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Date: Wed, 20 Dec 2023 09:40:30 -0500
From: Steven Rostedt <rostedt@goodmis.org>
To: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, Mark
 Rutland <mark.rutland@arm.com>, Mathieu Desnoyers
 <mathieu.desnoyers@efficios.com>, Andrew Morton
 <akpm@linux-foundation.org>, Tzvetomir Stoyanov <tz.stoyanov@gmail.com>,
 Vincent Donnefort <vdonnefort@google.com>, Kent Overstreet
 <kent.overstreet@gmail.com>
Subject: Re: [PATCH v5 03/15] ring-buffer: Add interface for configuring
 trace sub buffer size
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On Wed, 20 Dec 2023 23:26:19 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> On Tue, 19 Dec 2023 13:54:17 -0500
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > +/**
> > + * ring_buffer_subbuf_order_set - set the size of ring buffer sub page.
> > + * @buffer: The ring_buffer to set the new page size.
> > + * @order: Order of the system pages in one sub buffer page
> > + *
> > + * By default, one ring buffer pages equals to one system page. This API can be
> > + * used to set new size of the ring buffer page. The size must be order of
> > + * system page size, that's why the input parameter @order is the order of
> > + * system pages that are allocated for one ring buffer page:
> > + *  0 - 1 system page
> > + *  1 - 2 system pages
> > + *  3 - 4 system pages
> > + *  ...  
> 
> Don't we have the max order of the pages?

Actually there is. I think it's 7?

Honestly, anything over 5 is probably too much. But hey.

> 
> > + *
> > + * Returns 0 on success or < 0 in case of an error.
> > + */
> > +int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order)
> > +{
> > +	int psize;
> > +
> > +	if (!buffer || order < 0)
> > +		return -EINVAL;
> > +
> > +	if (buffer->subbuf_order == order)
> > +		return 0;
> > +
> > +	psize = (1 << order) * PAGE_SIZE;
> > +	if (psize <= BUF_PAGE_HDR_SIZE)
> > +		return -EINVAL;
> > +
> > +	buffer->subbuf_order = order;
> > +	buffer->subbuf_size = psize - BUF_PAGE_HDR_SIZE;
> > +
> > +	/* Todo: reset the buffer with the new page size */
> > +  
> 
> I just wonder why there is no reallocate the sub buffers here.
> Is it OK to change the sub buffer page size and order while
> using the ring buffer?

Currently we disable the ring buffer to do the update.

-- Steve