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    SubjectRe: [PATCH 0/2] mm/page_alloc: Remote per-cpu lists drain support
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    Hi Mel,

    On Thu, 2022-03-03 at 11:45 +0000, Mel Gorman wrote:
    > On Tue, Feb 08, 2022 at 11:07:48AM +0100, Nicolas Saenz Julienne wrote:
    > > This series replaces mm/page_alloc's per-cpu page lists drain mechanism with
    > > one that allows accessing the lists remotely. Currently, only a local CPU is
    > > permitted to change its per-cpu lists, and it's expected to do so, on-demand,
    > > whenever a process demands it by means of queueing a drain task on the local
    > > CPU. This causes problems for NOHZ_FULL CPUs and real-time systems that can't
    > > take any sort of interruption and to some lesser extent inconveniences idle and
    > > virtualised systems.
    > >
    >
    > I know this has been sitting here for a long while. Last few weeks have
    > not been fun.
    >
    > > Note that this is not the first attempt at fixing this per-cpu page lists:
    > > - The first attempt[1] tried to conditionally change the pagesets locking
    > > scheme based the NOHZ_FULL config. It was deemed hard to maintain as the
    > > NOHZ_FULL code path would be rarely tested. Also, this only solves the issue
    > > for NOHZ_FULL setups, which isn't ideal.
    > > - The second[2] unanimously switched the local_locks to per-cpu spinlocks. The
    > > performance degradation was too big.
    > >
    >
    > For unrelated reasons I looked at using llist to avoid locks entirely. It
    > turns out it's not possible and needs a lock. We know "local_locks to
    > per-cpu spinlocks" took a large penalty so I considered alternatives on
    > how a lock could be used. I found it's possible to both remote drain
    > the lists and avoid the disable/enable of IRQs entirely as long as a
    > preempting IRQ is willing to take the zone lock instead (should be very
    > rare). The IRQ part is a bit hairy though as softirqs are also a problem
    > and preempt-rt needs different rules and the llist has to sort PCP
    > refills which might be a loss in total. However, the remote draining may
    > still be interesting. The full series is at
    > https://git.kernel.org/pub/scm/linux/kernel/git/mel/linux.git/ mm-pcpllist-v1r2
    >
    > It's still waiting on tests to complete and not all the changelogs are
    > complete which is why it's not posted.
    >
    > This is a comparison of vanilla vs "local_locks to per-cpu spinlocks"
    > versus the git series up to "mm/page_alloc: Remotely drain per-cpu lists"
    > for the page faulting microbench I originally complained about. The test
    > machine is a 2-socket CascadeLake machine.
    >
    > pft timings
    > 5.17.0-rc5 5.17.0-rc5 5.17.0-rc5
    > vanilla mm-remotedrain-v2r1 mm-pcpdrain-v1r1
    > Amean elapsed-1 32.54 ( 0.00%) 33.08 * -1.66%* 32.82 * -0.86%*
    > Amean elapsed-4 8.66 ( 0.00%) 9.24 * -6.72%* 8.69 * -0.38%*
    > Amean elapsed-7 5.02 ( 0.00%) 5.43 * -8.16%* 5.05 * -0.55%*
    > Amean elapsed-12 3.07 ( 0.00%) 3.38 * -10.00%* 3.09 * -0.72%*
    > Amean elapsed-21 2.36 ( 0.00%) 2.38 * -0.89%* 2.19 * 7.39%*
    > Amean elapsed-30 1.75 ( 0.00%) 1.87 * -6.50%* 1.62 * 7.59%*
    > Amean elapsed-48 1.71 ( 0.00%) 2.00 * -17.32%* 1.71 ( -0.08%)
    > Amean elapsed-79 1.56 ( 0.00%) 1.62 * -3.84%* 1.56 ( -0.02%)
    > Amean elapsed-80 1.57 ( 0.00%) 1.65 * -5.31%* 1.57 ( -0.04%)
    >
    > Note the local_lock conversion took 1 1-17% penalty while the git tree
    > takes a negligile penalty while still allowing remote drains. It might
    > have some potential while being less complex than the RCU approach.

    I've been made aware of a problem with the spin_trylock() approach. It doesn't
    work for UP since in that context spin_lock() is a NOOP (well, it only disables
    preemption). So nothing prevents a race with an IRQ.

    It's kinda funny, breakages generally happen the other way around.

    Regards,

    --
    Nicolás Sáenz

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