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 S965401AbcIWHeo (ORCPT <rfc822;w@1wt.eu>);
        Fri, 23 Sep 2016 03:34:44 -0400
Received: from merlin.infradead.org ([205.233.59.134]:47898 "EHLO
        merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S965273AbcIWHem (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 23 Sep 2016 03:34:42 -0400
Date: Fri, 23 Sep 2016 09:34:35 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Mikulas Patocka <mpatocka@redhat.com>,
        Mike Snitzer <snitzer@redhat.com>, Alasdair Kergon <agk@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org,
        dm-devel@redhat.com, Joe Thornber <ejt@redhat.com>
Subject: Re: [RFC][PATCH] dm: Remove dm_bufio_cond_resched()
Message-ID: <20160923073435.GL2794@worktop>
References: <20160913084520.GA5012@twins.programming.kicks-ass.net>
 <20160913133959.GA22833@redhat.com>
 <20160919105325.GW5016@twins.programming.kicks-ass.net>
 <alpine.LRH.2.02.1609221645510.4447@file01.intranet.prod.int.rdu2.redhat.com>
 <alpine.DEB.2.20.1609222257050.5640@nanos>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <alpine.DEB.2.20.1609222257050.5640@nanos>
User-Agent: Mutt/1.5.22.1 (2013-10-16)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Thu, Sep 22, 2016 at 10:59:30PM +0200, Thomas Gleixner wrote:
> On Thu, 22 Sep 2016, Mikulas Patocka wrote:
> > On Mon, 19 Sep 2016, Peter Zijlstra wrote:
> > 
> > > On Tue, Sep 13, 2016 at 09:39:59AM -0400, Mike Snitzer wrote:
> > > > So I'm not sure how this dm-bufio local cond_resched() wrapper still got
> > > > in... happy to take your patch.
> > > > 
> > > > Please respond with whatever SOB you'd like applied to the patch header.
> > > 
> > > Sorry, for the delay, here goes.
> > 
> > Why not change it to might_sleep()? - that would be almost equivalent to 
> 
> You mean might_resched(). might_sleep() is not even remotely equivalent.

It is, might_sleep() implies might_resched(). In fact, that's all what
PREEMPT_VOLUNTARY is, make the might_sleep() debug test imply a resched
point.

> > If we call the cond_resched() function in tight loops such as walking all 
> > buffers in a list, there may be performance penalty due to the call, so 
> > the call should be done only if it is really needed (i.e. in 
> > CONFIG_PREEMPT_VOLUNTARY case).
> 
> Makes sense.

Is anybody still using PREEMPT_NONE? Most workloads also care about
latency to some extend. Lots of code has explicit cond_resched() and
doesn't worry.