Re: [PATCH] timekeeping: move multigrain ctime floor handling into timekeeper

[Christian Brauner <brauner@kernel.org>]

On Thu, Sep 12, 2024 at 08:39:32AM GMT, Jeff Layton wrote:
> On Thu, 2024-09-12 at 14:31 +0200, Christian Brauner wrote:
> > On Wed, Sep 11, 2024 at 08:56:56AM GMT, Jeff Layton wrote:
> > > The kernel test robot reported a performance regression in some
> > > will-it-scale tests due to the multigrain timestamp patches. The data
> > > showed that coarse_ctime() was slowing down current_time(), which is
> > > called frequently in the I/O path.
> > > 
> > > Add ktime_get_coarse_real_ts64_with_floor(), which returns either the
> > > coarse time or the floor as a realtime value. This avoids some of the
> > > conversion overhead of coarse_ctime(), and recovers some of the
> > > performance in these tests.
> > > 
> > > The will-it-scale pipe1_threads microbenchmark shows these averages on
> > > my test rig:
> > > 
> > > 	v6.11-rc7:			83830660 (baseline)
> > > 	v6.11-rc7 + mgtime series:	77631748 (93% of baseline)
> > > 	v6.11-rc7 + mgtime + this:	81620228 (97% of baseline)
> > > 
> > > Reported-by: kernel test robot <oliver.sang@intel.com>
> > > Closes: https://lore.kernel.org/oe-lkp/202409091303.31b2b713-oliver.sang@intel.com
> > > Suggested-by: Arnd Bergmann <arnd@kernel.org>
> > > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > > ---
> > > Arnd suggested moving this into the timekeeper when reviewing an earlier
> > > version of this series, and that turns out to be better for performance.
> > > 
> > > I'm not sure how this should go in (if acceptable). The multigrain
> > > timestamp patches that this would affect are in Christian's tree, so
> > > that may be best if the timekeeper maintainers are OK with this
> > > approach.
> > 
> > We will need this as otherwise we can't really merge the multigrain
> > timestamp work with known performance regressions?
> 
> Yes, I think we'll need something here. Arnd suggested an alternative
> way to do this that might be even better. I'm not 100% sure that it'll
> work though since the approach is a bit different.
> 
> I'd still like to see this go in for v6.12, so what I'd probably prefer
> is to take this patch initially (with the variable name change that
> John suggested), and then we can work on the alternative approach in
> the meantime
> 
> Would that be acceptable?

It would be ok with me but we should get a nodd from the time keeper folks.