FROMLIST: psi: introduce psi monitor

FROMLIST: psi: introduce psi monitor

Psi monitor aims to provide a low-latency short-term pressure
detection mechanism configurable by users. It allows users to
monitor psi metrics growth and trigger events whenever a metric
raises above user-defined threshold within user-defined time window.

Time window and threshold are both expressed in usecs. Multiple psi
resources with different thresholds and window sizes can be monitored
concurrently.

Psi monitors activate when system enters stall state for the monitored
psi metric and deactivate upon exit from the stall state. While system
is in the stall state psi signal growth is monitored at a rate of 10 times
per tracking window. Min window size is 500ms, therefore the min monitoring
interval is 50ms. Max window size is 10s with monitoring interval of 1s.

When activated psi monitor stays active for at least the duration of one
tracking window to avoid repeated activations/deactivations when psi
signal is bouncing.

Notifications to the users are rate-limited to one per tracking window.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
(not upstream yet, latest version published at: https://lore.kernel.org/patchwork/patch/1052418/)

Conflicts:
        include/linux/psi.h
        kernel/cgroup/cgroup.c
        kernel/sched/psi.c

(1. replaced __poll_t with unsigned int
2. replaced EPOLLERR/EPOLLPRI with POLLERR/POLLPRI (values are the same)
3. include <linux/cgroup-defs.h> in include/linux/psi.h
4. include <uapi/linux/sched/types.h> in kernel/sched/psi.c)

Bug: 127712811
Bug: 129157727
Test: lmkd in PSI mode
Change-Id: I1688f047e98e1f109627dad72a33d2f70e575268
Signed-off-by: Suren Baghdasaryan <surenb@google.com>