Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.

[Mel Gorman]

On Fri, Oct 02, 2020 at 07:41:20AM -0700, Paul E. McKenney wrote:
> On Fri, Oct 02, 2020 at 11:19:52AM +0100, Mel Gorman wrote:
> > On Fri, Oct 02, 2020 at 11:58:58AM +0200, Peter Zijlstra wrote:
> > > > It's enabled by default by enough distros that adding too many checks
> > > > is potentially painful. Granted it would be missed by most benchmarking
> > > > which tend to control allocations from userspace but a lot of performance
> > > > problems I see are the "death by a thousand cuts" variety.
> > > 
> > > Oh quite agreed, aka death by accounting. But if people are enabling
> > > DEBUG options in production kernels, there's something wrong, no?
> > > 
> > 
> > You'd think but historically I believe DEBUG_VM was enabled for some
> > distributions because it made certain classes of problems easier to debug
> > early. There is also a recent trend for enabling various DEBUG options for
> > "hardening" even when they protect very specific corner cases or are for
> > intended for kernel development. I've pushed back where I have an opinion
> > that matters but it's generally corrosive.
> > 
> > > Should we now go add CONFIG_REALLY_DEBUG_STAY_AWAY_ALREADY options?
> > 
> > It's heading in that direction :(
> 
> Given that you guys have just reiterated yet again that you are very
> unhappy with either a GFP_ flag or a special function like the one that
> Peter Zijlstra put together, it would be very helpful if you were to at
> least voice some level of support for Thomas Gleixner's patchset, which,
> if accepted, will allow me to solve at least 50% of the problem.
> 

I read through the series and didn't find anything problematic that
had not been covered already. Minimally, avoiding surprises about what
preemptible() means in different contexts is nice. While I have not
run it through a test grid to check, I'd be very surprised if this was
problematic from a performance perspective on a preempt-disabled kernels.
Last I checked, the difference between PREEMPT_NONE and PREEMPT_VOLUNTARY
was less than 2% *at worst* and I don't think that was due to the preempt
accounting.

-- 
Mel Gorman
SUSE Labs