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

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Message-ID: <20160923124624.GL5008@twins.programming.kicks-ass.net>
Date:   Fri, 23 Sep 2016 14:46:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mike Snitzer <snitzer@...hat.com>
Cc:     Mike Galbraith <umgwanakikbuti@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mikulas Patocka <mpatocka@...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, Sep 23, 2016 at 08:42:51AM -0400, Mike Snitzer wrote:
> On Fri, Sep 23 2016 at  8:26am -0400,
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Fri, Sep 23, 2016 at 02:17:10PM +0200, Mike Galbraith wrote:
> > > On Fri, 2016-09-23 at 10:00 +0200, Thomas Gleixner wrote:
> > > > On Fri, 23 Sep 2016, Peter Zijlstra wrote:
> > > 
> > > > > 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.
> > > 
> > > SUSE definitely uses it.  I had presumed that was enterprise standard.
> > 
> > Hmm, I thought most distros defaulted to PREEMPT_VOLUNTARY.
> 
> So what is the concensus on this?  Switch dm-bufio's cond_resched calls
> (in peter's patch) to might_sleep()?  Or continue using cond_resched but
> fix cond_resched to do the might_sleep() equivalent if PREEMPT_NONE?

I'd go with the one I posted and look again if ever a performance issue
shows up.