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sched/cputime: Fix mul_u64_u64_div_u64() precision for cputime

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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: 3dc167ba57 ("sched/cputime: Improve cputime_adjust()")
Signed-off-by: Zheng Zucheng <zhengzucheng@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/20240726023235.217771-1-zhengzucheng@huawei.com
This commit is contained in:
Zheng Zucheng 2024-07-26 02:32:35 +00:00 committed by Peter Zijlstra
parent 8400291e28
commit 77baa5bafc
1 changed files with 6 additions and 0 deletions
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6
kernel/sched/cputime.c
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@ -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:
/*