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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Fri, 23 Sep 2016 09:34:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
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 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.