From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D6E001CD2C; Tue, 27 May 2025 03:15:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1748315753; cv=none; b=P8xJViwraa79YR8aFCQdYbpI/OrRVix9HXqhnNSIvNk7OwCo+BMUuQWt/XQoa5zsC+E3K74ZqZXub95N0iHzFnIW8fztMRJzyBbRBd3xINDE3YWG5nJIQlk2JGypaCcxk+lpwqpPRIZo6AS+MRZCYqiKZ+lekrvs4gpMjqSAgck= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1748315753; c=relaxed/simple; bh=IVG8hcaS0HMjvnDTtNlgb60z2Nuvm9LQjKKz3uFlBkU=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=J2bNTq6u9HRbktAnJjyUlCC+tZRyG8vGQ+EIdnI3Xx5LcJ4SDQNUTiIeppn2Ui7s1p3RmQoZlNpO6/HNrS6hGvL6IqVfkkuP43J1c5oVuguQGAogRymFsdE9SxPqCtWIvSJoDwWuQXWf+U86k9SwAc0uCu5vpXIQx7j6eZIBdVs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=rQf6w47o; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="rQf6w47o" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 60697C4CEE7; Tue, 27 May 2025 03:15:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1748315752; bh=IVG8hcaS0HMjvnDTtNlgb60z2Nuvm9LQjKKz3uFlBkU=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=rQf6w47oz1XGQ4g2lZld5xAtoZrVqHMKIB/c0G55sj//106lgfmYQFEiqhdG1QCDV WidOU8l5ESm8ZbLNR5teOBg+x7WFJXb1KAj3536jtFQeqzppg3jb9dNMZXSCnKd2V1 sLxPJ3nvKgxZALMjOsWE4qXMjsAuONA14K1CzbxSGMnNnRH3i8zYGLw9dP/ncL9hRR EuchWUy8QKZuVKNLzhJI6RsHipCT8c32SibXEis/iksUMgjKIYQsX4M/GUatYTXh9C V9QXYxff2alHwq0wq53hZGuEr04ckZfbKvGek0boR055n8/yqUnCGpPh5MhEqdjGg4 EFn32kFhxGFCA== Date: Tue, 27 May 2025 12:15:49 +0900 From: Masami Hiramatsu (Google) To: Masami Hiramatsu (Google) Cc: Steven Rostedt , Mathieu Desnoyers , linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org Subject: Re: [PATCH v3 1/2] tracing: ring_buffer: Rewind persistent ring buffer when reboot Message-Id: <20250527121549.2d40608e9463199d85bf9ba4@kernel.org> In-Reply-To: <20250527091743.d036018195b7668aea0753c0@kernel.org> References: <174792927500.496143.10668210464102033012.stgit@mhiramat.tok.corp.google.com> <174792928413.496143.17979267710069360561.stgit@mhiramat.tok.corp.google.com> <20250523165425.0275c9a1@gandalf.local.home> <20250523172857.02ab4a75@gandalf.local.home> <20250526120342.cdcfca9ac3c2161d48c03af5@kernel.org> <20250527091743.d036018195b7668aea0753c0@kernel.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Tue, 27 May 2025 09:17:43 +0900 Masami Hiramatsu (Google) wrote: > On Mon, 26 May 2025 12:03:42 +0900 > Masami Hiramatsu (Google) wrote: > > > On Fri, 23 May 2025 17:28:57 -0400 > > Steven Rostedt wrote: > > > > > On Fri, 23 May 2025 16:54:25 -0400 > > > Steven Rostedt 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) -- Masami Hiramatsu (Google)