219994] 039: RDX: 0000000080004008 RSI: 0000000000000006 RDI: 0000000000000001
> [  137.227560] 039: RBP: ffff8897c3e1bb00 R08: 0000000000000000 R09: 0000000000000000
> [  137.235126] 039: R10: 0000000000000001 R11: 0000000000000001 R12: ffffffff81001fd1
> [  137.242694] 039: R13: 0000000000000044 R14: 0000000000000000 R15: ffffc9005835fcac
> [  137.250259] 039:  ? ___preempt_schedule+0x16/0x18
> [  137.254969] 039:  preempt_schedule_common+0x32/0x80
> [  137.259846] 039:  ___preempt_schedule+0x16/0x18
> [  137.264379] 039:  rcutorture_one_extend+0x33a/0x510 [rcutorture]
> [  137.270397] 039:  rcu_torture_one_read+0x18c/0x450 [rcutorture]
> [  137.276334] 039:  rcu_torture_reader+0xac/0x1f0 [rcutorture]
> [  137.281998] 039:  ? rcu_torture_reader+0x1f0/0x1f0 [rcutorture]
> [  137.287920] 039:  kthread+0x106/0x140
> [  137.291591] 039:  ? rcu_torture_one_read+0x450/0x450 [rcutorture]
> [  137.297681] 039:  ? kthread_bind+0x10/0x10
> [  137.301783] 039:  ret_from_fork+0x3a/0x50
> 
> Signed-off-by: Scott Wood <swood@redhat.com>
> ---
> The prohibition on use_softirq should be able to be dropped once RT gets
> the latest RCU code, but the question of what use_softirq should default
> to on PREEMPT_RT remains.
> 
> v3: Use IS_ENABLED

Out of curiosity, does PREEMPT_RT use the NOCB callback offloading? If no,
should it use it? IIUC, that does make the work the softirq have to do less
work since the callbacks are executed in threaded context.

If yes, can RT tolerate use_softirq=false and what could a realistic softirq
running/completion time be that PREEMPT_RT can tolerate? I guess that can be
answered by running rcuperf on PREEMPT_RT with a NOCB configuration and
measuring softirq worst-case start/completion times.

I could run these tests myself but I am vacation for the next week or so.

thanks,

 - Joel


> ---
>  kernel/rcu/tree.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index fc8b00c61b32..ee0a5ec2c30f 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -98,9 +98,14 @@ struct rcu_state rcu_state = {
>  /* Dump rcu_node combining tree at boot to verify correct setup. */
>  static bool dump_tree;
>  module_param(dump_tree, bool, 0444);
> -/* By default, use RCU_SOFTIRQ instead of rcuc kthreads. */
> -static bool use_softirq = 1;
> +/*
> + * By default, use RCU_SOFTIRQ instead of rcuc kthreads.
> + * But, avoid RCU_SOFTIRQ on PREEMPT_RT due to pi/rq deadlocks.
> + */
> +static bool use_softirq = !IS_ENABLED(CONFIG_PREEMPT_RT_FULL);
> +#ifdef CONFIG_PREEMPT_RT_FULL
>  module_param(use_softirq, bool, 0444);
> +#endif
>  /* Control rcu_node-tree auto-balancing at boot time. */
>  static bool rcu_fanout_exact;
>  module_param(rcu_fanout_exact, bool, 0444);
> -- 
> 1.8.3.1
> 

Re: [PATCH RT v3 4/5] rcu: Disable use_softirq on PREEMPT_RT

[Joel Fernandes]

