Re: [RFC][PATCH] dm: Remove dm_bufio_cond_resched()

[Peter Zijlstra <peterz@infradead.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.