[PATCH v2 2/5] time/sched_clock: Broaden sched_clock()'s instrumentation coverage

From: Marco Elver
Date: Mon Nov 04 2024 - 11:20:19 EST


Most of sched_clock()'s implementation is ineligible for instrumentation
due to relying on sched_clock_noinstr().

Split the implementation off into an __always_inline function
__sched_clock(), which is then used by the noinstr and instrumentable
version, to allow more of sched_clock() to be covered by various
instrumentation.

This will allow instrumentation with the various sanitizers (KASAN,
KCSAN, KMSAN, UBSAN). For KCSAN, we know that raw seqcount_latch usage
without annotations will result in false positive reports: tell it that
all of __sched_clock() is "atomic" for the latch writer; later changes
in this series will take care of the readers.

Link: https://lore.kernel.org/all/20241030204815.GQ14555@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
Co-developed-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
---
v2:
* New patch.
---
kernel/time/sched_clock.c | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 85595fcf6aa2..29bdf309dae8 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -80,7 +80,7 @@ notrace int sched_clock_read_retry(unsigned int seq)
return raw_read_seqcount_latch_retry(&cd.seq, seq);
}

-unsigned long long noinstr sched_clock_noinstr(void)
+static __always_inline unsigned long long __sched_clock(void)
{
struct clock_read_data *rd;
unsigned int seq;
@@ -98,11 +98,23 @@ unsigned long long noinstr sched_clock_noinstr(void)
return res;
}

+unsigned long long noinstr sched_clock_noinstr(void)
+{
+ return __sched_clock();
+}
+
unsigned long long notrace sched_clock(void)
{
unsigned long long ns;
preempt_disable_notrace();
- ns = sched_clock_noinstr();
+ /*
+ * All of __sched_clock() is a seqcount_latch reader critical section,
+ * but relies on the raw helpers which are uninstrumented. For KCSAN,
+ * mark all accesses in __sched_clock() as atomic.
+ */
+ kcsan_nestable_atomic_begin();
+ ns = __sched_clock();
+ kcsan_nestable_atomic_end();
preempt_enable_notrace();
return ns;
}
--
2.47.0.163.g1226f6d8fa-goog