linux-kernel - Re: [PATCH v5 04/15] ring-buffer: Set new size of the ring buffer sub page

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Message-ID: <20231220115602.1903bf77@gandalf.local.home>
Date: Wed, 20 Dec 2023 11:56:02 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Mark
 Rutland <mark.rutland@....com>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Tzvetomir Stoyanov <tz.stoyanov@...il.com>,
 Vincent Donnefort <vdonnefort@...gle.com>, Kent Overstreet
 <kent.overstreet@...il.com>
Subject: Re: [PATCH v5 04/15] ring-buffer: Set new size of the ring buffer
 sub page

On Thu, 21 Dec 2023 01:34:56 +0900
Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:

> On Tue, 19 Dec 2023 13:54:18 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > From: "Tzvetomir Stoyanov (VMware)" <tz.stoyanov@...il.com>
> > 
> > There are two approaches when changing the size of the ring buffer
> > sub page:
> >  1. Destroying all pages and allocating new pages with the new size.
> >  2. Allocating new pages, copying the content of the old pages before
> >     destroying them.
> > The first approach is easier, it is selected in the proposed
> > implementation. Changing the ring buffer sub page size is supposed to
> > not happen frequently. Usually, that size should be set only once,
> > when the buffer is not in use yet and is supposed to be empty.
> > 
> > Link: https://lore.kernel.org/linux-trace-devel/20211213094825.61876-5-tz.stoyanov@gmail.com
> >   
> 
> OK, this actually reallocate the sub buffers when a new order is set.
> BTW, with this change, if we set a new order, the total buffer size will be
> changed too? Or reserve the total size? I think either is OK but it should
> be described in the document. (e.g. if it is changed, user should set the
> order first and set the total size later.)
> 

Patch 11 keeps the same size of the buffer. As I would think that would be
what the user would expect. And not only that, it breaks the latency
tracers if it doesn't keep the same size.

-- Steve