linux-kernel - Re: [RFC][PATCH] dm: Remove dm_bufio_cond

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Message-ID: <20160919104752.GV5016@twins.programming.kicks-ass.net>
Date:   Mon, 19 Sep 2016 12:47:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        dm-devel@...hat.com
Subject: Re: [RFC][PATCH] dm: Remove dm_bufio_cond_resched()

On Mon, Sep 19, 2016 at 05:49:07AM -0400, Mikulas Patocka wrote:
> 
> 
> On Tue, 13 Sep 2016, Peter Zijlstra wrote:
> 
> > Hi all,
> > 
> > While grepping for PREEMPT_VOLUNTARY I ran into dm_bufio_cond_resched()
> > and wondered WTH it was about.
> 
> cond_resched() calls _cond_resched() even if when we have a preemptive 
> kernel - with preemptive kernel, calling cond_resched is pointless because 
> rescheduling is done peemtively.
> 
> So, I added that dm_bufio_cond_resched(), that does nothing on peemptive 
> kernels (and also on PREEMPT_NONE kernels where the user doesn't care 
> about latency).
> 
> What is the reason why cond_resched() tests for rescheduling with 
> preemptive kernel? Why should I use cond_resched() in that case?

Because every body else does too. 'Fixing' something like that in the dm
code is entirely the wrong place. Also, you loose out on the
might_sleep() warning implied in it.

As it happens, I have a patch fixing that somewhere, let me try and get
it merged.

But thanks for the reminder, I'll go write a Changelog for this so that
people can commit.