Re: [PATCH tip/core/rcu 05/13] rcu: Always set .need_qs from __rcu_read_lock() for strict GPs
On Mon, Sep 07, 2020 at 08:11:55AM +0800, Boqun Feng wrote:
> On Fri, Sep 04, 2020 at 06:41:42AM -0700, Paul E. McKenney wrote:
> > On Fri, Sep 04, 2020 at 12:05:34PM +0800, Boqun Feng wrote:
> > > Hi Paul,
> > >
> > > On Mon, Aug 31, 2020 at 11:11:12AM -0700, paul...@kernel.org wrote:
> > > > From: "Paul E. McKenney"
> > > >
> > > > The ->rcu_read_unlock_special.b.need_qs field in the task_struct
> > > > structure indicates that the RCU core needs a quiscent state from the
> > > > corresponding task. The __rcu_read_unlock() function checks this (via
> > > > an eventual call to rcu_preempt_deferred_qs_irqrestore()), and if set
> > > > reports a quiscent state immediately upon exit from the outermost RCU
> > > > read-side critical section.
> > > >
> > > > Currently, this flag is only set when the scheduling-clock interrupt
> > > > decides that the current RCU grace period is too old, as in about
> > > > one full second too old. But if the kernel has been built with
> > > > CONFIG_RCU_STRICT_GRACE_PERIOD=y, we clearly do not want to wait that
> > > > long. This commit therefore sets the .need_qs field immediately at the
> > > > start of the RCU read-side critical section from within
> > > > __rcu_read_lock()
> > > > in order to unconditionally enlist help from __rcu_read_unlock().
> > > >
> > >
> > > So why not make rcu_preempt_deferred_qs_irqrestore() always treat
> > > need_qs is true if CONFIG_RCU_STRICT_GRACE_PERIOD = y? IOW:
> > >
> > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > > index 982fc5be5269..2a9f31545453 100644
> > > --- a/kernel/rcu/tree_plugin.h
> > > +++ b/kernel/rcu/tree_plugin.h
> > > @@ -449,6 +449,8 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct
> > > *t, unsigned long flags)
> > >* t->rcu_read_unlock_special cannot change.
> > >*/
> > > special = t->rcu_read_unlock_special;
> > > + if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread)
> > > + special.b.need_qs = true;
> > > rdp = this_cpu_ptr(_data);
> > > if (!special.s && !rdp->exp_deferred_qs) {
> > > local_irq_restore(flags);
> > >
> > > , and in this way, you can save one store for each rcu_read_lock() ;-)
> >
> > Because unless I am missing something subtle, if the .need_qs
> > flag is not set, execution is not guaranteed to reach
> > rcu_preempt_deferred_qs_irqrestore().
>
> Fair enough. Although I think we can also add IS_ENABLED(...) check to
> make the outermost rcu_read_unlock() to call rcu_read_unlock_special()
> unconditionally, but that's too much I think.
You are quite right that there are several ways to make this
work. So yes, one alternative would be the IS_ENABLED() check in
rcu_read_unlock() in conjunction with your suggested added check in
rcu_preempt_deferred_qs_irqrestore(). But from what I know at the moment,
the current state is favored. Smaller change and all that.
Thanx, Paul
> Regards,
> Boqun
>
> > Thanx, Paul
> >
> > > Regards,
> > > Boqun
> > >
> > > > But note the additional check for rcu_state.gp_kthread, which prevents
> > > > attempts to awaken RCU's grace-period kthread during early boot before
> > > > there is a scheduler. Leaving off this check results in early boot
> > > > hangs.
> > > > So early that there is no console output. Thus, this additional check
> > > > fails until such time as RCU's grace-period kthread has been created,
> > > > avoiding these empty-console hangs.
> > > >
> > > > Reported-by Jann Horn
> > > > Signed-off-by: Paul E. McKenney
> > > > ---
> > > > kernel/rcu/tree_plugin.h | 2 ++
> > > > 1 file changed, 2 insertions(+)
> > > >
> > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > > > index 44cf77d..668bbd2 100644
> > > > --- a/kernel/rcu/tree_plugin.h
> > > > +++ b/kernel/rcu/tree_plugin.h
> > > > @@ -376,6 +376,8 @@ void __rcu_read_lock(void)
> > > > rcu_preempt_read_enter();
> > > > if (IS_ENABLED(CONFIG_PROVE_LOCKING))
> > > > WARN_ON_ONCE(rcu_preempt_depth() > RCU_NEST_PMAX);
> > > > + if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) &&
> > > > rcu_state.gp_kthread)
> > > > + WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs,
> > > > true);
> > > > barrier(); /* critical section after entry code. */
> > > > }
> > > > EXPORT_SYMBOL_GPL(__rcu_read_lock);
> > > > --
> > > > 2.9.5
> > > >
Re: [PATCH tip/core/rcu 05/13] rcu: Always set .need_qs from __rcu_read_lock() for strict GPs
On Fri, Sep 04, 2020 at 06:41:42AM -0700, Paul E. McKenney wrote:
> On Fri, Sep 04, 2020 at 12:05:34PM +0800, Boqun Feng wrote:
> > Hi Paul,
> >
> > On Mon, Aug 31, 2020 at 11:11:12AM -0700, paul...@kernel.org wrote:
> > > From: "Paul E. McKenney"
> > >
> > > The ->rcu_read_unlock_special.b.need_qs field in the task_struct
> > > structure indicates that the RCU core needs a quiscent state from the
> > > corresponding task. The __rcu_read_unlock() function checks this (via
> > > an eventual call to rcu_preempt_deferred_qs_irqrestore()), and if set
> > > reports a quiscent state immediately upon exit from the outermost RCU
> > > read-side critical section.
