linux-kernel - Re: [PATCH v7 1/3] trace: Make removal of ring buffer pages atomic

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Date:	Tue, 08 May 2012 23:38:02 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Vaibhav Nagarnaik <vnagarnaik@...gle.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Laurent Chavey <chavey@...gle.com>,
	Justin Teravest <teravest@...gle.com>,
	David Sharp <dhsharp@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 1/3] trace: Make removal of ring buffer pages atomic

On Thu, 2012-05-03 at 18:59 -0700, Vaibhav Nagarnaik wrote:

> +		get_online_cpus();
> +		/*
> +		 * Fire off all the required work handlers
> +		 * Look out for offline CPUs
> +		 */
> +		for_each_buffer_cpu(buffer, cpu) {
> +			cpu_buffer = buffer->buffers[cpu];
> +			if (!cpu_buffer->nr_pages_to_update ||
> +			    !cpu_online(cpu))
> +				continue;
> +
> +			schedule_work_on(cpu, &cpu_buffer->update_pages_work);
> +		}
> +		/*
> +		 * This loop is for the CPUs that are not online.
> +		 * We can't schedule anything on them, but it's not necessary
> +		 * since we can change their buffer sizes without any race.
> +		 */
> +		for_each_buffer_cpu(buffer, cpu) {
> +			cpu_buffer = buffer->buffers[cpu];
> +			if (!cpu_buffer->nr_pages_to_update ||
> +			    cpu_online(cpu))
> +				continue;
> +
> +			rb_update_pages(cpu_buffer);
>  		}

BTW, why the two loops and not just:

		for_each_buffer_cpu(buffer, cpu) {
			cpu_buffer = buffer->buffers[cpu];
			if (!cpu_buffer->nr_pages_to_update)
				continue;

			if (cpu_online(cpu))
				schedule_work_on(cpu, &cpu_buffer->update_pages_work);
			else
				rb_update_pages(cpu_buffer);
		}

??


>  
>  		/* wait for all the updates to complete */
>  		for_each_buffer_cpu(buffer, cpu) {
>  			cpu_buffer = buffer->buffers[cpu];
> -			if (cpu_buffer->nr_pages_to_update) {
> -				update_pages_handler(cpu_buffer);
> -			}
> +			if (!cpu_buffer->nr_pages_to_update||

			      !cpu_buffer->nr_pages_to_update ||

-- Steve

> +			    !cpu_online(cpu))
> +				continue;
> +
> +			wait_for_completion(&cpu_buffer->update_completion);
> +			/* reset this value */
> +			cpu_buffer->nr_pages_to_update = 0;
>  		}


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