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

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