[PATCH v2 0/3] kfence: optimize timer scheduling

[Marco Elver]

We have observed that mostly-idle systems with KFENCE enabled wake up
otherwise idle CPUs, preventing such to enter a lower power state.
Debugging revealed that KFENCE spends too much active time in
toggle_allocation_gate().

While the first version of KFENCE was using all the right bits to be
scheduling optimal, and thus power efficient, by simply using
wait_event() + wake_up(), that code was unfortunately removed.

As KFENCE was exposed to various different configs and tests, the
scheduling optimal code slowly disappeared. First because of hung task
warnings, and finally because of deadlocks when an allocation is made by
timer code with debug objects enabled. Clearly, the "fixes" were not too
friendly for devices that want to be power efficient.

Therefore, let's try a little harder to fix the hung task and deadlock
problems that we have with wait_event() + wake_up(), while remaining as
scheduling friendly and power efficient as possible.

Crucially, we need to defer the wake_up() to an irq_work, avoiding any
potential for deadlock.

The result with this series is that on the devices where we observed a
power regression, power usage returns back to baseline levels.

Changelog
---------

v2:
* Replace kfence_timer_waiting with simpler waitqueue_active() check.

v1: https://lkml.kernel.org/r/20210419085027.761150-1-elver@xxxxxxxxxx

Marco Elver (3):
  kfence: await for allocation using wait_event
  kfence: maximize allocation wait timeout duration
  kfence: use power-efficient work queue to run delayed work

 lib/Kconfig.kfence |  1 +
 mm/kfence/core.c   | 58 ++++++++++++++++++++++++++++++++--------------
 2 files changed, 42 insertions(+), 17 deletions(-)

-- 
2.31.1.368.gbe11c130af-goog