There are several scenario in network processing that can run
extensively under heavy traffic. In such situation, RCU synchronization
might not observe desired quiescent states for indefinitely long period.
Create a helper to safely raise the desired RCU quiescent states for
such scenario.

Currently the frequency is locked at HZ/10, i.e. 100ms, which is
sufficient to address existing problems around RCU tasks. It's unclear
yet if there is any future scenario for it to be further tuned down.

Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Jesper Dangaard Brouer <hawk@kernel.org>
Signed-off-by: Yan Zhai <yan@cloudflare.com>
---
v3->v4: comment fixup

---
 include/linux/rcupdate.h | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 0746b1b0b663..da224706323e 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -247,6 +247,30 @@ do { \
 	cond_resched(); \
 } while (0)
 
+/**
+ * rcu_softirq_qs_periodic - Periodically report consolidated quiescent states
+ * @old_ts: last jiffies when QS was reported. Might be modified in the macro.
+ *
+ * This helper is for network processing in non-RT kernels, where there could
+ * be busy polling threads that block RCU synchronization indefinitely.  In
+ * such context, simply calling cond_resched is insufficient, so give it a
+ * stronger push to eliminate all potential blockage of all RCU types.
+ *
+ * NOTE: unless absolutely sure, this helper should in general be called
+ * outside of bh lock section to avoid reporting a surprising QS to updaters,
+ * who could be expecting RCU read critical section to end at local_bh_enable().
+ */
+#define rcu_softirq_qs_periodic(old_ts) \
+do { \
+	if (!IS_ENABLED(CONFIG_PREEMPT_RT) && \
+	    time_after(jiffies, (old_ts) + HZ / 10)) { \
+		preempt_disable(); \
+		rcu_softirq_qs(); \
+		preempt_enable(); \
+		(old_ts) = jiffies; \
+	} \
+} while (0)
+
 /*
  * Infrastructure to implement the synchronize_() primitives in
  * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
-- 
2.30.2

On Fri, Mar 15, 2024 at 12:55:03PM -0700, Yan Zhai wrote:
> There are several scenario in network processing that can run
> extensively under heavy traffic. In such situation, RCU synchronization
> might not observe desired quiescent states for indefinitely long period.
> Create a helper to safely raise the desired RCU quiescent states for
> such scenario.
> 
> Currently the frequency is locked at HZ/10, i.e. 100ms, which is
> sufficient to address existing problems around RCU tasks. It's unclear
> yet if there is any future scenario for it to be further tuned down.

I suggest something like the following for the commit log:

------------------------------------------------------------------------

When under heavy load, network processing can run CPU-bound for many tens
of seconds.  Even in preemptible kernels, this can block RCU Tasks grace
periods, which can cause trace-event removal to take more than a minute,
which is unacceptably long.

This commit therefore creates a new helper function that passes
through both RCU and RCU-Tasks quiescent states every 100 milliseconds.
This hard-coded value suffices for current workloads.

------------------------------------------------------------------------

> Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> Reviewed-by: Jesper Dangaard Brouer <hawk@kernel.org>
> Signed-off-by: Yan Zhai <yan@cloudflare.com>
> ---
> v3->v4: comment fixup
> 
> ---
>  include/linux/rcupdate.h | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 0746b1b0b663..da224706323e 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -247,6 +247,30 @@ do { \
>  	cond_resched(); \
>  } while (0)
>  
> +/**
> + * rcu_softirq_qs_periodic - Periodically report consolidated quiescent states
> + * @old_ts: last jiffies when QS was reported. Might be modified in the macro.
> + *
> + * This helper is for network processing in non-RT kernels, where there could
> + * be busy polling threads that block RCU synchronization indefinitely.  In
> + * such context, simply calling cond_resched is insufficient, so give it a
> + * stronger push to eliminate all potential blockage of all RCU types.
> + *
> + * NOTE: unless absolutely sure, this helper should in general be called
> + * outside of bh lock section to avoid reporting a surprising QS to updaters,
> + * who could be expecting RCU read critical section to end at local_bh_enable().
> + */

How about something like this for the kernel-doc comment?

