[PATCH v2] Optimize USDT discovery a little

eugene.loh
2025-06-25T06:03:12Z

We want to reduce the performance cost of USDT discovery if possible.

Specifically, a number of statements -- with probe descriptions such as
"dtrace:::BEGIN" that could never specify USDT probes -- will not get
their clause flags set with DT_CLSFLAG_USDT_EXCLUDE. So these statements
get considered unnecessarily during periodic probe discovery.

Therefore:

*) Expand ignore_clause(dtp, n, uprp) to support the case uprp==NULL.
This case is independent of any knowledge of a specific underlying
probe.

*) During probe discovery, check ignore_clause(dtp, i, NULL). This
sets the DT_CLSFLAG_USDT_[INCLUDE|EXCLUDE] flag and allows faster
exclusion of statements that do not need to be reconsidered.

To take advantage of this optimization, users should specify providers.
E.g., instead of "BEGIN" (which could conceivably be a USDT probe),
specify "dtrace:::BEGIN".

Signed-off-by: Eugene Loh <eugene.loh@oracle.com>
---
libdtrace/dt_prov_uprobe.c | 18 ++++++++++--------
1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/libdtrace/dt_prov_uprobe.c b/libdtrace/dt_prov_uprobe.c
index cf5cfd431..5ba50b678 100644
--- a/libdtrace/dt_prov_uprobe.c
+++ b/libdtrace/dt_prov_uprobe.c
@@ -504,7 +504,8 @@ clean_usdt_probes(dtrace_hdl_t *dtp)

/*
* Judge whether clause "n" could ever be called as a USDT probe
- * for this underlying probe.
+ * for this underlying probe. We can pass uprp==NULL to see if
+ * the clause can be excluded for every probe.
*/
static int
ignore_clause(dtrace_hdl_t *dtp, int n, const dt_probe_t *uprp)
@@ -512,6 +513,9 @@ ignore_clause(dtrace_hdl_t *dtp, int n, const dt_probe_t *uprp)
dtrace_stmtdesc_t *stp = dtp->dt_stmts[n];
dtrace_probedesc_t *pdp = &stp->dtsd_ecbdesc->dted_probe;

+ if (stp == NULL)
+ return 1;
+
/*
* Some clauses could never be called for a USDT probe,
* regardless of the underlying probe uprp. Cache this
@@ -525,7 +529,7 @@ ignore_clause(dtrace_hdl_t *dtp, int n, const dt_probe_t *uprp)
* neither '*' nor a digit, it cannot be a USDT probe.
*/
if (len > 1) {
- char lastchar = pdp->prv[len - 1];
+ char lastchar = (pdp->prv[0] != '\0' ? pdp->prv[len - 1] : '*');

if (lastchar != '*' && !isdigit(lastchar)) {
dt_stmt_clsflag_set(stp, DT_CLSFLAG_USDT_EXCLUDE);
@@ -555,6 +559,8 @@ ignore_clause(dtrace_hdl_t *dtp, int n, const dt_probe_t *uprp)
}
if (dt_stmt_clsflag_test(stp, DT_CLSFLAG_USDT_EXCLUDE) == 1)
return 1;
+ if (uprp == NULL)
+ return 0;

/*
* If we cannot ignore this statement, try to use uprp.
@@ -751,13 +757,9 @@ static int discover(dtrace_hdl_t *dtp)
*/
memset(&pcb, 0, sizeof(dt_pcb_t));
for (i = 0; i < dtp->dt_stmt_nextid; i++) {
- dtrace_stmtdesc_t *stp;
-
- stp = dtp->dt_stmts[i];
- if (stp == NULL)
+ if (ignore_clause(dtp, i, NULL))
continue;
- if (dt_stmt_clsflag_test(stp, DT_CLSFLAG_USDT_EXCLUDE) != 1)
- dt_pid_create_usdt_probes(&stp->dtsd_ecbdesc->dted_probe, dtp, &pcb);
+ dt_pid_create_usdt_probes(&dtp->dt_stmts[i]->dtsd_ecbdesc->dted_probe, dtp, &pcb);
}

return 0;
--
2.43.5