From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9282BEE4993 for ; Mon, 21 Aug 2023 15:16:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236212AbjHUPQq (ORCPT ); Mon, 21 Aug 2023 11:16:46 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45848 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232461AbjHUPQp (ORCPT ); Mon, 21 Aug 2023 11:16:45 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3E6B4EE; Mon, 21 Aug 2023 08:16:44 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id C7C87619A5; Mon, 21 Aug 2023 15:16:43 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3420CC433C7; Mon, 21 Aug 2023 15:16:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1692631003; bh=JKe9ztueuPiEiUDswLDf27Y+P0gk7mojOx2Je864hew=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=PIL+MW3ilXMy0PLP4sW2oUdQYwl9tKSR46weUmd+DaLgnaVy513PicA/5tN5MZckm TkczGLObUjIDLcWuuBKAIDfv7+zqnA0oiV/PgITDr75Fq7KfghYua2qv0o0lDbUJ9P 6HRIo/RpyOURkiHxcJ8HvEsu6+BK25G/8ZEIOL81jFkQOKAb3mS5uXXkvdRep0NZ1c imEngEmCjWQBIBRE7tKhyoOuXxJoQMcFzIGaXWHyqh3PNHAJCUUSrweq/OCvF1EBhk vLWNmlyRs2+OiMlxKfCYGmXIM+oammUeOqp6ouzXAxHR2CMmeGHHkha8XWZ5MiT41P 5/J6LeXCE/6qg== Date: Tue, 22 Aug 2023 00:16:39 +0900 From: Masami Hiramatsu (Google) To: "Masami Hiramatsu (Google)" Cc: Steven Rostedt , linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org Subject: Re: [PATCH] tracing: Fix to avoid wakeup loop in splice read of per-cpu buffer Message-Id: <20230822001639.7a3cbedc3ad6e3938ee76e16@kernel.org> In-Reply-To: <169262755804.106231.8245792908363050528.stgit@devnote2> References: <169262755804.106231.8245792908363050528.stgit@devnote2> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-trace-kernel@vger.kernel.org On Mon, 21 Aug 2023 23:19:18 +0900 "Masami Hiramatsu (Google)" wrote: > From: Masami Hiramatsu (Google) > > ftrace user can set 0 or small number to the 'buffer_percent' for quick > response for the ring buffer. In that case wait_on_pipe() will return > before filling a page of the ring buffer. That is too soon for splice() > because ring_buffer_read_page() will fail again. > This leads unnecessary loop in tracing_buffers_splice_read(). > > Set a minimum percentage of the buffer which is enough to fill a page to > wait_on_pipe() to avoid this situation. > > Fixes: 03329f993978 ("tracing: Add tracefs file buffer_percentage") > Signed-off-by: Masami Hiramatsu (Google) So this is what I found when I played with virtio-trace. I think now trace-cmd has richer feature, so it can be replaced with that, maybe what we need is a selftest that ensures trace_pipe_raw supports splice correctly. BTW, even with this fix, blocking splice still causes a strange behavior. If I set '400' to buffer_size_kb (so 100 pages) and '1' to buffer_percent, splice always returns 8192 (2 pages) to read. But I expected that should return 4096 (1 page). This means splice() waits longer than I thought. I think the fullfilled percentage calculation will be a bit wrong. Thank you, > --- > kernel/trace/trace.c | 12 +++++++++++- > 1 file changed, 11 insertions(+), 1 deletion(-) > > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c > index b8870078ef58..88448e8d8214 100644 > --- a/kernel/trace/trace.c > +++ b/kernel/trace/trace.c > @@ -8462,6 +8462,8 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos, > /* did we read anything? */ > if (!spd.nr_pages) { > long wait_index; > + size_t nr_pages; > + size_t full; > > if (ret) > goto out; > @@ -8472,7 +8474,15 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos, > > wait_index = READ_ONCE(iter->wait_index); > > - ret = wait_on_pipe(iter, iter->tr->buffer_percent); > + /* For splice, we have to ensure at least 1 page is filled */ > + nr_pages = ring_buffer_nr_pages(iter->array_buffer->buffer, iter->cpu_file); > + if (nr_pages * iter->tr->buffer_percent < 100) { > + full = nr_pages + 99; > + do_div(full, nr_pages); > + } else > + full = iter->tr->buffer_percent; > + > + ret = wait_on_pipe(iter, full); > if (ret) > goto out; > > -- Masami Hiramatsu (Google)