linux-kernel - Re: [PATCH v5 2/4] trace: Make removal of ring buffer pages atomic

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Date:	Mon, 23 Apr 2012 10:31:58 -0700
From:	Vaibhav Nagarnaik <vnagarnaik@...gle.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Michael Rubin <mrubin@...gle.com>,
	David Sharp <dhsharp@...gle.com>,
	Justin Teravest <teravest@...gle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/4] trace: Make removal of ring buffer pages atomic

On Fri, Apr 20, 2012 at 9:27 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Thu, 2012-02-02 at 12:00 -0800, Vaibhav Nagarnaik wrote:
>> +rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned int
>> nr_pages)
>>  {
>> -       struct buffer_page *bpage;
>> -       struct list_head *p;
>> -       unsigned i;
>> +       unsigned int nr_removed;
>> +       int page_entries;
>> +       struct list_head *tail_page, *to_remove, *next_page;
>> +       unsigned long head_bit;
>> +       struct buffer_page *last_page, *first_page;
>> +       struct buffer_page *to_remove_page, *tmp_iter_page;
>>
> Also, please use the "upside down x-mas tree" for the declarations:
>
> ie.
>
>       struct list_head *tail_page, *to_remove, *next_page;
>       struct buffer_page *to_remove_page, *tmp_iter_page;
>       struct buffer_page *last_page, *first_page;
>       unsigned int nr_removed;
>       unsigned long head_bit;
>       int page_entries;
>
> See, it looks easier to read then what you had.
>
>> +             /* fire off all the required work handlers */
>> +             for_each_buffer_cpu(buffer, cpu) {
>> +                     cpu_buffer = buffer->buffers[cpu];
>> +                     if (!cpu_buffer->nr_pages_to_update)
>> +                             continue;
>> +                     schedule_work_on(cpu, &cpu_buffer->update_pages_work);
>
> This locks up. I just tried the following, and it hung the task.
>
> Here:
>
> # cd /sys/kernel/debug/tracing
> # echo 1 > events/enable
> # sleep 10
> # echo 0 > event/enable
> # echo 0 > /sys/devices/system/cpu/cpu1/online
> # echo 100 > buffer_size_kb
>
> <locked!>
>
> I guess you could test if the cpu is online. And if so, then do the
> schedule_work_on(). You will need to get_online_cpus first.
>
> If the cpu is offline, just change it.
>
> -- Steve
>
> PS. The first patch looks good, but I think you need to add some more
> blank lines in your patches. You like to bunch a lot of text together,
> and that causes some eye strain.


Thanks for the feedback. I will update the patch with your suggestions.



Vaibhav Nagarnaik
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