[PATCH] tsacct: keep task begin times stable across suspend

[Hungyu Lin]

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.

Fixes: f3cef7a99469 ("[PATCH] csa: basic accounting over taskstats")
Reported-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20130611200600.GA15168@xxxxxxxxxx
Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxx>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.1911071335320.4256@xxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Hungyu Lin <dennylin0707@xxxxxxxxx>
---
 kernel/tsacct.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/kernel/tsacct.c b/kernel/tsacct.c
index 5c153106e642..89f118227c65 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 +39,10 @@ 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);
+	stats->ac_btime64 = btime.tv_sec;
 
 	if (tsk->flags & PF_EXITING)
 		stats->ac_exitcode = tsk->exit_code;
-- 
2.43.0