/**
 * rcu_softirq_qs_periodic - Report RCU and RCU-Tasks quiescent states
 * @old_ts: jiffies at start of processing.
 *
 * This helper is for long-running softirq handlers, such as those
 * in networking.  The caller should initialize the variable passed in
 * as @old_ts at the beginning of the softirq handler.  When invoked
 * frequently, this macro will invoke rcu_softirq_qs() every 100
 * milliseconds thereafter, which will provide both RCU and RCU-Tasks
 * quiescent states.  Note that this macro modifies its old_ts argument.
 *
 * Note that although cond_resched() provides RCU quiescent states,
 * it does not provide RCU-Tasks quiescent states.
 *
 * Because regions of code that have disabled softirq act as RCU
 * read-side critical sections, this macro should be invoked with softirq
 * (and preemption) enabled.
 *
 * This macro has no effect in CONFIG_PREEMPT_RT kernels.
 */

							Thanx, Paul

> +#define rcu_softirq_qs_periodic(old_ts) \
> +do { \
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT) && \
> +	    time_after(jiffies, (old_ts) + HZ / 10)) { \
> +		preempt_disable(); \
> +		rcu_softirq_qs(); \
> +		preempt_enable(); \
> +		(old_ts) = jiffies; \
> +	} \
> +} while (0)
> +
>  /*
>   * Infrastructure to implement the synchronize_() primitives in
>   * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
> -- 
> 2.30.2
> 
>

On Sat, Mar 16, 2024 at 12:41 AM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Fri, Mar 15, 2024 at 12:55:03PM -0700, Yan Zhai wrote:
> > There are several scenario in network processing that can run
> > extensively under heavy traffic. In such situation, RCU synchronization
> > might not observe desired quiescent states for indefinitely long period.
> > Create a helper to safely raise the desired RCU quiescent states for
> > such scenario.
> >
> > Currently the frequency is locked at HZ/10, i.e. 100ms, which is
> > sufficient to address existing problems around RCU tasks. It's unclear
> > yet if there is any future scenario for it to be further tuned down.
>
> I suggest something like the following for the commit log:
>
> ------------------------------------------------------------------------
>
> When under heavy load, network processing can run CPU-bound for many tens
> of seconds.  Even in preemptible kernels, this can block RCU Tasks grace
> periods, which can cause trace-event removal to take more than a minute,
> which is unacceptably long.
>
> This commit therefore creates a new helper function that passes
> through both RCU and RCU-Tasks quiescent states every 100 milliseconds.
> This hard-coded value suffices for current workloads.
>
> ------------------------------------------------------------------------
>
> > Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> > Reviewed-by: Jesper Dangaard Brouer <hawk@kernel.org>
> > Signed-off-by: Yan Zhai <yan@cloudflare.com>
> > ---
> > v3->v4: comment fixup
> >
> > ---
> >  include/linux/rcupdate.h | 24 ++++++++++++++++++++++++
> >  1 file changed, 24 insertions(+)
> >
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 0746b1b0b663..da224706323e 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -247,6 +247,30 @@ do { \
> >       cond_resched(); \
> >  } while (0)
> >
> > +/**
> > + * rcu_softirq_qs_periodic - Periodically report consolidated quiescent states
> > + * @old_ts: last jiffies when QS was reported. Might be modified in the macro.
> > + *
> > + * This helper is for network processing in non-RT kernels, where there could
> > + * be busy polling threads that block RCU synchronization indefinitely.  In
> > + * such context, simply calling cond_resched is insufficient, so give it a
> > + * stronger push to eliminate all potential blockage of all RCU types.
> > + *
> > + * NOTE: unless absolutely sure, this helper should in general be called
> > + * outside of bh lock section to avoid reporting a surprising QS to updaters,
> > + * who could be expecting RCU read critical section to end at local_bh_enable().
> > + */
>
> How about something like this for the kernel-doc comment?
>
> /**
>  * rcu_softirq_qs_periodic - Report RCU and RCU-Tasks quiescent states
>  * @old_ts: jiffies at start of processing.
>  *
>  * This helper is for long-running softirq handlers, such as those
>  * in networking.  The caller should initialize the variable passed in
>  * as @old_ts at the beginning of the softirq handler.  When invoked
>  * frequently, this macro will invoke rcu_softirq_qs() every 100
>  * milliseconds thereafter, which will provide both RCU and RCU-Tasks
>  * quiescent states.  Note that this macro modifies its old_ts argument.
>  *
>  * Note that although cond_resched() provides RCU quiescent states,
>  * it does not provide RCU-Tasks quiescent states.
>  *
>  * Because regions of code that have disabled softirq act as RCU
>  * read-side critical sections, this macro should be invoked with softirq
>  * (and preemption) enabled.
>  *
>  * This macro has no effect in CONFIG_PREEMPT_RT kernels.
>  */
>
It would be more accurate this way, I like it. Thanks!

