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

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Message-ID: <alpine.DEB.2.20.1609230956540.5640@nanos>
Date:   Fri, 23 Sep 2016 10:00:37 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Mikulas Patocka <mpatocka@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Alasdair Kergon <agk@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        dm-devel@...hat.com, Joe Thornber <ejt@...hat.com>
Subject: Re: [RFC][PATCH] dm: Remove dm_bufio_cond_resched()

On Fri, 23 Sep 2016, Peter Zijlstra wrote:
> 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.

Grr, how intuitive - NOT!
 
> > > 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.

Dunno. But I bet there are workloads which love it.

Thanks,

	tglx