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 11:48:50 +0200
From:   Daniel Bristot de Oliveira <bristot@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 29/05/2019 10:41, Peter Zijlstra wrote:
> 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.
> 

ack! I will change the methods (and remove some blank lines here and there... :-))

Thanks Peter!

-- Daniel