Yan

>                                                         Thanx, Paul
>
> > +#define rcu_softirq_qs_periodic(old_ts) \
> > +do { \
> > +     if (!IS_ENABLED(CONFIG_PREEMPT_RT) && \
> > +         time_after(jiffies, (old_ts) + HZ / 10)) { \
> > +             preempt_disable(); \
> > +             rcu_softirq_qs(); \
> > +             preempt_enable(); \
> > +             (old_ts) = jiffies; \
> > +     } \
> > +} while (0)
> > +
> >  /*
> >   * Infrastructure to implement the synchronize_() primitives in
> >   * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
> > --
> > 2.30.2
> >
> >

On Fri, Mar 15, 2024 at 10:40:56PM -0700, Paul E. McKenney wrote:
> On Fri, Mar 15, 2024 at 12:55:03PM -0700, Yan Zhai wrote:
> > There are several scenario in network processing that can run
> > extensively under heavy traffic. In such situation, RCU synchronization
> > might not observe desired quiescent states for indefinitely long period.
> > Create a helper to safely raise the desired RCU quiescent states for
> > such scenario.
> > 
> > Currently the frequency is locked at HZ/10, i.e. 100ms, which is
> > sufficient to address existing problems around RCU tasks. It's unclear
> > yet if there is any future scenario for it to be further tuned down.
> 
> I suggest something like the following for the commit log:
> 
> ------------------------------------------------------------------------
> 
> When under heavy load, network processing can run CPU-bound for many tens
> of seconds.  Even in preemptible kernels, this can block RCU Tasks grace
> periods, which can cause trace-event removal to take more than a minute,
> which is unacceptably long.
> 
> This commit therefore creates a new helper function that passes
> through both RCU and RCU-Tasks quiescent states every 100 milliseconds.
> This hard-coded value suffices for current workloads.

FWIW, this sounds good to me.

> 
> ------------------------------------------------------------------------
> 
> > Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> > Reviewed-by: Jesper Dangaard Brouer <hawk@kernel.org>
> > Signed-off-by: Yan Zhai <yan@cloudflare.com>
> > ---
> > v3->v4: comment fixup
> > 
> > ---
> >  include/linux/rcupdate.h | 24 ++++++++++++++++++++++++
> >  1 file changed, 24 insertions(+)
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 0746b1b0b663..da224706323e 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -247,6 +247,30 @@ do { \
> >  	cond_resched(); \
> >  } while (0)
> >  
> > +/**
> > + * rcu_softirq_qs_periodic - Periodically report consolidated quiescent states
> > + * @old_ts: last jiffies when QS was reported. Might be modified in the macro.
> > + *
> > + * This helper is for network processing in non-RT kernels, where there could
> > + * be busy polling threads that block RCU synchronization indefinitely.  In
> > + * such context, simply calling cond_resched is insufficient, so give it a
> > + * stronger push to eliminate all potential blockage of all RCU types.
> > + *
> > + * NOTE: unless absolutely sure, this helper should in general be called
> > + * outside of bh lock section to avoid reporting a surprising QS to updaters,
> > + * who could be expecting RCU read critical section to end at local_bh_enable().
> > + */
> 
> How about something like this for the kernel-doc comment?
> 
> /**
>  * rcu_softirq_qs_periodic - Report RCU and RCU-Tasks quiescent states
>  * @old_ts: jiffies at start of processing.
>  *
>  * This helper is for long-running softirq handlers, such as those
>  * in networking.  The caller should initialize the variable passed in
>  * as @old_ts at the beginning of the softirq handler.  When invoked
>  * frequently, this macro will invoke rcu_softirq_qs() every 100
>  * milliseconds thereafter, which will provide both RCU and RCU-Tasks
>  * quiescent states.  Note that this macro modifies its old_ts argument.
>  *
>  * Note that although cond_resched() provides RCU quiescent states,
>  * it does not provide RCU-Tasks quiescent states.
>  *
>  * Because regions of code that have disabled softirq act as RCU
>  * read-side critical sections, this macro should be invoked with softirq
>  * (and preemption) enabled.
>  *
>  * This macro has no effect in CONFIG_PREEMPT_RT kernels.
>  */

Considering the note about cond_resched(), does does cond_resched() actually
provide an RCU quiescent state for fully-preemptible kernels? IIUC for those
cond_resched() expands to:

	__might_resched();
	klp_sched_try_switch()

... and AFAICT neither reports an RCU quiescent state.

