linux-kernel - Re: [PATCH V4 05/12] trace/hwlat: Support hotplug operations

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Message-ID: <c87c24bd-253c-a645-1f29-83c558d8d4c5@redhat.com>
Date:   Mon, 21 Jun 2021 18:14:36 +0200
From:   Daniel Bristot de Oliveira <bristot@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Phil Auld <pauld@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Kate Carcia <kcarcia@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Clark Willaims <williams@...hat.com>,
        John Kacur <jkacur@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4 05/12] trace/hwlat: Support hotplug operations

On 6/21/21 5:25 PM, Steven Rostedt wrote:
> On Mon, 21 Jun 2021 13:34:44 +0200
> Daniel Bristot de Oliveira <bristot@...hat.com> wrote:
> 
>>> And of course, because get_online_cpus() is called within
>>> trace_types_lock, doing this check is going to cause a lock inversion.
>>>  
>> Yep! I tried to take the trace_type_lock here, and got the lockdep info about
>> this problem.
>>
>>> The only thing I could think of is to wake up a worker thread to do the
>>> work. That is, this just wakes the worker thread, then the worker grabs
>>> the trace_types_lock, iterates through the cpu mask of expect running
>>> threads, and then starts or kills them depending on the hwlat_busy
>>> value.  
>> So, it will not wait for the kworker to run?
> What wont wait?

For example, at the shutdown, should the hotplug callback wait for the workqueue
to run & kill the thread, or not?

-- Daniel

> -- Steve
>