linux-kernel - Re: [PATCH v6] Unified trace buffer

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Message-ID: <48DD50AB.10208@redhat.com>
Date:	Fri, 26 Sep 2008 17:14:19 -0400
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	prasad@...ux.vnet.ibm.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	David Wilder <dwilder@...ibm.com>, hch@....de,
	Martin Bligh <mbligh@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <srostedt@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: [PATCH v6] Unified trace buffer

Peter Zijlstra wrote:
> On Fri, 2008-09-26 at 14:05 -0400, Steven Rostedt wrote:
>> +static void
>> +rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned
>> nr_pages)
>> +{
>> +       struct page *page;
>> +       struct list_head *p;
>> +       unsigned i;
>> +
>> +       atomic_inc(&cpu_buffer->record_disabled);
> 
> You probably want synchronize_sched() here (and similar other places) to
> ensure any active writer on the corresponding cpu is actually stopped.

Would it really be done in the buffer layer?
I think it should be done by each tracer, because buffer layer
can't ensure truly active writers have stopped.

Thank you,

> 
> Which suggests you want to use something like ring_buffer_lock_cpu() and
> implement that as above.
> 
>> +       for (i = 0; i < nr_pages; i++) {
>> +               BUG_ON(list_empty(&cpu_buffer->pages));
>> +               p = cpu_buffer->pages.next;
>> +               page = list_entry(p, struct page, lru);
>> +               list_del_init(&page->lru);
>> +               __free_page(page);
>> +       }
>> +       BUG_ON(list_empty(&cpu_buffer->pages));
>> +
>> +       __ring_buffer_reset_cpu(cpu_buffer);
>> +
>> +       check_pages(cpu_buffer);
>> +
>> +       atomic_dec(&cpu_buffer->record_disabled);
>> +
>> +}
> 

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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