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Subject: Re: [PATCH V4 05/12] trace/hwlat: Support hotplug operations
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Phil Auld <pauld@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Kate Carcia <kcarcia@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        Clark Willaims <williams@redhat.com>,
        John Kacur <jkacur@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
References: <cover.1623746916.git.bristot@redhat.com>
 <8899f8a8bec38bc600f7a2c61bc6ca664aa7beeb.1623746916.git.bristot@redhat.com>
 <20210618124503.388fe4d4@oasis.local.home>
 <20210618150020.689439d4@oasis.local.home>
From:   Daniel Bristot de Oliveira <bristot@redhat.com>
Message-ID: <c4b86b0e-b45d-3039-f49c-0dc53e1adcbd@redhat.com>
Date:   Mon, 21 Jun 2021 13:34:44 +0200
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On 6/18/21 9:00 PM, Steven Rostedt wrote:
> On Fri, 18 Jun 2021 12:45:03 -0400
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 
>>> +/*
>>> + * hwlat_cpu_init - CPU hotplug online callback function
>>> + */
>>> +static int hwlat_cpu_init(unsigned int cpu)
>>> +{
>>> +	struct trace_array *tr = hwlat_trace;
>>> +  
>> You need to take the trace_types_lock here, between testing the
>> hwlat_busy and starting the threads. Otherwise, between the two, the
>> hwlat tracer could be turned off while a CPU is coming on line, and
>> then you just started a per cpu thread, while the hwlat tracer is not
>> enabled.
> 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?

-- Daniel

> 
> -- Steve
>