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Reply-To: xlpang@linux.alibaba.com
Subject: Re: [PATCH] sched/cputime: Ensure correct utime and stime proportion
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Tejun Heo <tj@kernel.org>, linux-kernel@vger.kernel.org
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From:   Xunlei Pang <xlpang@linux.alibaba.com>
Message-ID: <bd275aff-61dd-557f-5b91-da4ce82f9501@linux.alibaba.com>
Date:   Thu, 5 Jul 2018 21:21:15 +0800
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On 7/5/18 6:46 PM, Peter Zijlstra wrote:
> On Wed, Jun 27, 2018 at 08:22:42PM +0800, Xunlei Pang wrote:
>> tick-based whole utime is utime_0, tick-based whole stime
>> is stime_0, scheduler time is rtime_0. 
> 
>> For a long time, the process runs mainly in userspace with
>> run-sleep patterns, and because two different clocks, it
>> is possible to have the following condition:
>>   rtime_0 < utime_0 (as with little stime_0)
> 
> I don't follow... what?
> 
> Why are you, and why do you think it makes sense to, compare rtime_0
> against utime_0 ?
> 
> The [us]time_0 are, per your earlier definition, ticks. They're not an
> actual measure of time. Do not compare the two, that makes no bloody
> sense.
> 

[us]time_0 is task_struct:utime{stime}, I cited directly from
cputime_adjust(), both in nanoseconds. I assumed "rtime_0 < utime_0"
here to simple the following proof to help explain the problem we met.