[tip: timers/urgent] tsacct: Keep task begin times stable across suspend

[tip-bot2 for Hungyu Lin]

The following commit has been merged into the timers/urgent branch of tip:

Commit-ID:     c49b3c20a2d0b22700e2100a527e68e829ece7f0
Gitweb:        https://git.kernel.org/tip/c49b3c20a2d0b22700e2100a527e68e829ece7f0
Author:        Hungyu Lin <dennylin0707@xxxxxxxxx>
AuthorDate:    Fri, 30 Oct 2026 05:42:27 
Committer:     Thomas Gleixner <tglx@xxxxxxxxxx>
CommitterDate: Mon, 09 Nov 2026 20:15:58 +01:00

tsacct: Keep task begin times stable across suspend

bacct_add_tsk() derives ac_btime and ac_btime64 by subtracting a
CLOCK_MONOTONIC elapsed duration from CLOCK_REALTIME. MONOTONIC stops
during suspend while REALTIME continues, so querying an existing task
after resume moves its reported begin time forward by the time spent
suspended.

Calculate the begin time from the wall-clock boot timestamp and the
task's CLOCK_BOOTTIME start timestamp instead. This accounts for suspend
and adds the nanoseconds before truncating to seconds. Keep ac_etime and
ac_tgetime on CLOCK_MONOTONIC and retain the existing rebasing when the
wall clock is adjusted.

The new begin time can be negative: setting CLOCK_REALTIME is bounded
by CLOCK_MONOTONIC, which excludes suspend, rather than CLOCK_BOOTTIME.
A permitted backward clock adjustment after suspend can therefore put
the task's begin time before the epoch. Clamp ac_btime64 at zero too,
instead of wrapping in the unsigned field, and retain its positive range.

Fixes: f3cef7a99469 ("[PATCH] csa: basic accounting over taskstats")
Reported-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxx>
Signed-off-by: Hungyu Lin <dennylin0707@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/20130611200600.GA15168@xxxxxxxxxx
Link: https://lore.kernel.org/r/alpine.DEB.2.21.1911071335320.4256@xxxxxxxxxxxxxxxxxxxxxxx
Link: https://patch.msgid.link/20261030054227.35453-3-dennylin0707@xxxxxxxxx
---
 kernel/tsacct.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/tsacct.c b/kernel/tsacct.c
index 5c15310..4832068 100644
--- a/kernel/tsacct.c
+++ b/kernel/tsacct.c
@@ -24,7 +24,7 @@ void bacct_add_tsk(struct user_namespace *user_ns,
 	const struct cred *tcred;
 	u64 utime, stime, utimescaled, stimescaled;
 	u64 now_ns, delta;
-	time64_t btime;
+	struct timespec64 btime;
 
 	BUILD_BUG_ON(TS_COMM_LEN < TASK_COMM_LEN);
 
@@ -39,9 +39,11 @@ void bacct_add_tsk(struct user_namespace *user_ns,
 	do_div(delta, NSEC_PER_USEC);
 	stats->ac_etime = delta;
 	/* Convert to seconds for btime (note y2106 limit) */
-	btime = ktime_get_real_seconds() - div_u64(delta, USEC_PER_SEC);
-	stats->ac_btime = clamp_t(time64_t, btime, 0, U32_MAX);
-	stats->ac_btime64 = btime;
+	getboottime64(&btime);
+	btime = timespec64_add(btime, ns_to_timespec64(tsk->start_boottime));
+	stats->ac_btime = clamp_t(time64_t, btime.tv_sec, 0, U32_MAX);
+	/* Setting the clock back after suspend can make btime negative. */
+	stats->ac_btime64 = max_t(time64_t, btime.tv_sec, 0);
 
 	if (tsk->flags & PF_EXITING)
 		stats->ac_exitcode = tsk->exit_code;