So maybe it's worth dropping the note?

Seperately, what's the rationale for not doing this on PREEMPT_RT? Does that
avoid the problem through other means, or are people just not running effected
workloads on that?

Mark.

> 
> 							Thanx, Paul
> 
> > +#define rcu_softirq_qs_periodic(old_ts) \
> > +do { \
> > +	if (!IS_ENABLED(CONFIG_PREEMPT_RT) && \
> > +	    time_after(jiffies, (old_ts) + HZ / 10)) { \
> > +		preempt_disable(); \
> > +		rcu_softirq_qs(); \
> > +		preempt_enable(); \
> > +		(old_ts) = jiffies; \
> > +	} \
> > +} while (0)
> > +
> >  /*
> >   * Infrastructure to implement the synchronize_() primitives in
> >   * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
> > -- 
> > 2.30.2
> > 
> >

On Mon, Mar 18, 2024 at 5:59 AM Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Fri, Mar 15, 2024 at 10:40:56PM -0700, Paul E. McKenney wrote:
> > On Fri, Mar 15, 2024 at 12:55:03PM -0700, Yan Zhai wrote:
> > > There are several scenario in network processing that can run
> > > extensively under heavy traffic. In such situation, RCU synchronization
> > > might not observe desired quiescent states for indefinitely long period.
> > > Create a helper to safely raise the desired RCU quiescent states for
> > > such scenario.
> > >
> > > Currently the frequency is locked at HZ/10, i.e. 100ms, which is
> > > sufficient to address existing problems around RCU tasks. It's unclear
> > > yet if there is any future scenario for it to be further tuned down.
> >
> > I suggest something like the following for the commit log:
> >
> > ------------------------------------------------------------------------
> >
> > When under heavy load, network processing can run CPU-bound for many tens
> > of seconds.  Even in preemptible kernels, this can block RCU Tasks grace
> > periods, which can cause trace-event removal to take more than a minute,
> > which is unacceptably long.
> >
> > This commit therefore creates a new helper function that passes
> > through both RCU and RCU-Tasks quiescent states every 100 milliseconds.
> > This hard-coded value suffices for current workloads.
>
> FWIW, this sounds good to me.
>
> >
> > ------------------------------------------------------------------------
> >
> > > Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> > > Reviewed-by: Jesper Dangaard Brouer <hawk@kernel.org>
> > > Signed-off-by: Yan Zhai <yan@cloudflare.com>
> > > ---
> > > v3->v4: comment fixup
> > >
> > > ---
> > >  include/linux/rcupdate.h | 24 ++++++++++++++++++++++++
> > >  1 file changed, 24 insertions(+)
> > >
> > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > > index 0746b1b0b663..da224706323e 100644
> > > --- a/include/linux/rcupdate.h
> > > +++ b/include/linux/rcupdate.h
> > > @@ -247,6 +247,30 @@ do { \
> > >     cond_resched(); \
> > >  } while (0)
> > >
> > > +/**
> > > + * rcu_softirq_qs_periodic - Periodically report consolidated quiescent states
> > > + * @old_ts: last jiffies when QS was reported. Might be modified in the macro.
> > > + *
> > > + * This helper is for network processing in non-RT kernels, where there could
> > > + * be busy polling threads that block RCU synchronization indefinitely.  In
> > > + * such context, simply calling cond_resched is insufficient, so give it a
> > > + * stronger push to eliminate all potential blockage of all RCU types.
> > > + *
> > > + * NOTE: unless absolutely sure, this helper should in general be called
> > > + * outside of bh lock section to avoid reporting a surprising QS to updaters,
> > > + * who could be expecting RCU read critical section to end at local_bh_enable().
> > > + */
> >
> > How about something like this for the kernel-doc comment?
> >
> > /**
> >  * rcu_softirq_qs_periodic - Report RCU and RCU-Tasks quiescent states
> >  * @old_ts: jiffies at start of processing.
> >  *
> >  * This helper is for long-running softirq handlers, such as those
> >  * in networking.  The caller should initialize the variable passed in
> >  * as @old_ts at the beginning of the softirq handler.  When invoked
> >  * frequently, this macro will invoke rcu_softirq_qs() every 100
> >  * milliseconds thereafter, which will provide both RCU and RCU-Tasks
> >  * quiescent states.  Note that this macro modifies its old_ts argument.
> >  *
> >  * Note that although cond_resched() provides RCU quiescent states,
> >  * it does not provide RCU-Tasks quiescent states.
> >  *
> >  * Because regions of code that have disabled softirq act as RCU
> >  * read-side critical sections, this macro should be invoked with softirq
> >  * (and preemption) enabled.
> >  *
> >  * This macro has no effect in CONFIG_PREEMPT_RT kernels.
> >  */
>
> Considering the note about cond_resched(), does does cond_resched() actually
> provide an RCU quiescent state for fully-preemptible kernels? IIUC for those
> cond_resched() expands to:
>
>         __might_resched();
>         klp_sched_try_switch()
>
> ... and AFAICT neither reports an RCU quiescent state.
>
> So maybe it's worth dropping the note?
>
> Seperately, what's the rationale for not doing this on PREEMPT_RT? Does that
> avoid the problem through other means, or are people just not running effected
> workloads on that?
>
It's a bit anti-intuition but yes the RT kernel avoids the problem.
This is because "schedule()" reports task RCU QS actually, and on RT
kernel cond_resched() call won't call "__cond_resched()" or
"__schedule(PREEMPT)" as you already pointed out, which would clear
need-resched flag. This then allows "schedule()" to be called on hard
IRQ exit time by time.