On Thu, Sep 12, 2019 at 05:38:43PM -0400, Joel Fernandes wrote:
> Hi Scott,
> 
> Would you mind CC'ing rcu@vger.kernel.org on RCU related patches? I added it
> for this time.
> 
> On Wed, Sep 11, 2019 at 05:57:28PM +0100, Scott Wood wrote:
> > Besides restoring behavior that used to be default on RT, this avoids
> > a deadlock on scheduler locks:
[snip]
> > [  136.995194] 039:  May be due to missing lock nesting notation
> > 
> > [  137.001115] 039: 3 locks held by rcu_torture_rea/13474:
> > [  137.006341] 039:  #0:
> > [  137.008707] 039: 000000005f25146d
> > [  137.012024] 039:  (
> > [  137.014131] 039: &p->pi_lock
> > [  137.017015] 039: ){-...}
> > [  137.019558] 039: , at: try_to_wake_up+0x39/0x920
> > [  137.024175] 039:  #1:
> > [  137.026540] 039: 0000000011c8e51d
> > [  137.029859] 039:  (
> > [  137.031966] 039: &rq->lock
> > [  137.034679] 039: ){-...}
> > [  137.037217] 039: , at: try_to_wake_up+0x241/0x920
> > [  137.041924] 039:  #2:
> > [  137.044291] 039: 00000000098649b9
> > [  137.047610] 039:  (
> > [  137.049714] 039: rcu_read_lock
> > [  137.052774] 039: ){....}
> > [  137.055314] 039: , at: cpuacct_charge+0x33/0x1e0
> > [  137.059934] 039:
> > stack backtrace:
> > [  137.063425] 039: CPU: 39 PID: 13474 Comm: rcu_torture_rea Kdump: loaded Tainted: G            E     5.2.9-rt3.dbg+ #174
> > [  137.074197] 039: Hardware name: Intel Corporation S2600BT/S2600BT, BIOS SE5C620.86B.01.00.0763.022420181017 02/24/2018
> > [  137.084886] 039: Call Trace:
> > [  137.087773] 039:  <IRQ>
> > [  137.090226] 039:  dump_stack+0x5e/0x8b
> > [  137.093997] 039:  __lock_acquire+0x725/0x1100
> > [  137.098358] 039:  lock_acquire+0xc0/0x240
> > [  137.102374] 039:  ? try_to_wake_up+0x39/0x920
> > [  137.106737] 039:  _raw_spin_lock_irqsave+0x47/0x90
> > [  137.111534] 039:  ? try_to_wake_up+0x39/0x920
> > [  137.115910] 039:  try_to_wake_up+0x39/0x920
> > [  137.120098] 039:  rcu_read_unlock_special+0x65/0xb0
> > [  137.124977] 039:  __rcu_read_unlock+0x5d/0x70
> > [  137.129337] 039:  cpuacct_charge+0xd9/0x1e0
> > [  137.133522] 039:  ? cpuacct_charge+0x33/0x1e0
> > [  137.137880] 039:  update_curr+0x14b/0x420
> > [  137.141894] 039:  enqueue_entity+0x42/0x370
> > [  137.146080] 039:  enqueue_task_fair+0xa9/0x490
> > [  137.150528] 039:  activate_task+0x5a/0xf0
> > [  137.154539] 039:  ttwu_do_activate+0x4e/0x90
> > [  137.158813] 039:  try_to_wake_up+0x277/0x920
> > [  137.163086] 039:  irq_exit+0xb6/0xf0
[snip]
> > Signed-off-by: Scott Wood <swood@redhat.com>
> > ---
> > The prohibition on use_softirq should be able to be dropped once RT gets
> > the latest RCU code, but the question of what use_softirq should default
> > to on PREEMPT_RT remains.
> > 
> > v3: Use IS_ENABLED
> 
> Out of curiosity, does PREEMPT_RT use the NOCB callback offloading? If no,
> should it use it? IIUC, that does make the work the softirq have to do less
> work since the callbacks are executed in threaded context.
> 
> If yes, can RT tolerate use_softirq=false and what could a realistic softirq

s/use_softirq=false/use_softirq=true/

thanks,

 - Joel

Re: [PATCH RT v3 4/5] rcu: Disable use_softirq on PREEMPT_RT

[Sebastian Andrzej Siewior]

On 2019-09-12 17:38:43 [-0400], Joel Fernandes wrote:
> > The prohibition on use_softirq should be able to be dropped once RT gets
> > the latest RCU code, but the question of what use_softirq should default
> > to on PREEMPT_RT remains.
> > 
> > v3: Use IS_ENABLED

I'm going to pick it up.

> Out of curiosity, does PREEMPT_RT use the NOCB callback offloading? If no,
> should it use it? IIUC, that does make the work the softirq have to do less
> work since the callbacks are executed in threaded context.

It can use it, it is recommended to do so and to move those threads out
of the CPU(s) that are dedicated for RT workload. But then there are
those with a single CPU.

> If yes, can RT tolerate use_softirq=false and what could a realistic softirq
> running/completion time be that PREEMPT_RT can tolerate? I guess that can be
> answered by running rcuperf on PREEMPT_RT with a NOCB configuration and
> measuring softirq worst-case start/completion times.

It depends. RT as of now. The softirq can be preempted this includes the
RCU softirq. That means that a wakeup of a high priority RT task can
preempt the RCU softirq. So this might not be an issue.
the softirq takes a global per-CPU lock which means we can only have one
softirq vector running at a time (we used to have the possibility of
multiple vectors running in parallel but that is gone now). So this
enforces the behaviour we have in !RT as of today.  If you rely on
"timely" executed softirq then long running RCU-softirq might be bad for
you. But so is everything else that might cause long running softirqs.

What I don't know is how RCU-boosting and preempted softirq works.

> I could run these tests myself but I am vacation for the next week or so.
> 
> thanks,
> 
>  - Joel

Sebastian