linux-kernel - Re: [PATCH 1/1] cputime: Make the reported utime+stime correspond to the actual runtime.

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Message-ID: <CAKdL+dT7ey_=Bhfzv97kHe8-YfS+GVFyokGO0HRKWgtyPzYNkg@mail.gmail.com>
Date:	Sat, 13 Jun 2015 13:17:32 +0200
From:	Fredrik Markström <fredrik.markstrom@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...hat.com, linux-kernel@...r.kernel.org,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 1/1] cputime: Make the reported utime+stime correspond to
 the actual runtime.

Resending beause it bounced of linux-kernel (google inbox sends
everything as html) !

Thanks for your quick response, I'll elaborate on the Changelog.

Regarding the global spinlock I considered adding it to task_struct
and signal_struct. My reasoning not to do it, flawed or not, was that
I thought the risk for congestion and cache line bouncing would be
small given the following assumptions:
1. As far as I understand neither of the callers are typically called
very frequently. (procfs, k_getrusage, wait_task_zombie and sys_times)
2 The  and the time spent in the lock region is small.

Did I have bad luck when thinking :) or do you still think it's better
to add the locks to the structs above ?

/Fredrik

On Fri, Jun 12, 2015 at 12:16 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, 2015-06-12 at 10:55 +0200, Fredrik Markstrom wrote:
>> The scaling mechanism might sometimes cause top to report >100%
>> (sometimes > 1000%) cpu usage for a single thread. This patch makes
>> sure that stime+utime corresponds to the actual runtime of the thread.
>
> This Changelog is inadequate, it does not explain the actual problem.
>
>> +static DEFINE_SPINLOCK(prev_time_lock);
>
> global (spin)locks are bad.

-- 
/Fredrik
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