Implement sleep strategy using the IO subsystem

See merge request !214

Design goals
============

* Wakeup either on external newWork notifications or on local IO completions
  -> Sleep strategy is sound without the IO completer
* Do as less as possible in a system saturated with work
* Pass a hint where to find new work to suspended workers

Algorithm
=========

Data:
	Global:
		hint pipe
		sleepers count
	Per worker:
		dispatch hint buffer
		in flight flag

Sleep:
	if we have no sleep request in flight
		Atomic increment sleep count
		Remember that we are sleeping
		Prepare read cqe from the hint pipe to dispatch hint buffer
	Prevent the completer from reaping completions on this worker's IoContext
	Wait until IO completions occurred

NotifyEmper(n):
	if observed sleepers <= 0
		return

	// Determine how many we are responsible to wake
	do
		toWakeup = min(observed sleepers, n)
	while (!CAS(sleepers, toWakeup))

	write toWakeup hints to the hint pipe

NotifyAnywhere(n):
	// Ensure all n notifications take effect
	while (!CAS(sleepers, observed sleepers - n))
		if observed sleeping <= -n
			return

	toWakeup = min(observed sleeping, n)
	write toWakeup hints to the hint pipe

onNewWorkCompletion:
	reset in flight flag
	allow completer to reap completions on this IoContext

Notes
=====

* We must decrement the sleepers count on the notifier side to
  prevent multiple notifiers to observe all the same amount of sleepers,
  trying to wake up the same sleepers by writing to the pipe and jamming it up
  with unconsumed hints and thus blocking in the notify write resulting
  in a deadlock.
* The CAS loops on the notifier side are needed because decrementing
  and incrementing the excess is racy: Two notifier can observe the
  sum of both their excess decrement and increment to much resulting in a
  broken counter.
* Add the dispatch hint code in AbstractWorkStealingScheduler::nextFiber.
  This allows workers to check the dispatch hint after there
  where no local work to execute.
  This is a trade-off where we trade slower wakeup - a just awoken worker
  will check for local work - against a faster dispatch hot path when
  we have work to do in our local WSQ.
* The completer tread must not reap completions on the IoContexts of
  sleeping workers because this introduces a race for cqes and a possible
  lost wakeup if the completer consumes the completions before the worker
  is actually waiting for them.
* When notifying sleeping workers from anywhere we must ensure that all
  notifications take effect. This is needed for example when terminating
  the runtime to prevent sleep attempt from worker thread which are
  about to sleep but have not incremented the sleeper count yet.
  We achieve this by always decrementing the sleeper count by the notification
  count.

Thanks to Florian Schmaus <flow@cs.fau.de> for spotting bugs and suggesting
improvements.

[IO] document and adjust tests to changes in io_uring's broken link handling

Closes #20

See merge request !212

[io/tests] gracefully terminate the runtime on success

See merge request !213

This allows the examination of stats which get printed in the Runtime
destructor.
Also remove useless stdlib includes (Why does iwyu not catch those?).

Until kernel commit cf10960426515 io_uring submitted sqe until it encountered
an invalid one. Now io_uring will catch the error on subimssion and
will cancel all sqe's contained in the link prior to the invalid one.

[Runtime] use pthread_getaffinity_np to determine default worker count

Closes #19

See merge request !211

The value reported by std::thread::hardware_concurrency() can be more
than the available CPUs to the current process for example if EMPER runs
in a container or qemu.
GNU nproc(1) does the right thing and we can take inspiration from them.
The solution is to use pthread_getaffinity_np which fills a CPU set with
all CPU a pthread can be scheduled on.
We use this more precise CPU count if possible.

[gitlab-ci] Bump flowdalic/debian-testing-dev to 1.12

See merge request !209

[Future] Fix clang-tidy issue in MadviseFuture

See merge request !208

[gitlab-ci] Bump debian-testing-dev image to 1.10

See merge request !199

[IO] add a madvise Future

See merge request !206

Fix most llvm-12 clang-tidy warnings in IO code

See merge request !205

Batch dequeue from AnywhereQueue

See merge request !196

Thanks llvm 12 :)

errno was never set on error and thus the DIE_MSG_ERRNO did not properly
report what happened.

Found during changing Future member types.

The posix versions (close/openat) return int as well.
And this fixes a narrowing conversion in c_emper.cpp: emper_openat and emper_close

Use int32_t as Future returnValue instead of ssize_t since
io_uring uses int32_t not ssize_t as results value therefore it is
pointless to use a bigger type.

This shrinks the future size and prevents useless and implementation defined
narrowing conversions from ssize_t to int32_t.

Use union for flags and offset.

Ignore int to pointer cast clang-tidy warning.

[Debug] maybe-uninitialized should be ignored for gcc >= 11.1.0

See merge request !204

[check-license] Do not pass --max-args to xargs

See merge request !203

GCC 11.1 fixes

See merge request !202

[CountingPrivateSemaphore] Fix assert

See merge request !200

xargs: warning: options --max-args and --replace/-I/-i are mutually exclusive, ignoring previous --max-args value

gcc 11.1 on my arch system complains that the calls to free
does not match the allocator function new[].

As the code comment above the assert indicates, the atomic exchange
operation may either return nullptr or a valid Context pointer. And
this is what the assert() should assert. :)

Reported-by: Florian Fischer <florian.fl.fischer@fau.de>

[Common] Call abort() instead of exit() in die()

See merge request !198

Calling abort() has multiple advantages. First, it better matches the
semantic of DIE(_MSG) as it signals an abnormal process termination,
whereas exit() is a normal process termination (even though
potentially with an error exit code). Secondly, abort() throws
SIGABRT, which in turn triggers a coredump, hence the program's state
can be inspected afterwards.

[LAWS/Fiber] Introduce the concept of Multi-Fiber

See merge request !195