Yan

> Mark.
>
> >
> >                                                       Thanx, Paul
> >
> > > +#define rcu_softirq_qs_periodic(old_ts) \
> > > +do { \
> > > +   if (!IS_ENABLED(CONFIG_PREEMPT_RT) && \
> > > +       time_after(jiffies, (old_ts) + HZ / 10)) { \
> > > +           preempt_disable(); \
> > > +           rcu_softirq_qs(); \
> > > +           preempt_enable(); \
> > > +           (old_ts) = jiffies; \
> > > +   } \
> > > +} while (0)
> > > +
> > >  /*
> > >   * Infrastructure to implement the synchronize_() primitives in
> > >   * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
> > > --
> > > 2.30.2
> > >
> > >

On Mon, Mar 18, 2024 at 9:32 PM Yan Zhai <yan@cloudflare.com> wrote:
>
> On Mon, Mar 18, 2024 at 5:59 AM Mark Rutland <mark.rutland@arm.com> wrote:
> >
> > On Fri, Mar 15, 2024 at 10:40:56PM -0700, Paul E. McKenney wrote:
> > > On Fri, Mar 15, 2024 at 12:55:03PM -0700, Yan Zhai wrote:
> > > > There are several scenario in network processing that can run
> > > > extensively under heavy traffic. In such situation, RCU synchronization
> > > > might not observe desired quiescent states for indefinitely long period.
> > > > Create a helper to safely raise the desired RCU quiescent states for
> > > > such scenario.
> > > >
> > > > Currently the frequency is locked at HZ/10, i.e. 100ms, which is
> > > > sufficient to address existing problems around RCU tasks. It's unclear
> > > > yet if there is any future scenario for it to be further tuned down.
> > >
> > > I suggest something like the following for the commit log:
> > >
> > > ------------------------------------------------------------------------
> > >
> > > When under heavy load, network processing can run CPU-bound for many tens
> > > of seconds.  Even in preemptible kernels, this can block RCU Tasks grace
> > > periods, which can cause trace-event removal to take more than a minute,
> > > which is unacceptably long.
> > >
> > > This commit therefore creates a new helper function that passes
> > > through both RCU and RCU-Tasks quiescent states every 100 milliseconds.
> > > This hard-coded value suffices for current workloads.
> >
> > FWIW, this sounds good to me.
> >
> > >
> > > ------------------------------------------------------------------------
> > >
> > > > Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> > > > Reviewed-by: Jesper Dangaard Brouer <hawk@kernel.org>
> > > > Signed-off-by: Yan Zhai <yan@cloudflare.com>
> > > > ---
> > > > v3->v4: comment fixup
> > > >
> > > > ---
> > > >  include/linux/rcupdate.h | 24 ++++++++++++++++++++++++
> > > >  1 file changed, 24 insertions(+)
> > > >
> > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > > > index 0746b1b0b663..da224706323e 100644
> > > > --- a/include/linux/rcupdate.h
> > > > +++ b/include/linux/rcupdate.h
> > > > @@ -247,6 +247,30 @@ do { \
> > > >     cond_resched(); \
> > > >  } while (0)
> > > >
> > > > +/**
> > > > + * rcu_softirq_qs_periodic - Periodically report consolidated quiescent states
> > > > + * @old_ts: last jiffies when QS was reported. Might be modified in the macro.
> > > > + *
> > > > + * This helper is for network processing in non-RT kernels, where there could
> > > > + * be busy polling threads that block RCU synchronization indefinitely.  In
> > > > + * such context, simply calling cond_resched is insufficient, so give it a
