linux-kernel - Re: Ping: [PATCH v15 00/13] support "task

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Date:   Wed, 7 Sep 2016 17:11:06 -0400
From:   Francis Giraldeau <francis.giraldeau@...il.com>
To:     Chris Metcalf <cmetcalf@...lanox.com>,
        Gilad Ben Yossef <giladb@...lanox.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Christoph Lameter <cl@...ux.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...capital.net>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        linux-doc@...r.kernel.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Ping: [PATCH v15 00/13] support "task_isolation" mode

On 2016-08-29 12:27 PM, Chris Metcalf wrote:
> On 8/16/2016 5:19 PM, Chris Metcalf wrote:
>> Here is a respin of the task-isolation patch set.
>
> No concerns have been raised yet with the v15 version of the patch series
> in the two weeks since I posted it, and I think I have addressed all
> previously-raised concerns (or perhaps people have just given up arguing
> with me).

There is a cycle with header include in the v15 patch set on x86_64 that cause a compilation error. You will find the patch (and other fixes) in the following branch:

    https://github.com/giraldeau/linux/commits/dataplane-x86-fix-inc

In the test file, it is indicated to change timer-tick.c to disable the 1Hz tick, is there a reason why the change is not in the patch set? I added this trivial change in the branch dataplane-x86-fix-inc above.

The syscall test fails on x86:

    $ sudo ./isolation
    [...]
    test_syscall: FAIL (0x100)
    test_syscall (SIGUSR1): FAIL (0x100)

I wanted to debug this problem with gdb and a KVM virtual machine. However, the TSC clock source is detected as non reliable, even with the boot parameter tsc=reliable, and therefore prctl(PR_SET_TASK_ISOLATION, PR_TASK_ISOLATION_ENABLE) always returns EAGAIN. Is there a trick to run task isolation in a VM (at least for debugging purposes)?

BTW, this was causing the test to enter an infinite loop. If the clock source is not reliable, maybe a different error code should be returned, because this situation not transient. In the mean time, I added a check for 100 max retries in the test (prctl_safe()).

When running only the test_jitter(), the isolation mode is lost:

    [ 6741.566048] isolation/9515: task_isolation mode lost due to irq_work

With ftrace (events/workqueue/workqueue_execute_start), I get a bit more info:

     kworker/1:1-676   [001] ....  6610.097128: workqueue_execute_start: work struct ffff8801a784ca20: function dbs_work_handler

The governor was ondemand, so I tried to set the frequency scaling governor to performance, but that does not solve the issue. Is there a way to suppress this irq_work? Should we run the isolated task with high real-time priority, such that it never get preempted?

Thanks!

Francis