linux-kernel - [PATCH v2 -next] sched/cputime: Fix mul_u64_u64_div

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Message-ID: <20240726023235.217771-1-zhengzucheng@huawei.com>
Date: Fri, 26 Jul 2024 02:32:35 +0000
From: Zheng Zucheng <zhengzucheng@...wei.com>
To: <mingo@...hat.com>, <peterz@...radead.org>, <juri.lelli@...hat.com>,
	<vincent.guittot@...aro.org>, <dietmar.eggemann@....com>,
	<rostedt@...dmis.org>, <bsegall@...gle.com>, <mgorman@...e.de>,
	<bristot@...hat.com>, <vschneid@...hat.com>, <oleg@...hat.com>
CC: <linux-kernel@...r.kernel.org>
Subject: [PATCH v2 -next] sched/cputime: Fix mul_u64_u64_div_u64() precision for cputime

In extreme test scenarios:
the 14th field utime in /proc/xx/stat is greater than sum_exec_runtime,
utime = 18446744073709518790 ns, rtime = 135989749728000 ns

In cputime_adjust() process, stime is greater than rtime due to
mul_u64_u64_div_u64() precision problem.
before call mul_u64_u64_div_u64(),
stime = 175136586720000, rtime = 135989749728000, utime = 1416780000.
after call mul_u64_u64_div_u64(),
stime = 135989949653530

unsigned reversion occurs because rtime is less than stime.
utime = rtime - stime = 135989749728000 - 135989949653530
		      = -199925530
		      = (u64)18446744073709518790

Trigger condition:
1). User task run in kernel mode most of time
2). ARM64 architecture
3). TICK_CPU_ACCOUNTING=y
    CONFIG_VIRT_CPU_ACCOUNTING_NATIVE is not set

Fix mul_u64_u64_div_u64() conversion precision by reset stime to rtime

v2:
 - Add comment
 - Update trigger condition

Fixes: 3dc167ba5729 ("sched/cputime: Improve cputime_adjust()")
Cc: <stable@...r.kernel.org>
Signed-off-by: Zheng Zucheng <zhengzucheng@...wei.com>
---
 kernel/sched/cputime.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index aa48b2ec879d..4feef0d4e449 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -582,6 +582,12 @@ void cputime_adjust(struct task_cputime *curr, struct prev_cputime *prev,
 	}
 
 	stime = mul_u64_u64_div_u64(stime, rtime, stime + utime);
+	/*
+	 * Because mul_u64_u64_div_u64() can approximate on some
+	 * achitectures; enforce the constraint that: a*b/(b+c) <= a.
+	 */
+	if (unlikely(stime > rtime))
+		stime = rtime;
 
 update:
 	/*
-- 
2.34.1