> > > > + * stronger push to eliminate all potential blockage of all RCU types.
> > > > + *
> > > > + * NOTE: unless absolutely sure, this helper should in general be called
> > > > + * outside of bh lock section to avoid reporting a surprising QS to updaters,
> > > > + * who could be expecting RCU read critical section to end at local_bh_enable().
> > > > + */
> > >
> > > How about something like this for the kernel-doc comment?
> > >
> > > /**
> > >  * rcu_softirq_qs_periodic - Report RCU and RCU-Tasks quiescent states
> > >  * @old_ts: jiffies at start of processing.
> > >  *
> > >  * This helper is for long-running softirq handlers, such as those
> > >  * in networking.  The caller should initialize the variable passed in
> > >  * as @old_ts at the beginning of the softirq handler.  When invoked
> > >  * frequently, this macro will invoke rcu_softirq_qs() every 100
> > >  * milliseconds thereafter, which will provide both RCU and RCU-Tasks
> > >  * quiescent states.  Note that this macro modifies its old_ts argument.
> > >  *
> > >  * Note that although cond_resched() provides RCU quiescent states,
> > >  * it does not provide RCU-Tasks quiescent states.
> > >  *
> > >  * Because regions of code that have disabled softirq act as RCU
> > >  * read-side critical sections, this macro should be invoked with softirq
> > >  * (and preemption) enabled.
> > >  *
> > >  * This macro has no effect in CONFIG_PREEMPT_RT kernels.
> > >  */
> >
> > Considering the note about cond_resched(), does does cond_resched() actually
> > provide an RCU quiescent state for fully-preemptible kernels? IIUC for those
> > cond_resched() expands to:
> >
> >         __might_resched();
> >         klp_sched_try_switch()
> >
> > ... and AFAICT neither reports an RCU quiescent state.
> >
> > So maybe it's worth dropping the note?
> >
> > Seperately, what's the rationale for not doing this on PREEMPT_RT? Does that
> > avoid the problem through other means, or are people just not running effected
> > workloads on that?
> >
> It's a bit anti-intuition but yes the RT kernel avoids the problem.
> This is because "schedule()" reports task RCU QS actually, and on RT
> kernel cond_resched() call won't call "__cond_resched()" or
> "__schedule(PREEMPT)" as you already pointed out, which would clear
> need-resched flag. This then allows "schedule()" to be called on hard
> IRQ exit time by time.
>

And these are excellent questions that I should originally include in
the comment. Thanks for bringing it up.
Let me send another version tomorrow, allowing more thoughts on this if any.

thanks
Yan

> Yan
>
> > Mark.
> >
> > >
> > >                                                       Thanx, Paul
> > >
> > > > +#define rcu_softirq_qs_periodic(old_ts) \
> > > > +do { \
> > > > +   if (!IS_ENABLED(CONFIG_PREEMPT_RT) && \
> > > > +       time_after(jiffies, (old_ts) + HZ / 10)) { \
> > > > +           preempt_disable(); \
> > > > +           rcu_softirq_qs(); \
> > > > +           preempt_enable(); \
> > > > +           (old_ts) = jiffies; \
> > > > +   } \
> > > > +} while (0)
> > > > +
> > > >  /*
> > > >   * Infrastructure to implement the synchronize_() primitives in
> > > >   * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
> > > > --
> > > > 2.30.2
> > > >
> > > >