> > >
> > > Currently, this flag is only set when the scheduling-clock interrupt
> > > decides that the current RCU grace period is too old, as in about
> > > one full second too old. But if the kernel has been built with
> > > CONFIG_RCU_STRICT_GRACE_PERIOD=y, we clearly do not want to wait that
> > > long. This commit therefore sets the .need_qs field immediately at the
> > > start of the RCU read-side critical section from within __rcu_read_lock()
> > > in order to unconditionally enlist help from __rcu_read_unlock().
> > >
> >
> > So why not make rcu_preempt_deferred_qs_irqrestore() always treat
> > need_qs is true if CONFIG_RCU_STRICT_GRACE_PERIOD = y? IOW:
> >
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 982fc5be5269..2a9f31545453 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -449,6 +449,8 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct
> > *t, unsigned long flags)
> > * t->rcu_read_unlock_special cannot change.
> > */
> > special = t->rcu_read_unlock_special;
> > + if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread)
> > + special.b.need_qs = true;
> > rdp = this_cpu_ptr(_data);
> > if (!special.s && !rdp->exp_deferred_qs) {
> > local_irq_restore(flags);
> >
> > , and in this way, you can save one store for each rcu_read_lock() ;-)
>
> Because unless I am missing something subtle, if the .need_qs
> flag is not set, execution is not guaranteed to reach
> rcu_preempt_deferred_qs_irqrestore().
>
Fair enough. Although I think we can also add IS_ENABLED(...) check to
make the outermost rcu_read_unlock() to call rcu_read_unlock_special()
unconditionally, but that's too much I think.
Regards,
Boqun
> Thanx, Paul
>
> > Regards,
> > Boqun
> >
> > > But note the additional check for rcu_state.gp_kthread, which prevents
> > > attempts to awaken RCU's grace-period kthread during early boot before
> > > there is a scheduler. Leaving off this check results in early boot hangs.
> > > So early that there is no console output. Thus, this additional check
> > > fails until such time as RCU's grace-period kthread has been created,
> > > avoiding these empty-console hangs.
> > >
> > > Reported-by Jann Horn
> > > Signed-off-by: Paul E. McKenney
> > > ---
> > > kernel/rcu/tree_plugin.h | 2 ++
> > > 1 file changed, 2 insertions(+)
> > >
> > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > > index 44cf77d..668bbd2 100644
> > > --- a/kernel/rcu/tree_plugin.h
> > > +++ b/kernel/rcu/tree_plugin.h
> > > @@ -376,6 +376,8 @@ void __rcu_read_lock(void)
> > > rcu_preempt_read_enter();
> > > if (IS_ENABLED(CONFIG_PROVE_LOCKING))
> > > WARN_ON_ONCE(rcu_preempt_depth() > RCU_NEST_PMAX);
> > > + if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread)
> > > + WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, true);
> > > barrier(); /* critical section after entry code. */
> > > }
> > > EXPORT_SYMBOL_GPL(__rcu_read_lock);
> > > --
> > > 2.9.5
> > >
Re: [PATCH tip/core/rcu 05/13] rcu: Always set .need_qs from __rcu_read_lock() for strict GPs
On Fri, Sep 04, 2020 at 12:05:34PM +0800, Boqun Feng wrote:
> Hi Paul,
>
> On Mon, Aug 31, 2020 at 11:11:12AM -0700, paul...@kernel.org wrote:
> > From: "Paul E. McKenney"
> >
> > The ->rcu_read_unlock_special.b.need_qs field in the task_struct
> > structure indicates that the RCU core needs a quiscent state from the
> > corresponding task. The __rcu_read_unlock() function checks this (via
> > an eventual call to rcu_preempt_deferred_qs_irqrestore()), and if set
> > reports a quiscent state immediately upon exit from the outermost RCU
> > read-side critical section.
> >
> > Currently, this flag is only set when the scheduling-clock interrupt
> > decides that the current RCU grace period is too old, as in about
> > one full second too old. But if the kernel has been built with
> > CONFIG_RCU_STRICT_GRACE_PERIOD=y, we clearly do not want to wait that
> > long. This commit therefore sets the .need_qs field immediately at the
> > start of the RCU read-side critical section from within __rcu_read_lock()
> > in order to unconditionally enlist help from __rcu_read_unlock().
