linux-kernel - Re: [BUG nohz]: wrong user and system time accounting

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Date:   Wed, 05 Apr 2017 10:26:51 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Luiz Capitulino <lcapitulino@...hat.com>,
        Frederic Weisbecker <fweisbec@...il.com>
Cc:     Wanpeng Li <kernellwp@...il.com>, Mike Galbraith <efault@....de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [BUG nohz]: wrong user and system time accounting

On Tue, 2017-04-04 at 13:36 -0400, Luiz Capitulino wrote:
> 
> On further debugging this, I realized that I had overlooked
> something:
> the timer interrupt in this trace is not the tick, but cyclictest's
> timer
> (remember that the test-case consists of pinning cyclictest and a
> task
> hogging the CPU to the same CPU).
> 
> I'm running cyclictest with -i 200. If I increase this to -i 1000,
> then
> I seem unable to reproduce the issue (caution: even with -i 200 it
> doesn't always happen. But it does usually happen after I restart the
> test-case a few times. However, I've never been able to reproduce
> with -i 1000).
> 
> Now, if it's really cyclictest that's causing the timer interrupts to
> get aligned, I guess this might not have a solution? (note: I haven't
> been able to reproduce this on bare-metal).

With any sample (tick) based timekeeping, it is possible
to construct workloads that avoid the sampling and result
in skewed statistics as a result.

However, given that local users can already DoS the system
in all kinds of ways, skewed statistics are probably not
that high up on the list of importance.

If there were a way to do accurate accounting (true vtime
accounting) without increasing the overhead of every
syscall and interrupt noticeably, that might be worth it,
but syscall overhead is likely to be a more important
factor than the accuracy of statistics.

I don't know if doing TSC reads and subtraction/addition
only, and delaying the conversion to cputime until a later
point would slow down system calls measurably, compared
with reading jiffies and comparing it against a cached
value of jiffies, nor do I know whether spending time
implementing and testing that would be worthwhile :)

-- 
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