BACKPORT: fs: kernfs: add poll file operation

BACKPORT: fs: kernfs: add poll file operation

Patch series "psi: pressure stall monitors", v3.

Android is adopting psi to detect and remedy memory pressure that results
in stuttering and decreased responsiveness on mobile devices.

Psi gives us the stall information, but because we're dealing with
latencies in the millisecond range, periodically reading the pressure
files to detect stalls in a timely fashion is not feasible.  Psi also
doesn't aggregate its averages at a high enough frequency right now.

This patch series extends the psi interface such that users can configure
sensitive latency thresholds and use poll() and friends to be notified
when these are breached.

As high-frequency aggregation is costly, it implements an aggregation
method that is optimized for fast, short-interval averaGing, and makes the
aggregation frequency adaptive, such that high-frequency updates only
happen while monitored stall events are actively occurring.

With these patches applied, Android can monitor for, and ward off,
mounting memory shortages before they cause problems for the user.  For
example, using memory stall monitors in userspace low memory killer daemon
(lmkd) we can detect mounting pressure and kill less important processes
before device becomes visibly sluggish.  In our memory stress testing psi
memory monitors produce roughly 10x less false positives compared to
vmpressure signals.  Having ability to specify multiple triggers for the
same psi metric allows other parts of Android framework to monitor memory
state of the device and act accordingly.

The new interface is straightforward.  The user opens one of the pressure
files for writing and writes a trigger description into the file
descriptor that defines the stall state - some or full, and the maximum
stall time over a given window of time.  E.g.:

        /* Signal when stall time exceeds 100ms of a 1s window */
        char trigger[] = "full 100000 1000000";
        fd = open("/proc/pressure/memory");
        write(fd, trigger, sizeof(trigger));
        while (poll() >= 0) {
                ...
        }
        close(fd);

When the monitored stall state is entered, psi adapts its aggregation
frequency according to what the configured time window requires in order
to emit event signals in a timely fashion.  Once the stalling subsides,
aggregation reverts back to normal.

The trigger is associated with the open file descriptor.  To stop
monitoring, the user only needs to close the file descriptor and the
trigger is discarded.

Patches 1-4 prepare the psi code for polling support.  Patch 5 implements
the adaptive polling logic, the pressure growth detection optimized for
short intervals, and hooks up write() and poll() on the pressure files.

The patches were developed in collaboration with Johannes Weiner.

This patch (of 5):

Kernfs has a standardized poll/notification mechanism for waking all
pollers on all fds when a filesystem node changes.  To allow polling for
custom events, add a .poll callback that can override the default.

This is in preparation for pollable cgroup pressure files which have
per-fd trigger configurations.

Link: http://lkml.kernel.org/r/20190124211518.244221-2-surenb@google.com
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
(cherry picked from commit: 147e1a97c4a0bdd43f55a582a9416bb9092563a9)

Conflicts:
        fs/kernfs/file.c
        include/linux/kernfs.h

1. replaced __poll_t with unsigned int.
2. replaced EPOLLERR/EPOLLPRI with POLLERR/POLLPRI (values are the same)

Bug: 127712811
Test: lmkd in PSI mode
Change-Id: Ic2bed334d05aec62f4e695f263893c3057921c55
Signed-off-by: Suren Baghdasaryan <surenb@google.com>