linux-kernel - Re: [GIT PULL] isolation: 1Hz residual tick offloading v4

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Message-ID: <xunyvabfjylg.fsf@redhat.com>
Date:   Tue, 22 May 2018 22:10:19 +0300
From:   Yauheni Kaliuta <yauheni.kaliuta@...hat.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Luiz Capitulino <lcapitulino@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Lameter <cl@...ux.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Mike Galbraith <efault@....de>, Rik van Riel <riel@...riel.com>
Subject: Re: [GIT PULL] isolation: 1Hz residual tick offloading v4

Hi, Frederic!

>>>>> On Mon, 29 Jan 2018 02:10:26 +0100, Frederic Weisbecker  wrote:
 > On Wed, Jan 24, 2018 at 10:46:08AM -0500, Luiz Capitulino wrote:

[...]

 >> Since the 1Hz tick offload worked for you, I must be missing
 >> a way to disable this timer or the kernel is thinking my CPU
 >> has unstable TSC (which it doesn't AFAIK).

 > It's beyond the scope of this patchset but indeed that's
 > right, I run my kernels with tsc=reliable because my CPUs
 > don't have the TSC_RELIABLE flag.  That's the only way I found
 > to shutdown the tick completely on my test machine, otherwise
 > I keep having that clocksource watchdog.

[...]

Thanks, it helps. But I have accounting problem:

if I run user busy loop on the nohz cpu, the task accounting works
correctly (top shows the task takes 100% cpu), but cpu accounting is
wrong (cpu is 100% idle, in the per-core view as well).

If I understand correctly, the stats are updated by account_user_time()
-> task_group_account_field() but there is no call for it in case of
offloading (it is called from irqtime_account_process_tick,
account_process_tick, vtime_user_exit).

Moreover, task_group_account_field() uses __this_cpu_add() which will be
wrong for offloading.

For testing I used kcpustat_cpu(task_cpu(p)) in
task_group_account_field() and added call account_user_time(curr, delta)
to the sched_tick_remote() what fixes it for me, but what would be the
proper fix?

-- 
WBR,
Yauheni Kaliuta