Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Wed, 29 May 2019 10:41:18 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Daniel Bristot de Oliveira <bristot@redhat.com>
Cc:     linux-kernel@vger.kernel.org, williams@redhat.com,
        daniel@bristot.me, "Steven Rostedt (VMware)" <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Matthias Kaehlcke <mka@chromium.org>,
        "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Yangtao Li <tiny.windzz@gmail.com>,
        Tommaso Cucinotta <tommaso.cucinotta@santannapisa.it>
Subject: Re: [RFC 3/3] preempt_tracer: Use a percpu variable to control
 traceble calls
Message-ID: <20190529084118.GG2623@hirez.programming.kicks-ass.net>
References: <cover.1559051152.git.bristot@redhat.com>
 <9b0698774be3bb406e2b8b2c12dc1fb91532bff0.1559051152.git.bristot@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <9b0698774be3bb406e2b8b2c12dc1fb91532bff0.1559051152.git.bristot@redhat.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.