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    SubjectRe: [PATCH v3] rcu: Allow to eliminate softirq processing from rcutree
    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.

    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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