linux-kernel - Re: [PATCH v3 1/2] tracing: ring_buffer: Rewind persistent ring buffer when reboot

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Message-Id: <20250527121549.2d40608e9463199d85bf9ba4@kernel.org>
Date: Tue, 27 May 2025 12:15:49 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Masami Hiramatsu (Google) <mhiramat@...nel.org>
Cc: Steven Rostedt <rostedt@...dmis.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] tracing: ring_buffer: Rewind persistent ring
 buffer when reboot

On Tue, 27 May 2025 09:17:43 +0900
Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:

> On Mon, 26 May 2025 12:03:42 +0900
> Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:
> 
> > On Fri, 23 May 2025 17:28:57 -0400
> > Steven Rostedt <rostedt@...dmis.org> wrote:
> > 
> > > On Fri, 23 May 2025 16:54:25 -0400
> > > Steven Rostedt <rostedt@...dmis.org> wrote:
> > > 
> > > > I think we also need this:
> > > > 
> > > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > > > index 7d837963dd1e..456efebc396a 100644
> > > > --- a/kernel/trace/ring_buffer.c
> > > > +++ b/kernel/trace/ring_buffer.c
> > > > @@ -3638,6 +3638,9 @@ rb_move_tail(struct ring_buffer_per_cpu *cpu_buffer,
> > > >  
> > > >  	rb_reset_tail(cpu_buffer, tail, info);
> > > >  
> > > > +	/* The new page should have zero committed */
> > > > +	rb_init_page(next_page->page);
> > > > +
> > > >  	/* Commit what we have for now. */
> > > >  	rb_end_commit(cpu_buffer);
> > > >  	/* rb_end_commit() decs committing */
> > > 
> > > No we don't need it ;-)
> > > 
> > > I'm looking deeper into the code and we have this:
> > > 
> > > 		/*
> > > 		 * No need to worry about races with clearing out the commit.
> > > 		 * it only can increment when a commit takes place. But that
> > > 		 * only happens in the outer most nested commit.
> > > 		 */
> > > 		local_set(&next_page->page->commit, 0);
> > > 
> > > When the tail page gets moved.
> > 
> > Yeah, when the page gets written, it is cleared.
> 
> What about this? It seems if we keep the reader->page->commit, when we
> catch up the tail page, rb_get_reader_page() returns new reader (== tail)
> because reader->read(==0) < reader->page->commit. But we should not return
> new reader.
> 
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 5034bae02f08..179142db9586 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -5522,6 +5522,12 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
>                 cpu_buffer->last_overrun = overwrite;
>         }
>  
> +       /* But if we catch up to the tail, do not return reader page. */
> +       if (cpu_buffer->commit_page == cpu_buffer->reader_page) {


Hmm, I found this might not work if it is called twice.
Maybe rb_get_reader_page() can check the reader_page->ts > head_page->ts
and return NULL (or reader_page if read < commit) at first.

Thank you,

> +               reader = NULL;
> +               goto out;
> +       }
> +
>         goto again;
>  
>   out:
> 
> 
> -- 
> Masami Hiramatsu (Google) <mhiramat@...nel.org>


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>