linux-kernel - [tip: locking/core] time/sched_clock: Broaden sched

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Message-ID: <173089006323.32228.18347923409718973574.tip-bot2@tip-bot2>
Date: Wed, 06 Nov 2024 10:47:43 -0000
From: "tip-bot2 for Marco Elver" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>,
 Marco Elver <elver@...gle.com>, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip: locking/core] time/sched_clock: Broaden sched_clock()'s
 instrumentation coverage

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     8ab40fc2b9086b915e46890bb9252dc7692f1da0
Gitweb:        https://git.kernel.org/tip/8ab40fc2b9086b915e46890bb9252dc7692f1da0
Author:        Marco Elver <elver@...gle.com>
AuthorDate:    Mon, 04 Nov 2024 16:43:06 +01:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Tue, 05 Nov 2024 12:55:35 +01:00

time/sched_clock: Broaden sched_clock()'s instrumentation coverage

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 reader; later changes
in this series will take care of the writers.

Co-developed-by: "Peter Zijlstra (Intel)" <peterz@...radead.org>
Signed-off-by: "Peter Zijlstra (Intel)" <peterz@...radead.org>
Signed-off-by: Marco Elver <elver@...gle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lore.kernel.org/r/20241104161910.780003-3-elver@google.com
---
 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 85595fc..29bdf30 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;
 }