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Index: head/sys/kern/subr_smr.c
===================================================================
--- head/sys/kern/subr_smr.c
+++ head/sys/kern/subr_smr.c
@@ -209,6 +209,26 @@
return (goal);
}
+smr_seq_t
+smr_advance_deferred(smr_t smr, int limit)
+{
+ smr_seq_t goal;
+ smr_t csmr;
+
+ critical_enter();
+ csmr = zpcpu_get(smr);
+ if (++csmr->c_deferred >= limit) {
+ goal = SMR_SEQ_INVALID;
+ csmr->c_deferred = 0;
+ } else
+ goal = smr_shared_current(csmr->c_shared) + SMR_SEQ_INCR;
+ critical_exit();
+ if (goal != SMR_SEQ_INVALID)
+ return (goal);
+
+ return (smr_advance(smr));
+}
+
/*
* Poll to determine whether all readers have observed the 'goal' write
* sequence number.
@@ -255,6 +275,17 @@
* c_seq can only reference time after this wr_seq.
*/
s_wr_seq = atomic_load_acq_int(&s->s_wr_seq);
+
+ /*
+ * This may have come from a deferred advance. Consider one
+ * increment past the current wr_seq valid and make sure we
+ * have advanced far enough to succeed. We simply add to avoid
+ * an additional fence.
+ */
+ if (goal == s_wr_seq + SMR_SEQ_INCR) {
+ atomic_add_int(&s->s_wr_seq, SMR_SEQ_INCR);
+ s_wr_seq = goal;
+ }
/*
* Detect whether the goal is valid and has already been observed.
Index: head/sys/sys/smr.h
===================================================================
--- head/sys/sys/smr.h
+++ head/sys/sys/smr.h
@@ -64,6 +64,7 @@
struct smr {
smr_seq_t c_seq; /* Current observed sequence. */
smr_shared_t c_shared; /* Shared SMR state. */
+ int c_deferred; /* Deferred advance counter. */
};
/*
@@ -144,6 +145,13 @@
* required to ensure that all modifications are visible to readers.
*/
smr_seq_t smr_advance(smr_t smr);
+
+/*
+ * Advances the write sequence number only after N calls. Returns
+ * the correct goal for a wr_seq that has not yet occurred. Used to
+ * minimize shared cacheline invalidations for frequent writers.
+ */
+smr_seq_t smr_advance_deferred(smr_t smr, int limit);
/*
* Returns true if a goal sequence has been reached. If

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