Issue
=====

On asymmetric CPU capacity topologies, we currently rely on wake_cap() to
drive select_task_rq_fair() towards either:

- its slow-path (find_idlest_cpu()) if either the previous or
  current (waking) CPU has too little capacity for the waking task
- its fast-path (select_idle_sibling()) otherwise

Commit:

  3273163c ("sched/fair: Let asymmetric CPU configurations balance at wake-up")

points out that this relies on the assumption that "[...]the CPU capacities
within an SD_SHARE_PKG_RESOURCES domain (sd_llc) are homogeneous".

This assumption no longer holds on newer generations of big.LITTLE
systems (DynamIQ), which can accommodate CPUs of different compute capacity
within a single LLC domain. To hopefully paint a better picture, a regular
big.LITTLE topology would look like this:

  +---------+ +---------+
  |   L2    | |   L2    |
  +----+----+ +----+----+
  |CPU0|CPU1| |CPU2|CPU3|
  +----+----+ +----+----+
      ^^^         ^^^
    LITTLEs      bigs

which would result in the following scheduler topology:

  DIE [         ] <- sd_asym_cpucapacity
  MC  [   ] [   ] <- sd_llc
       0 1   2 3

Conversely, a DynamIQ topology could look like:

  +-------------------+
  |        L3         |
  +----+----+----+----+
  | L2 | L2 | L2 | L2 |
  +----+----+----+----+
  |CPU0|CPU1|CPU2|CPU3|
  +----+----+----+----+
     ^^^^^     ^^^^^
    LITTLEs    bigs

which would result in the following scheduler topology:

  MC [       ] <- sd_llc, sd_asym_cpucapacity
      0 1 2 3

What this means is that, on DynamIQ systems, we could pass the wake_cap()
test (IOW presume the waking task fits on the CPU capacities of some LLC
domain), thus go through select_idle_sibling().
This function operates on an LLC domain, which here spans both bigs and
LITTLEs, so it could very well pick a CPU of too small capacity for the
task, despite there being fitting idle CPUs - it very much depends on the
CPU iteration order, on which we have absolutely no guarantees
capacity-wise.

Implementation
==============

Introduce yet another select_idle_sibling() helper function that takes CPU
capacity into account. The policy is to pick the first idle CPU which is
big enough for the task (task_util * margin < cpu_capacity). If no
idle CPU is big enough, we pick the idle one with the highest capacity.

Unlike other select_idle_sibling() helpers, this one operates on the
sd_asym_cpucapacity sched_domain pointer, which is guaranteed to span all
known CPU capacities in the system. As such, this will work for both
"legacy" big.LITTLE (LITTLEs & bigs split at MC, joined at DIE) and for
newer DynamIQ systems (e.g. LITTLEs and bigs in the same MC domain).

Note that this limits the scope of select_idle_sibling() to
select_idle_capacity() for asymmetric CPU capacity systems - the LLC domain
will not be scanned, and no further heuristic will be applied.
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Quentin Perret <qperret@google.com>
Link: https://lkml.kernel.org/r/20200206191957.12325-2-valentin.schneider@arm.com