From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 4EE4A30DD30; Thu, 21 May 2026 03:46:43 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779335206; cv=none; b=PhEZsSMXj6IIR5g4q5VrLWTp4XQogoKzAtryEgHbzTABQBvRSkGJJh9EHbCvJfGuW2dppjZIM24u/m7vMzmlESECS4g3h5j1Ari/503jRIEtDOr4UkPnFVdMI2i2/ZWeEvq/jucBnXXSK1eaa/L25kBATRMAXPUnzBxSEnBmkew= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779335206; c=relaxed/simple; bh=vfYSD69Q3rD4e9jkq8XgIX/wilniO7gBdcEfUt264EI=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=hAzR6bq7fqc1bdgrjLubazDb8HHVXESKJWM5WRadNfFua+6qGmJbFlr1f1DsCgIypMRrRpdlMITPl+VQma8lK450/4iRtfKkMw7U0IuCWcDJbfpDyGgsmI8/imdvXdvqxOB3KBTMl3KNN9fbYz9NDha04fHDhx7rGK2CZ5y7YxU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=eScHYmno; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="eScHYmno" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 17A8D1F000E9; Thu, 21 May 2026 03:46:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1779335203; bh=qkhZOf3D5ilURBV43WvvgE2pB6mKClKLalFB6ckHHss=; h=Date:From:To:Cc:Subject:In-Reply-To:References; b=eScHYmnoGa2wpjOyQfh5394DjpLHb5MH+NIncZZSdConkGSYJir4feiGqjq01agEd vmA396BBTL2OOio+Ol0qZie9oEuFQLZgRIRyNmhcblE3uRXEOwfONOrME8Lz+mfIDl PSf29Jn7yz3NicmngjbX/biejlzm4O+j9s9/NJGscZcn6WiZ7kw1SNyu85HsFDNyEO a4wi8GaVcUEwESP6zViGEpokmIKmnxDDeBpy6MN5HWmYRDDvS78hWG/I892flWq2+c kh6vebo69h7Yh3AXEXVEohIbioiFmvQOfY738ZO2/ZW97MsH6F3rn8+MfWTuiHVV8g SF0vWlopwObig== Date: Thu, 21 May 2026 12:46:40 +0900 From: Masami Hiramatsu (Google) To: Steven Rostedt Cc: LKML , Linux trace kernel , Masami Hiramatsu , Mathieu Desnoyers Subject: Re: [PATCH v2] ring-buffer: Fix reporting of missed events in iterator Message-Id: <20260521124640.daadbf2ecbe3c1e6b6560134@kernel.org> In-Reply-To: <20260520220801.4fd09d13@fedora> References: <20260520220801.4fd09d13@fedora> 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 Wed, 20 May 2026 22:08:01 -0400 Steven Rostedt wrote: > From c3651ad3ac95b331c7aa010d163704a3702855da Mon Sep 17 00:00:00 2001 > From: Steven Rostedt > Date: Wed, 20 May 2026 14:28:17 -0400 > Subject: [PATCH] ring-buffer: Fix reporting of missed events in iterator > > When tracing is active while reading the trace file, if the iterator > reading the buffer detects that the writer has passed the iterator head, > it will reset and set a "missed events" flag. This flag is passed to the > output processing to show the user that events were missed: > > CPU:4 [LOST EVENTS] > > The problem is that the flag is reset after it is checked in > ring_buffer_iter_dropped(). But the "trace" file iterates over all the CPU > ring buffers and it will check if they are dropped when figuring out which > buffer to print next. This prematurely clears the missed_events flag if > the CPU buffer with the missed events is not the one that is printed next. > > On the iteration where the CPU buffer with the missed events is printed, > the check if it had missed events would return false and the output does > not show that events were missed. > > Do not reset the missed_events flag when checking if there were missed > events, but instead clear it when moving the iterator head to the next > event. > Looks good to me. Acked-by: Masami Hiramatsu (Google) > Cc: stable@vger.kernel.org > Fixes: c9b7a4a72ff64 ("ring-buffer/tracing: Have iterator acknowledge dropped events") > Signed-off-by: Steven Rostedt > --- > Changes since v1: https://patch.msgid.link/20260520142817.4050abab@gandalf.local.home > > - Added clearing iter->missed_events in rb_iter_reset() (Sashiko) > > kernel/trace/ring_buffer.c | 8 +++----- > 1 file changed, 3 insertions(+), 5 deletions(-) > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > index 7288383b1f27..7b07d2004cc6 100644 > --- a/kernel/trace/ring_buffer.c > +++ b/kernel/trace/ring_buffer.c > @@ -5429,6 +5429,7 @@ static void rb_iter_reset(struct ring_buffer_iter *iter) > iter->head_page = cpu_buffer->reader_page; > iter->head = cpu_buffer->reader_page->read; > iter->next_event = iter->head; > + iter->missed_events = 0; > > iter->cache_reader_page = iter->head_page; > iter->cache_read = cpu_buffer->read; > @@ -6108,10 +6109,7 @@ ring_buffer_peek(struct trace_buffer *buffer, int cpu, u64 *ts, > */ > bool ring_buffer_iter_dropped(struct ring_buffer_iter *iter) > { > - bool ret = iter->missed_events != 0; > - > - iter->missed_events = 0; > - return ret; > + return iter->missed_events != 0; > } > EXPORT_SYMBOL_GPL(ring_buffer_iter_dropped); > > @@ -6273,7 +6271,7 @@ void ring_buffer_iter_advance(struct ring_buffer_iter *iter) > unsigned long flags; > > raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags); > - > + iter->missed_events = 0; > rb_advance_iter(iter); > > raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags); > -- > 2.53.0 > -- Masami Hiramatsu (Google)