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 6232573177 for ; Tue, 12 Dec 2023 14:20:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="igwB604A" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 21DE8C433C8; Tue, 12 Dec 2023 14:20:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1702390812; bh=32D5OibO0S828mD8/iI+qNHoBvt9+ZXoMSHftKjKbcg=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=igwB604Aqp997pDV3+MxQMAZQ8IXZXq1CqOBA77lXN+g4O9RD5iF+PYljarvD0ncU jdk7NfWgqwlRa3FRPA6Rs1evDijZ+/w91IrGXq42omc5sizXOFzbOeETGSbXBg/KP+ 4eIwFRbLWO6CtzCzLCX7TZktFG3YR0w19MW7Ii1kCdmMhy2WcFp9OJ+1/0NfMb9Cpr TNznoeq9hT94hdn8f1UPN31RwvbF5QhcqcAX8h9bqKvuiQBLRXFJhb92bJ/BiifWSX UcED2EHvD6ss0IYeauDQwL3g8PWNHtJhTbt4BexGrPd8OHEBP6okSRhLf/Jakrg9Pp coC3ECf/UFCLQ== Date: Tue, 12 Dec 2023 23:20:08 +0900 From: Masami Hiramatsu (Google) To: Steven Rostedt Cc: LKML , Linux Trace Kernel , Masami Hiramatsu , Mark Rutland , Mathieu Desnoyers Subject: Re: [PATCH v2] ring-buffer: Never use absolute timestamp for first event Message-Id: <20231212232008.5b79e5d7fcb8967604ae5c3a@kernel.org> In-Reply-To: <20231212071837.5fdd6c13@gandalf.local.home> References: <20231212071837.5fdd6c13@gandalf.local.home> X-Mailer: Sylpheed 3.7.0 (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, 12 Dec 2023 07:18:37 -0500 Steven Rostedt wrote: > From: "Steven Rostedt (Google)" > > On 32bit machines, the 64 bit timestamps are broken up into 32 bit words > to keep from using local64_cmpxchg(), as that is very expensive on 32 bit > architectures. > > On 32 bit architectures, reading these timestamps can happen in a middle > of an update. In this case, the read returns "false", telling the caller > that the timestamp is in the middle of an update, and it needs to assume > it is corrupted. The code then accommodates this. > > When first reserving space on the ring buffer, a "before_stamp" and > "write_stamp" are read. If they do not match, or if either is in the > process of being updated (false was returned from the read), an absolute > timestamp is added and the delta is not used, as that requires reading > theses timestamps without being corrupted. > > The one case that this does not matter is if the event is the first event > on the sub-buffer, in which case, the event uses the sub-buffer's > timestamp and doesn't need the other stamps for calculating them. > > After some work to consolidate the code, if the before or write stamps are > in the process of updating, an absolute timestamp will be added regardless > if the event is the first event on the sub-buffer. This is wrong as it > should not care about the success of these reads if it is the first event > on the sub-buffer. > > Fix up the parenthesis so that even if the timestamps are corrupted, if > the event is the first event on the sub-buffer (w == 0) it still does not > force an absolute timestamp. > > It's actually likely that w is not zero, but move it out of the unlikeyl() > and test it first. It should be in hot cache anyway, and there's no reason > to do the rest of the test for the first event on the sub-buffer. And this > prevents having to test all the 'or' statements in that case. > > Cc: stable@vger.kernel.org > Fixes: 58fbc3c63275c ("ring-buffer: Consolidate add_timestamp to remove some branches") > Signed-off-by: Steven Rostedt (Google) > --- > Changes since v2: https://lore.kernel.org/linux-trace-kernel/20231211115949.4692e429@gandalf.local.home > > - Move the test to 'w' out of the unlikely and do it first. > It's already in hot cache, and the rest of test shouldn't be done > if 'w' is zero. > > kernel/trace/ring_buffer.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > index b416bdf6c44a..095b86081ea8 100644 > --- a/kernel/trace/ring_buffer.c > +++ b/kernel/trace/ring_buffer.c > @@ -3581,7 +3581,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer, > * absolute timestamp. > * Don't bother if this is the start of a new page (w == 0). > */ > - if (unlikely(!a_ok || !b_ok || (info->before != info->after && w))) { > + if (w && unlikely(!a_ok || !b_ok || info->before != info->after)) { > info->add_timestamp |= RB_ADD_STAMP_FORCE | RB_ADD_STAMP_EXTEND; > info->length += RB_LEN_TIME_EXTEND; > } else { After this else, } else { info->delta = info->ts - info->after; The code is using info_after, but it is not sure 'a_ok'. Does this mean if 'w == 0 && !a_ok' this doesn't work correctly? What will be the expected behavior when w == 0 here? Thank you, > -- > 2.42.0 > -- Masami Hiramatsu (Google)