futex: Misc cleanups

Date	Thu, 2 Jan 2014 07:05:17 -0800

From: Jason Low <jason.low2@hp.com>

- Remove unnecessary head variables.
- Delete unused parameter in queue_unlock().

Cc: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Darren Hart <dvhart@linux.intel.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Jeff Mahoney <jeffm@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Scott Norton <scott.norton@hp.com>
Cc: Tom Vaden <tom.vaden@hp.com>
Cc: Aswin Chandramouleeswaran <aswin@hp.com>
Cc: Waiman Long <Waiman.Long@hp.com>
Signed-off-by: Jason Low <jason.low2@hp.com>
Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>

Signed-off-by: Paul Reioux <reioux@gmail.com>
Signed-off-by: Lens-F <tnoah12@gmail.com>

Conflicts:
	kernel/futex.c
futex: add comments removed by htc

Signed-off-by: Lens-F <tnoah12@gmail.com>
futex: Larger hash table

Date	Thu, 2 Jan 2014 07:05:18 -0800

From: Davidlohr Bueso <davidlohr@hp.com>

Currently, the futex global hash table suffers from it's fixed, smallish
(for today's standards) size of 256 entries, as well as its lack of NUMA
awareness. Large systems, using many futexes, can be prone to high amounts
of collisions; where these futexes hash to the same bucket and lead to
extra contention on the same hb->lock. Furthermore, cacheline bouncing is a
reality when we have multiple hb->locks residing on the same cacheline and
different futexes hash to adjacent buckets.
This patch keeps the current static size of 16 entries for small systems,
or otherwise, 256 * ncpus (or larger as we need to round the number to a
power of 2). Note that this number of CPUs accounts for all CPUs that can
ever be available in the system, taking into consideration things like
hotpluging. While we do impose extra overhead at bootup by making the hash
table larger, this is a one time thing, and does not shadow the benefits
of this patch.

Furthermore, as suggested by tglx, by cache aligning the hash buckets we can
avoid access across cacheline boundaries and also avoid massive cache line
bouncing if multiple cpus are hammering away at different hash buckets which
happen to reside in the same cache line.

Also, similar to other core kernel components (pid, dcache, tcp), by using
alloc_large_system_hash() we benefit from its NUMA awareness and thus the
table is distributed among the nodes instead of in a single one.

For a custom microbenchmark that pounds on the uaddr hashing -- making the wait
path fail at futex_wait_setup() returning -EWOULDBLOCK for large amounts of
futexes, we can see the following benefits on a 80-core, 8-socket 1Tb server:

+---------+--------------------+------------------------+-----------------------+-------------------------------+
| threads | baseline (ops/sec) | aligned-only (ops/sec) | large table (ops/sec) | large table+aligned (ops/sec) |
+---------+--------------------+------------------------+-----------------------+-------------------------------+
|     512 |		 32426 | 50531  (+55.8%)	| 255274  (+687.2%)	| 292553  (+802.2%)		|
|     256 |		 65360 | 99588  (+52.3%)	| 443563  (+578.6%)	| 508088  (+677.3%)		|
|     128 |		125635 | 200075 (+59.2%)	| 742613  (+491.1%)	| 835452  (+564.9%)		|
|      80 |		193559 | 323425 (+67.1%)	| 1028147 (+431.1%)	| 1130304 (+483.9%)		|
|      64 |		247667 | 443740 (+79.1%)	| 997300  (+302.6%)	| 1145494 (+362.5%)		|
|      32 |		628412 | 721401 (+14.7%)	| 965996  (+53.7%)	| 1122115 (+78.5%)		|
+---------+--------------------+------------------------+-----------------------+-------------------------------+
Cc: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Darren Hart <dvhart@linux.intel.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Jeff Mahoney <jeffm@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Scott Norton <scott.norton@hp.com>
Cc: Tom Vaden <tom.vaden@hp.com>
Cc: Aswin Chandramouleeswaran <aswin@hp.com>
Reviewed-by: Waiman Long <Waiman.Long@hp.com>
Reviewed-and-tested-by: Jason Low <jason.low2@hp.com>
Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Signed-off-by: Paul Reioux <reioux@gmail.com>

