[rcu,01/12] srcu: Rename srcu_might_be_idle() to srcu_should_expedite()

Commit Message

Paul E. McKenney Oct. 9, 2024, 6:07 p.m. UTC

SRCU auto-expedites grace periods that follow a sufficiently long idle
period, and the srcu_might_be_idle() function is used to make this
decision.  However, the upcoming light-weight SRCU readers will not do
auto-expediting because doing so would cause the grace-period machinery
to invoke synchronize_rcu_expedited() twice, with IPIs all around.
However, software-engineering considerations force this determination
to remain in srcu_might_be_idle().

This commit therefore changes the name of srcu_might_be_idle() to
srcu_should_expedite(), thus moving from what it currently does to why
it does it, this latter being more future-proof.

Signed-off-by: Paul E. McKenney <[e-mail]>
Cc: Alexei Starovoitov <[e-mail]>
Cc: Andrii Nakryiko <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Kent Overstreet <[e-mail]>
Cc: <[e-mail]>
---
 kernel/rcu/srcutree.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

Comments

Neeraj Upadhyay Oct. 14, 2024, 8:56 a.m. UTC | #1

On 10/9/2024 11:37 PM, Paul E. McKenney wrote:
> SRCU auto-expedites grace periods that follow a sufficiently long idle
> period, and the srcu_might_be_idle() function is used to make this
> decision.  However, the upcoming light-weight SRCU readers will not do
> auto-expediting because doing so would cause the grace-period machinery
> to invoke synchronize_rcu_expedited() twice, with IPIs all around.
> However, software-engineering considerations force this determination
> to remain in srcu_might_be_idle().
> 
> This commit therefore changes the name of srcu_might_be_idle() to
> srcu_should_expedite(), thus moving from what it currently does to why
> it does it, this latter being more future-proof.
> 
> Signed-off-by: Paul E. McKenney <[e-mail]>
> Cc: Alexei Starovoitov <[e-mail]>
> Cc: Andrii Nakryiko <[e-mail]>
> Cc: Peter Zijlstra <[e-mail]>
> Cc: Kent Overstreet <[e-mail]>
> Cc: <[e-mail]>
> ---

Reviewed-by: Neeraj Upadhyay <[e-mail]>


- Neeraj

Patch

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 31706e3293bce..9ff4ded609ba5 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1139,7 +1139,8 @@  static void srcu_flip(struct srcu_struct *ssp)
 }
 
 /*
- * If SRCU is likely idle, return true, otherwise return false.
+ * If SRCU is likely idle, in other words, the next SRCU grace period
+ * should be expedited, return true, otherwise return false.
  *
  * Note that it is OK for several current from-idle requests for a new
  * grace period from idle to specify expediting because they will all end
@@ -1159,7 +1160,7 @@  static void srcu_flip(struct srcu_struct *ssp)
  * negligible when amortized over that time period, and the extra latency
  * of a needlessly non-expedited grace period is similarly negligible.
  */
-static bool srcu_might_be_idle(struct srcu_struct *ssp)
+static bool srcu_should_expedite(struct srcu_struct *ssp)
 {
 	unsigned long curseq;
 	unsigned long flags;
@@ -1469,14 +1470,15 @@  EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
  * Implementation of these memory-ordering guarantees is similar to
  * that of synchronize_rcu().
  *
- * If SRCU is likely idle, expedite the first request.  This semantic
- * was provided by Classic SRCU, and is relied upon by its users, so TREE
- * SRCU must also provide it.  Note that detecting idleness is heuristic
- * and subject to both false positives and negatives.
+ * If SRCU is likely idle as determined by srcu_should_expedite(),
+ * expedite the first request.  This semantic was provided by Classic SRCU,
+ * and is relied upon by its users, so TREE SRCU must also provide it.
+ * Note that detecting idleness is heuristic and subject to both false
+ * positives and negatives.
  */
 void synchronize_srcu(struct srcu_struct *ssp)
 {
-	if (srcu_might_be_idle(ssp) || rcu_gp_is_expedited())
+	if (srcu_should_expedite(ssp) || rcu_gp_is_expedited())
 		synchronize_srcu_expedited(ssp);
 	else
 		__synchronize_srcu(ssp, true);