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    SubjectRe: [PATCH v2 00/11] fold per-CPU vmstats remotely
    On Fri, Feb 24, 2023 at 10:34:10AM +0800, Hillf Danton wrote:
    > On Thu, Feb 09, 2023 at 12:01:50PM -0300, Marcelo Tosatti wrote:
    > > This patch series addresses the following two problems:
    > >
    > > 1. A customer provided some evidence which indicates that
    > > the idle tick was stopped; albeit, CPU-specific vmstat
    > > counters still remained populated.
    > >
    > > Thus one can only assume quiet_vmstat() was not
    > > invoked on return to the idle loop. If I understand
    > > correctly, I suspect this divergence might erroneously
    > > prevent a reclaim attempt by kswapd. If the number of
    > > zone specific free pages are below their per-cpu drift
    > > value then zone_page_state_snapshot() is used to
    > > compute a more accurate view of the aforementioned
    > > statistic. Thus any task blocked on the NUMA node
    > > specific pfmemalloc_wait queue will be unable to make
    > > significant progress via direct reclaim unless it is
    > > killed after being woken up by kswapd
    > > (see throttle_direct_reclaim())
    > >
    > > 2. With a SCHED_FIFO task that busy loops on a given CPU,
    > > and kworker for that CPU at SCHED_OTHER priority,
    > > queuing work to sync per-vmstats will either cause that
    > > work to never execute, or stalld (i.e. stall daemon)
    > > boosts kworker priority which causes a latency
    > > violation
    > >
    > > By having vmstat_shepherd flush the per-CPU counters to the
    > > global counters from remote CPUs.
    > >
    > > This is done using cmpxchg to manipulate the counters,
    > > both CPU locally (via the account functions),
    > > and remotely (via cpu_vm_stats_fold).
    >
    > Frankly another case of bandaid[1] ?
    >
    > [1] https://lore.kernel.org/lkml/20230223150624.GA29739@lst.de/

    Only if you disable per-CPU vmstat counters for isolated CPUs
    (then maintenance of the data structures in isolated CPUs is
    not necessary).

    Which would be terrible for performance, however.


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