On Fri, Mar 22, 2019 at 12:35:53AM -0700, Paul E. McKenney wrote:
> On Thu, Mar 21, 2019 at 04:32:44PM -0700, Paul E. McKenney wrote:
> > On Wed, Mar 20, 2019 at 04:46:01PM -0700, Paul E. McKenney wrote:
> > > On Wed, Mar 20, 2019 at 10:13:33PM +0100, Sebastian Andrzej Siewior wrote:
> > > > Running RCU out of softirq is a problem for some workloads that would
> > > > like to manage RCU core processing independently of other softirq
> > > > work, for example, setting kthread priority. This commit therefore
> > > > introduces the `rcunosoftirq' option which moves the RCU core work
> > > > from softirq to a per-CPU/per-flavor SCHED_OTHER kthread named rcuc.
> > > > The SCHED_OTHER approach avoids the scalability problems that appeared
> > > > with the earlier attempt to move RCU core processing to from softirq
> > > > to kthreads. That said, kernels built with RCU_BOOST=y will run the
> > > > rcuc kthreads at the RCU-boosting priority.
> > > >
> > > > Reported-by: Thomas Gleixner <t...@linutronix.de>
> > > > Tested-by: Mike Galbraith <efa...@gmx.de>
> > > > Signed-off-by: Sebastian Andrzej Siewior <bige...@linutronix.de>
> > >
> > > Thank you! I reverted v2 and applied this one with the same sort of
> > > update. Testing is going well thus far aside from my failing to add
> > > the required "=0" after the rcutree.use_softirq. I will probably not
> > > be the only one who will run afoul of this, so I updated the commit log
> > > and the documentation accordingly, as shown below.
> >
> > And I took a look, please see updates/questions interspersed.
> >
> > I didn't find anything substantive, but still I get hangs. Which is
> > the normal situation. ;-)
> >
> > Will fire off more testing...
>
> And despite my protestations about restrictions involving the scheduler
> and rcu_read_unlock(), with the patch below TREE01, TREE02, TREE03, and
> TREE09 pass an hour of rcutorture with rcutree.use_softirq=0. Without
> this patch, seven-minute runs get hard hangs and this:
>
> [ 18.417315] BUG: spinlock recursion on CPU#5, rcu_torture_rea/763
> [ 18.418624] lock: 0xffff9d207eb61940, .magic: dead4ead, .owner:
> rcu_torture_rea/763, .owner_cpu: 5
> [ 18.420418] CPU: 5 PID: 763 Comm: rcu_torture_rea Not tainted 5.1.0-rc1+ #1
> [ 18.421786] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> Bochs 01/01/2011
> [ 18.423375] Call Trace:
> [ 18.423880] <IRQ>
> [ 18.424284] dump_stack+0x46/0x5b
> [ 18.424953] do_raw_spin_lock+0x8d/0x90
> [ 18.425699] try_to_wake_up+0x2cd/0x4f0
> [ 18.426493] invoke_rcu_core_kthread+0x63/0x80
> [ 18.427337] rcu_read_unlock_special+0x41/0x80
> [ 18.428212] __rcu_read_unlock+0x48/0x50
> [ 18.428984] cpuacct_charge+0x96/0xd0
> [ 18.429725] ? cpuacct_charge+0x2e/0xd0
> [ 18.430463] update_curr+0x112/0x240
> [ 18.431172] enqueue_task_fair+0xa9/0x1220
> [ 18.432009] ttwu_do_activate+0x49/0xa0
> [ 18.432741] sched_ttwu_pending+0x75/0xa0
> [ 18.433583] scheduler_ipi+0x53/0x150
> [ 18.434291] reschedule_interrupt+0xf/0x20
> [ 18.435137] </IRQ
>
> I clearly need to audit the setting of ->rcu_read_unlock_special.
>
> Again, the patch below is bad for expedited grace periods, so it is
> experimental.
And this was just me being slow. A prior RCU read-side critical
section might have been preempted, but have had something (bh, irq,
preempt) disabled at rcu_read_unlock() time. Then the task remains
queued until the next full-up quiescent state.
So this hack is what I have for the time being. I will be looking
into it more...
Thanx, Paul
> ------------------------------------------------------------------------
>
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index ca972b0b2467..d133fa837426 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -607,12 +607,9 @@ static void rcu_read_unlock_special(struct task_struct
> *t)
> if (preempt_bh_were_disabled || irqs_were_disabled) {
> WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> /* Need to defer quiescent state until everything is enabled. */
> - if (irqs_were_disabled) {
> + if (irqs_were_disabled && use_softirq) {
> /* Enabling irqs does not reschedule, so... */
> - if (use_softirq)
> - raise_softirq_irqoff(RCU_SOFTIRQ);
> - else
> - invoke_rcu_core();
> + raise_softirq_irqoff(RCU_SOFTIRQ);
> } else {
> /* Enabling BH or preempt does reschedule, so... */
> set_tsk_need_resched(current);