Signed-off-by: Lens-F <tnoah12@gmail.com>
futex: Document ordering guarantees

Date	Thu, 2 Jan 2014 07:05:19 -0800

From: Thomas Gleixner <tglx@linutronix.de>

That's essential, if you want to hack on futexes.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Darren Hart <dvhart@linux.intel.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Jeff Mahoney <jeffm@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Scott Norton <scott.norton@hp.com>
Cc: Tom Vaden <tom.vaden@hp.com>
Cc: Aswin Chandramouleeswaran <aswin@hp.com>
Cc: Waiman Long <Waiman.Long@hp.com>
Cc: Jason Low <jason.low2@hp.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Signed-off-by: Paul Reioux <reioux@gmail.com>
Signed-off-by: Lens-F <tnoah12@gmail.com>
futex: Avoid taking hb lock if nothing to wakeup

Date	Thu, 2 Jan 2014 07:05:20 -0800

From: Davidlohr Bueso <davidlohr@hp.com>

In futex_wake() there is clearly no point in taking the hb->lock if we know
beforehand that there are no tasks to be woken. While the hash bucket's plist
head is a cheap way of knowing this, we cannot rely 100% on it as there is a
racy window between the futex_wait call and when the task is actually added to
the plist. To this end, we couple it with the spinlock check as tasks trying to
enter the critical region are most likely potential waiters that will be added
to the plist, thus preventing tasks sleeping forever if wakers don't acknowledge
all possible waiters.

Furthermore, the futex ordering guarantees are preserved, ensuring that waiters
either observe the changed user space value before blocking or is woken by a
concurrent waker. For wakers, this is done by relying on the barriers in
get_futex_key_refs() -- for archs that do have implicit mb in atomic_inc() we
explicitly add them through a new futex_get_mm function. For waiters we rely
on the fact that spin_lock calls already update the head counter, so spinners
are visible even if the lock hasn't been acquired yet.

For more details please refer to the updated comments in the code and related
discussion: https://lkml.org/lkml/2013/11/26/556

Special thanks to tglx for careful review and feedback.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Darren Hart <dvhart@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Jeff Mahoney <jeffm@suse.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Scott Norton <scott.norton@hp.com>
Cc: Tom Vaden <tom.vaden@hp.com>
Cc: Aswin Chandramouleeswaran <aswin@hp.com>
Cc: Waiman Long <Waiman.Long@hp.com>
Cc: Jason Low <jason.low2@hp.com>
Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Signed-off-by: Paul Reioux <reioux@gmail.com>
Signed-off-by: Lens-F <tnoah12@gmail.com>
kernel/futex.c: Linux 3.4 compatibility fix up

Signed-off-by: Paul Reioux <reioux@gmail.com>
Signed-off-by: Lens-F <tnoah12@gmail.com>
freezer: shorten freezer sleep time using exponential backoff

Date	Wed, 1 May 2013 18:34:59 -0700

All tasks can easily be frozen in under 10 ms, switch to using
an initial 1 ms sleep followed by exponential backoff until
8 ms.  Also convert the printed time to ms instead of centiseconds.

Signed-off-by: Colin Cross <ccross@android.com>
modified for Mako hybrid from LKML

Signed-off-by: Paul Reioux <reioux@gmail.com>
Signed-off-by: Lens-F <tnoah12@gmail.com>

Conflicts:
	kernel/power/process.c
freezer: add new freezable helpers using freezer_do_not_count()

Date	Wed, 1 May 2013 18:35:01 -0700

Freezing tasks will wake up almost every userspace task from
where it is blocking and force it to run until it hits a
call to try_to_sleep(), generally on the exit path from the syscall
it is blocking in.  On resume each task will run again, usually
restarting the syscall and running until it hits the same
blocking call as it was originally blocked in.

To allow tasks to avoid running on every suspend/resume cycle,
this patch adds additional freezable wrappers around blocking calls
that call freezer_do_not_count().  Combined with the previous patch,
these tasks will not run during suspend or resume unless they wake
up for another reason, in which case they will run until they hit
the try_to_freeze() in freezer_count(), and then continue processing
the wakeup after tasks are thawed.  This patch also converts the
existing wait_event_freezable* wrappers to use freezer_do_not_count().

Additional patches will convert the most common locations that
userspace blocks in to use freezable helpers.

Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: Lens-F <tnoah12@gmail.com>