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

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Message-ID: <b21d9fa7-45be-8346-db28-4c42847e2e0b@huawei.com>
Date: Thu, 25 Jul 2024 22:49:46 +0800
From: zhengzucheng <zhengzucheng@...wei.com>
To: Peter Zijlstra <peterz@...radead.org>
CC: <mingo@...hat.com>, <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>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] sched/cputime: Fix mul_u64_u64_div_u64() precision
 for cputime

Sorry, I made a mistake here. CONFIG_VIRT_CPU_ACCOUNTING_NATIVE is not set.

在 2024/7/25 22:05, Peter Zijlstra 写道:
> On Thu, Jul 25, 2024 at 12:03:15PM +0000, Zheng Zucheng wrote:
>> 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 scenario:
>> 1. User task run in kernel mode most of time.
>> 2. The ARM64 architecture && CONFIG_VIRT_CPU_ACCOUNTING_NATIVE=y &&
>>     TICK_CPU_ACCOUNTING=y
>>
>> Fix mul_u64_u64_div_u64() conversion precision by reset stime to rtime
>>
>> Fixes: 3dc167ba5729 ("sched/cputime: Improve cputime_adjust()")
>> Signed-off-by: Zheng Zucheng <zhengzucheng@...wei.com>
>> ---
>>   kernel/sched/cputime.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
>> index aa48b2ec879d..365c74e95537 100644
>> --- a/kernel/sched/cputime.c
>> +++ b/kernel/sched/cputime.c
>> @@ -582,6 +582,8 @@ void cputime_adjust(struct task_cputime *curr, struct prev_cputime *prev,
>>   	}
>>   
>>   	stime = mul_u64_u64_div_u64(stime, rtime, stime + utime);
>> +	if (unlikely(stime > rtime))
>> +		stime = rtime;
> But but but... for CONFIG_VIRT_CPU_ACCOUNTING_NATIVE=y the code you're
> patching is not compiled!
>
>
> .