> >
>
> So why not make rcu_preempt_deferred_qs_irqrestore() always treat
> need_qs is true if CONFIG_RCU_STRICT_GRACE_PERIOD = y? IOW:
>
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 982fc5be5269..2a9f31545453 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -449,6 +449,8 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t,
> unsigned long flags)
>* t->rcu_read_unlock_special cannot change.
>*/
> special = t->rcu_read_unlock_special;
> + if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread)
> + special.b.need_qs = true;
> rdp = this_cpu_ptr(_data);
> if (!special.s && !rdp->exp_deferred_qs) {
> local_irq_restore(flags);
>
> , and in this way, you can save one store for each rcu_read_lock() ;-)
Because unless I am missing something subtle, if the .need_qs
flag is not set, execution is not guaranteed to reach
rcu_preempt_deferred_qs_irqrestore().
Thanx, Paul
> Regards,
> Boqun
>
> > But note the additional check for rcu_state.gp_kthread, which prevents
> > attempts to awaken RCU's grace-period kthread during early boot before
> > there is a scheduler. Leaving off this check results in early boot hangs.
> > So early that there is no console output. Thus, this additional check
> > fails until such time as RCU's grace-period kthread has been created,
> > avoiding these empty-console hangs.
> >
> > Reported-by Jann Horn
> > Signed-off-by: Paul E. McKenney
> > ---
> > kernel/rcu/tree_plugin.h | 2 ++
> > 1 file changed, 2 insertions(+)
> >
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 44cf77d..668bbd2 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -376,6 +376,8 @@ void __rcu_read_lock(void)
> > rcu_preempt_read_enter();
> > if (IS_ENABLED(CONFIG_PROVE_LOCKING))
> > WARN_ON_ONCE(rcu_preempt_depth() > RCU_NEST_PMAX);
> > + if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread)
> > + WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, true);
> > barrier(); /* critical section after entry code. */
> > }
> > EXPORT_SYMBOL_GPL(__rcu_read_lock);
> > --
> > 2.9.5
> >
Re: [PATCH tip/core/rcu 05/13] rcu: Always set .need_qs from __rcu_read_lock() for strict GPs
Hi Paul,
On Mon, Aug 31, 2020 at 11:11:12AM -0700, paul...@kernel.org wrote:
> From: "Paul E. McKenney"
>
> The ->rcu_read_unlock_special.b.need_qs field in the task_struct
> structure indicates that the RCU core needs a quiscent state from the
> corresponding task. The __rcu_read_unlock() function checks this (via
> an eventual call to rcu_preempt_deferred_qs_irqrestore()), and if set
> reports a quiscent state immediately upon exit from the outermost RCU
> read-side critical section.
>
> Currently, this flag is only set when the scheduling-clock interrupt
> decides that the current RCU grace period is too old, as in about
> one full second too old. But if the kernel has been built with
> CONFIG_RCU_STRICT_GRACE_PERIOD=y, we clearly do not want to wait that
> long. This commit therefore sets the .need_qs field immediately at the
> start of the RCU read-side critical section from within __rcu_read_lock()
> in order to unconditionally enlist help from __rcu_read_unlock().
>
So why not make rcu_preempt_deferred_qs_irqrestore() always treat
need_qs is true if CONFIG_RCU_STRICT_GRACE_PERIOD = y? IOW:
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 982fc5be5269..2a9f31545453 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -449,6 +449,8 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t,
unsigned long flags)
* t->rcu_read_unlock_special cannot change.
*/
special = t->rcu_read_unlock_special;
+ if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread)
+ special.b.need_qs = true;
rdp = this_cpu_ptr(_data);
if (!special.s && !rdp->exp_deferred_qs) {
local_irq_restore(flags);
, and in this way, you can save one store for each rcu_read_lock() ;-)
Regards,
Boqun
> But note the additional check for rcu_state.gp_kthread, which prevents
> attempts to awaken RCU's grace-period kthread during early boot before
> there is a scheduler. Leaving off this check results in early boot hangs.
> So early that there is no console output. Thus, this additional check
> fails until such time as RCU's grace-period kthread has been created,
> avoiding these empty-console hangs.
>
> Reported-by Jann Horn
> Signed-off-by: Paul E. McKenney
> ---
> kernel/rcu/tree_plugin.h | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 44cf77d..668bbd2 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -376,6 +376,8 @@ void __rcu_read_lock(void)
> rcu_preempt_read_enter();
> if (IS_ENABLED(CONFIG_PROVE_LOCKING))
> WARN_ON_ONCE(rcu_preempt_depth() > RCU_NEST_PMAX);
> + if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread)
> + WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, true);
> barrier(); /* critical section after entry code. */
> }
> EXPORT_SYMBOL_GPL(__rcu_read_lock);
> --
> 2.9.5
>
