From 5d4298e75f252b162008ad0475f29349023573cb Mon Sep 17 00:00:00 2001 From: Michael Paquier Date: Mon, 11 Nov 2024 09:02:30 +0900 Subject: [PATCH] pg_stat_statements: Avoid some locking during PGSS entry scans A single PGSS entry's spinlock is used to be able to modify "counters" without holding pgss->lock exclusively, as mentioned at the top of pg_stat_statements.c and within pgssEntry. Within a single pgssEntry, stats_since and minmax_stats_since are never modified without holding pgss->lock exclusively, so there is no need to hold an entry's spinlock when reading stats_since and minmax_stats_since, as done when scanning all the PGSS entries for function calls of pg_stat_statements(). This also restores the consistency between the code and the comments about the entry's spinlock usage. This change is a performance improvement (it can be argued that this is a logic bug), so there is no need for a backpatch. This saves two instructions from being read while holding an entry's spinlock. Author: Karina Litskevich Reviewed-by: Michael Paquier, wenhui qiu Discussion: https://postgr.es/m/CACiT8ibhCmzbcOxM0v4pRLH3abk-95LPkt7_uC2JMP+miPjxsg@mail.gmail.com --- contrib/pg_stat_statements/pg_stat_statements.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/contrib/pg_stat_statements/pg_stat_statements.c b/contrib/pg_stat_statements/pg_stat_statements.c index 1798e1d016f..49c657b3e07 100644 --- a/contrib/pg_stat_statements/pg_stat_statements.c +++ b/contrib/pg_stat_statements/pg_stat_statements.c @@ -1869,9 +1869,14 @@ pg_stat_statements_internal(FunctionCallInfo fcinfo, /* copy counters to a local variable to keep locking time short */ SpinLockAcquire(&entry->mutex); tmp = entry->counters; + SpinLockRelease(&entry->mutex); + + /* + * The spinlock is not required when reading these two as they are + * always updated when holding pgss->lock exclusively. + */ stats_since = entry->stats_since; minmax_stats_since = entry->minmax_stats_since; - SpinLockRelease(&entry->mutex); /* Skip entry if unexecuted (ie, it's a pending "sticky" entry) */ if (IS_STICKY(tmp)) -- 2.30.2