Messages in this thread | | | Date | Fri, 22 Mar 2019 05:43:10 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v3] rcu: Allow to eliminate softirq processing from rcutree |
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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 <tglx@linutronix.de> > > > > Tested-by: Mike Galbraith <efault@gmx.de> > > > > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@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);
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