r333831:

rwlock: diff-reduction of runlock compared to sx sunlock

Undo LOCK_PROFILING pessimisation after r313454 and r313455

With the option used to compile the kernel both sx and rw shared ops would
always go to the slow path which added avoidable overhead even when the
facility is disabled.

Furthermore the increased time spent doing uncontested shared lock acquire
would be bogusly added to total wait time, somewhat skewing the results.

Restore old behaviour of going there only when profiling is enabled.

This change is a no-op for kernels without LOCK_PROFILING (which is the
default).

sx: fix adaptive spinning broken in r327397

The condition was flipped.

In particular heavy multithreaded kernel builds on zfs started suffering
due to nested sx locks.

For instance make -s -j 128 buildkernel:

before: 3326.67s user 1269.62s system 6981% cpu 1:05.84 total
after: 3365.55s user 911.27s system 6871% cpu 1:02.24 total

locks: fix a corner case in r327399

If there were exactly rowner_retries/asx_retries (by default: 10) transitions
between read and write state and the waiters still did not get the lock, the
next owner -> reader transition would result in the code correctly falling
back to turnstile/sleepq where it would incorrectly think it was waiting
for a writer and decide to leave turnstile/sleepq to loop back. From this
point it would take ts/sq trips until the lock gets released.

The bug sometimes manifested itself in stalls during -j 128 package builds.

Refactor the code to fix the bug, while here remove some of the gratituous
differences between rw and sx locks.

sx: don't do an atomic op in upgrade if it cananot succeed

The code already pays the cost of reading the lock to obtain the waiters
flag. Checking whether there is more than one reader is not a problem and
avoids dirtying the line.

This also fixes a small corner case: if waiters were to show up between
reading the flag and upgrading the lock, the operation would fail even
though it should not. No correctness change here though.

mtx: tidy up recursion handling in thread lock

Normally after grabbing the lock it has to be verified we got the right one
to begin with. However, if we are recursing, it must not change thus the
check can be avoided. In particular this avoids a lock read for non-recursing
case which found out the lock was changed.

While here avoid an irq trip of this happens.

locks: slightly depessimize lockstat

The slow path is always taken when lockstat is enabled. This induces
rdtsc (or other) calls to get the cycle count even when there was no
contention.

Still go to the slow path to not mess with the fast path, but avoid
the heavy lifting unless necessary.

This reduces sys and real time during -j 80 buildkernel:
before: 3651.84s user 1105.59s system 5394% cpu 1:28.18 total
after: 3685.99s user 975.74s system 5450% cpu 1:25.53 total
disabled: 3697.96s user 411.13s system 5261% cpu 1:18.10 total

So note this is still a significant hit.

LOCK_PROFILING results are not affected.

rw: whack avoidable re-reads in try_upgrade

locks: extend speculative spin waiting for readers to drain

Now that 10 years have passed since the original limit of 10000 was
committed, bump it a little bit.

Spinning waiting for writers is semi-informed in the sense that we always
know if the owner is running and base the decision to spin on that.
However, no such information is provided for read-locking. In particular
this means that it is possible for a write-spinner to completely waste cpu
time waiting for the lock to be released, while the reader holding it was
preempted and is now waiting for the spinner to go off cpu.

Nonetheless, in majority of cases it is an improvement to spin instead of
instantly giving up and going to sleep.

The current approach is pretty simple: snatch the number of current readers
and performs that many pauses before checking again. The total number of
pauses to execute is limited to 10k. If the lock is still not free by
that time, go to sleep.

Given the previously noted problem of not knowing whether spinning makes
any sense to begin with the new limit has to remain rather conservative.
But at the very least it should also be related to the machine. Waiting
for writers uses parameters selected based on the number of activated
hardware threads. The upper limit of pause instructions to be executed
in-between re-reads of the lock is typically 16384 or 32678. It was
selected as the limit of total spins. The lower bound is set to
already present 10000 as to not change it for smaller machines.

Bumping the limit reduces system time by few % during benchmarks like
buildworld, buildkernel and others. Tested on 2 and 4 socket machines
(Broadwell, Skylake).

Figuring out how to make a more informed decision while not pessimizing
the fast path is left as an exercise for the reader.

fix uninitialized variable warning in reader locks