linux-kernel - Re: [RFC 3/3] preempt_tracer: Use a percpu variable to control traceble calls

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Date:   Wed, 29 May 2019 10:41:18 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     linux-kernel@...r.kernel.org, williams@...hat.com,
        daniel@...stot.me, "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Yangtao Li <tiny.windzz@...il.com>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>
Subject: Re: [RFC 3/3] preempt_tracer: Use a percpu variable to control
 traceble calls

On Tue, May 28, 2019 at 05:16:24PM +0200, Daniel Bristot de Oliveira wrote:
>  #if defined(CONFIG_PREEMPT) && (defined(CONFIG_DEBUG_PREEMPT) || \
>  				defined(CONFIG_TRACE_PREEMPT_TOGGLE))
> +
> +DEFINE_PER_CPU(int, __traced_preempt_count) = 0;
>  /*
>   * If the value passed in is equal to the current preempt count
>   * then we just disabled preemption. Start timing the latency.
>   */
>  void preempt_latency_start(int val)
>  {
> -	if (preempt_count() == val) {
> +	int curr = this_cpu_read(__traced_preempt_count);

We actually have this_cpu_add_return();

> +
> +	if (!curr) {
>  		unsigned long ip = get_lock_parent_ip();
>  #ifdef CONFIG_DEBUG_PREEMPT
>  		current->preempt_disable_ip = ip;
>  #endif
>  		trace_preempt_off(CALLER_ADDR0, ip);
>  	}
> +
> +	this_cpu_write(__traced_preempt_count, curr + val);
>  }
>  
>  static inline void preempt_add_start_latency(int val)
> @@ -3200,8 +3206,12 @@ NOKPROBE_SYMBOL(preempt_count_add);
>   */
>  void preempt_latency_stop(int val)
>  {
> -	if (preempt_count() == val)
> +	int curr = this_cpu_read(__traced_preempt_count) - val;

this_cpu_sub_return();

> +
> +	if (!curr)
>  		trace_preempt_on(CALLER_ADDR0, get_lock_parent_ip());
> +
> +	this_cpu_write(__traced_preempt_count, curr);
>  }

Can't say I love this, but it is miles better than the last patch.