linux-kernel - Re: [RFC PATCH 0/7] Early task context tracking

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Date:   Mon, 8 Apr 2019 14:54:54 +0200
From:   Daniel Bristot de Oliveira <bristot@...hat.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>, Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>,
        Romulo Silva de Oliveira <romulo.deoliveira@...c.br>,
        paulmck@...ux.vnet.ibm.com, Clark Williams <williams@...hat.com>,
        x86@...nel.org
Subject: Re: [RFC PATCH 0/7] Early task context tracking

On 4/4/19 7:40 PM, Joel Fernandes wrote:
>> Currently, recursion control uses the preempt_counter to
>> identify the current context. The NMI/HARD/SOFT IRQ counters
>> are set in the preempt_counter in the irq_enter/exit functions.
> Just started looking.
> 
> Thinking out loud... can we not just update the preempt_count as early on
> entry and as late on exit, as possible, and fix it that way? (Haven't fully
> yet looked into what could break if we did that.)
> 
> I also feel the context tracking should be unified, right now we already have
> two methods AFAIK - preempt_count and lockdep. Now this is yet another third.
> Granted lockdep cannot be enabled in production, but still. It will be nice
> to unify these tracking methods and if there is a single point of all such
> context tracking that works well, and even better if we can just fix
> preempt_count and use that for non-debugging usecases.
> 
> Also I feel in_interrupt() etc should be updated to rely on such tracking
> methods if something other than preempt_count is used..

Hi Joel,

I agree with you that it is important to have a single method to identify the
context.

I did the RFC using a specific percpu variable to make things simpler. Also
because I tried to move set/unset of the preempt_counter and my dev VM stopped
booting. So it looked, somehow, risky to move the preempt_counter.

Still, if people believe it is better to use the preempt_counter... I am not
against...

-- Daniel


> thanks,
> 
>  - Joel
> 
>