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    SubjectRe: [PATCH] sched: Resolve sd_idle and first_idle_cpu Catch-22 - v1
    On Fri, 2011-02-04 at 13:25 -0800, Venkatesh Pallipadi wrote:
    > Consider a system with { [ (A B) (C D) ] [ (E F) (G H) ] },
    > () denoting SMT siblings, [] cores on same socket and {} system wide
    > Further, A, C and D are idle, B is busy and one of EFGH has excess load.
    > With sd_idle logic, a check in rebalance_domains() converts tick
    > based load balance requests from CPU A to busy load balance for core
    > and above domains (lower rate of balance and higher load_idx).

    the if (load_balance())
    idle = CPU_NOT_IDLE;
    bit, right?

    > With first_idle_cpu logic, when CPU C or D tries to balance across domains
    > the logic finds CPU A as first idle CPU in the group and nominates CPU A to
    > idle balance across sockets.


    > But, sd_idle above would not allow CPU A to do cross socket idle balance
    > as CPU A switches its higher level balancing to busy balance.

    Because it fails the sd->flags & SD_SHARE_CPUPOWER test at the beginning
    of load_balance() and hence sd_idle will remain 0, right?

    I'm just not quite sure how we then end up returning !0 for
    load_balance(), both branches returning -1 seem conditional on
    SD_SHARE_CPUPOWER but the [ (A B) (C D) ], domain doesn't have that set.

    > So, this can result is no cross socket balancing for extended periods.

    Which is bad

    > The fix here adds additional check to detect sd_idle logic in
    > first_idle_cpu code path. We will now nominate (in order or preference):
    > * First fully idle CPU
    > * First semi-idle CPU
    > * First CPU
    > Note that this solution works fine for 2 SMT siblings case and won't be
    > perfect in picking proper semi-idle in case of more than 2 SMT threads.

    All these SMT exceptions make my head hurt, can't we clean that up
    instead of making them worse?

    Why is SMT treaded differently from say a shared cache? In both cases we
    want to spread the load as wide as possible to provide as much of the
    resources to the few runnable tasks.

     \ /
      Last update: 2011-02-07 14:51    [W:0.021 / U:0.696 seconds]
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