Commit afa672a0 authored by Zheng Zucheng's avatar Zheng Zucheng
Browse files

sched/cputime: Fix mul_u64_u64_div_u64() precision for cputime 

mainline inclusion
from mainline-v6.11-rc2
commit 77baa5bafcbe1b2a15ef9c37232c21279c95481c
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/IAIN7D

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=77baa5bafcbe1b2a15ef9c37232c21279c95481c



--------------------------------

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

Fixes: 3dc167ba ("sched/cputime: Improve cputime_adjust()")
Signed-off-by: default avatarZheng Zucheng <zhengzucheng@huawei.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/20240726023235.217771-1-zhengzucheng@huawei.com


Signed-off-by: default avatarZheng Zucheng <zhengzucheng@huawei.com>
parent 13ac3d01

kernel/sched/cputime.c

+6 −0
Original line number Diff line number Diff line
@@ -579,6 +579,12 @@ 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:

	/*