[v3,net,1/3] rcu: add a helper to report consolidated flavor QS

Commit Message

Yan Zhai March 13, 2024, 4:25 p.m. UTC

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.

Reviewed-by: Jesper Dangaard Brouer <[e-mail]>
Signed-off-by: Yan Zhai <[e-mail]>
---
 include/linux/rcupdate.h | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

Comments

Jakub Kicinski March 14, 2024, 9:54 p.m. UTC | #1

On Wed, 13 Mar 2024 09:25:49 -0700 Yan Zhai wrote:
> +/**
> + * rcu_softirq_qs_periodic - Periodically report consolidated quiescent states

> +#define rcu_softirq_qs_periodic(old_ts) \

scripts/kernel-doc says:

include/linux/rcupdate.h:271: warning: Function parameter or struct member 'old_ts' not described in 'rcu_softirq_qs_periodic'

Yan Zhai March 15, 2024, 2:54 p.m. UTC | #2

On Thu, Mar 14, 2024 at 4:55 PM Jakub Kicinski <[e-mail]> wrote:
>
> On Wed, 13 Mar 2024 09:25:49 -0700 Yan Zhai wrote:
> > +/**
> > + * rcu_softirq_qs_periodic - Periodically report consolidated quiescent states
>
> > +#define rcu_softirq_qs_periodic(old_ts) \
>
> scripts/kernel-doc says:
>
TIL, thanks. Let me send v4 to amend the text.

Yan

> include/linux/rcupdate.h:271: warning: Function parameter or struct member 'old_ts' not described in 'rcu_softirq_qs_periodic'
> --
> pw-bot: cr

Patch

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 0746b1b0b663..e91ae38c33e3 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -247,6 +247,29 @@  do { \
 	cond_resched(); \
 } while (0)
 
+/**
+ * rcu_softirq_qs_periodic - Periodically report consolidated quiescent states
+ *
+ * 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 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.