From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1752579AbcF2Mp0 (ORCPT <rfc822;w@1wt.eu>);
	Wed, 29 Jun 2016 08:45:26 -0400
Received: from LGEAMRELO12.lge.com ([156.147.23.52]:45504 "EHLO
	lgeamrelo12.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751767AbcF2MpY (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 29 Jun 2016 08:45:24 -0400
X-Original-SENDERIP: 156.147.1.127
X-Original-MAILFROM: byungchul.park@lge.com
X-Original-SENDERIP: 10.177.222.33
X-Original-MAILFROM: byungchul.park@lge.com
Date: Wed, 29 Jun 2016 21:43:43 +0900
From: Byungchul Park <byungchul.park@lge.com>
To: peterz@infradead.org, mingo@kernel.org
Cc: linux-kernel@vger.kernel.org, npiggin@suse.de, walken@google.com,
        ak@suse.de, xinhui.pan@linux.vnet.ibm.com
Subject: Re: [RFC 12/12] x86/dumpstack: Optimize save_stack_trace
Message-ID: <20160629124343.GQ2279@X58A-UD3R>
References: <1466398527-1122-1-git-send-email-byungchul.park@lge.com>
 <1466398527-1122-13-git-send-email-byungchul.park@lge.com>
 <57679B57.40905@linux.vnet.ibm.com>
 <000101d1cac8$6f321c40$4d9654c0$@lge.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <000101d1cac8$6f321c40$4d9654c0$@lge.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, Jun 20, 2016 at 04:50:37PM +0900, byungchul.park wrote:
> > -----Original Message-----
> > From: xinhui [mailto:xinhui.pan@linux.vnet.ibm.com]
> > Sent: Monday, June 20, 2016 4:29 PM
> > To: Byungchul Park; peterz@infradead.org; mingo@kernel.org
> > Cc: linux-kernel@vger.kernel.org; npiggin@suse.de; walken@google.com;
> > ak@suse.de; tglx@inhelltoy.tec.linutronix.de
> > Subject: Re: [RFC 12/12] x86/dumpstack: Optimize save_stack_trace
> > 
> > 
> > On 2016年06月20日 12:55, Byungchul Park wrote:
> > > Currently, x86 implementation of save_stack_trace() is walking all stack
> > > region word by word regardless of what the trace->max_entries is.
> > > However, it's unnecessary to walk after already fulfilling caller's
> > > requirement, say, if trace->nr_entries >= trace->max_entries is true.
> > >
> > > For example, CONFIG_LOCKDEP_CROSSRELEASE implementation calls
> > > save_stack_trace() with max_entries = 5 frequently. I measured its
> > > overhead and printed its difference of sched_clock() with my QEMU x86
> > > machine.
> > >
> > > The latency was improved over 70% when trace->max_entries = 5.
> > >
> > [snip]
> > 
> > > +static int save_stack_end(void *data)
> > > +{
> > > +	struct stack_trace *trace = data;
> > > +	return trace->nr_entries >= trace->max_entries;
> > > +}
> > > +
> > >   static const struct stacktrace_ops save_stack_ops = {
> > >   	.stack		= save_stack_stack,
> > >   	.address	= save_stack_address,
> > then why not check the return value of ->address(), -1 indicate there is
> > no room to store any pointer.
> 
> Hello,
> 
> Indeed. It also looks good to me even though it has to propagate the condition
> between callback functions. I will modify it if it's better.

Do you also think it would be better to make it propagate the result of
->address() rather than add a new callback, say, end_walk?

> 
> Thank you.
> Byungchul
> 
> > 
> > >   	.walk_stack	= print_context_stack,
> > > +	.end_walk	= save_stack_end,
> > >   };
> > >
> > >   static const struct stacktrace_ops save_stack_ops_nosched = {
> > >