From:   Thomas Gleixner <tglx@linutronix.de>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Wei Li <liwei391@huawei.com>,
        Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>,
        Yu Liao <liaoyu15@huawei.com>, Hillf Danton <hdanton@sina.com>,
        Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH 7/7] selftests/proc: Remove idle time monotonicity
 assertions
In-Reply-To: <20230220124129.519477-8-frederic@kernel.org>
References: <20230220124129.519477-1-frederic@kernel.org>
 <20230220124129.519477-8-frederic@kernel.org>
Date:   Mon, 20 Feb 2023 22:53:17 +0100
Message-ID: <87k00covg2.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Mon, Feb 20 2023 at 13:41, Frederic Weisbecker wrote:

> Due to broken iowait task counting design (cf: comments above
> get_cpu_idle_time_us() and nr_iowait()), it is not possible to provide
> the guarantee that /proc/stat or /proc/uptime display monotonic idle
> time values.
>
> Remove the selftests that verify the related wrong assumption so that
> testers and maintainers don't spend more time on that.
>
> Reported-by: Yu Liao <liaoyu15@huawei.com>
> Reported-by: Thomas Gleixner <tglx@linutronix.de>

I did really not ask you to remove the selftests alltogether.

Those tests check uptime and idle time. I asked you to remove the idle
time monotonicity assertion.

uptime is really supposed to be monotonically increasing. It's based on
CLOCK_BOOTTIME. If that goes backwards then we surely have more trouble
than /proc/uptime.

But it would be a good thing to change the test in the following way:

       ut1 = parse("/proc/uptime");
       bt1 = clock_gettime(CLOCK_BOOTTIME);
       assert(ut1 <= bt1);
       ut2 = parse("/proc/uptime");
       bt2 = clock_gettime(CLOCK_BOOTTIME);
       assert(ut2 <= bt2);
       assert(ut1 <= ut2);
       ....

Hmm?

Thanks,

        tglx