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
