linux-kernel - Re: [RFC PATCH] LTTng relay buffer allocation, read, write

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Date:	Mon, 29 Sep 2008 15:54:56 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, Martin Bligh <mbligh@...gle.com>,
	prasad@...ux.vnet.ibm.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>, od@...e.com,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>, hch@....de,
	David Wilder <dwilder@...ibm.com>,
	Tom Zanussi <zanussi@...cast.net>
Subject: Re: [RFC PATCH] LTTng relay buffer allocation, read, write


On Mon, 29 Sep 2008, Steven Rostedt wrote:
>   We do not need to worry about SMP for writes.
>   That is, if you reserve data and atomically push the head forward
>    if the head goes past the end of page, One, you check if your
>    start of head (head - length) is still on the page. If it is
>    you add your padding (you already reserved it), then you atomically
>    push the head forward. Then you start the process again.
>   If the start of the head (head - length) is not on the page, that
>    means that an IRQ or NMI came in and pushed it before you.

I forgot to mention one important detail. The "head" index will stay
on the page frame. That way we do not need to figure out which 
head_page we are on. We grab the head_page, we atomically 
(local_inc_return) the head pointer on that page. If the return value is 
still on the page, we succeeded. We can also increment a value on this 
page frame that will prevent recording if we somehow overflowed the buffer 
before relinquishing the stack.

That is

  reserve_event()

     IRQ->
            reserve_event();

      [...]

     IRQ->reserve_event() came back to original head!

      Here we do not have a big enough buffer, and this is just stupid ;-)
      We would drop packets in this case, and should drop the guy on his
      head who came up with the too small buffer.

-- Steve

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