rwsem-spinlock: let rwsem write lock stealable

We(Linux Kernel Performance project) found a regression introduced by
commit 5a50508, which just convert all mutex lock to rwsem write lock.
The semantics is same, but the results is quite huge in some cases.
After investigation, we found the root cause: mutex support lock
stealing. Here is the link for the detailed regression report:
    https://lkml.org/lkml/2013/1/29/84

Ingo suggests to add write lock stealing to rwsem as well:
    "I think we should allow lock-steal between rwsem writers - that
     will not hurt fairness as most rwsem fairness concerns relate to
     reader vs. writer fairness"

And here is the rwsem-spinlock version.

With this patch, we got a double performance increase in one test box
with following aim7 workfile:
    FILESIZE: 1M
    POOLSIZE: 10M
    10 fork_test

some /usr/bin/time output w/o patch      some /usr/bin/time_output with patch
----------------------------------------------------------------------------
Percent of CPU this job got: 369%        Percent of CPU this job got: 537%
Voluntary context switches: 640595016    Voluntary context switches: 157915561
----------------------------------------------------------------------------
You will see we got a 45% increase of CPU usage and saves about 3/4
voluntary context switches.
Here is the .nr_running filed for all CPUs from /proc/sched_debug.

output w/o this patch:
----------------------
cpu 00:   0   0   ...   0   0   0   0   0   0   0   1   0   1 .... 0   0
cpu 01:   0   0   ...   1   0   0   0   0   0   1   1   0   1 .... 0   0
cpu 02:   0   0   ...   1   1   0   0   0   1   0   0   1   0 .... 1   1
cpu 03:   0   0   ...   0   1   0   0   0   1   1   0   1   1 .... 0   0
cpu 04:   0   1   ...   0   0   2   1   1   2   1   0   1   0 .... 1   0
cpu 05:   0   1   ...   0   0   2   1   1   2   1   1   1   1 .... 0   0
cpu 06:   0   0   ...   2   0   0   1   0   0   1   0   0   0 .... 0   0
cpu 07:   0   0   ...   2   0   0   0   1   0   1   1   0   0 .... 1   0
cpu 08:   0   0   ...   1   0   0   0   1   0   0   1   0   0 .... 0   1
cpu 09:   0   0   ...   1   0   0   0   1   0   0   1   0   0 .... 0   1
cpu 10:   0   0   ...   0   0   0   2   0   0   1   0   1   1 .... 1   2
cpu 11:   0   0   ...   0   0   0   2   2   0   1   0   1   0 .... 1   2
cpu 12:   0   0   ...   2   0   0   0   1   1   3   1   1   1 .... 1   0
cpu 13:   0   0   ...   2   0   0   0   1   1   3   1   1   0 .... 1   1
cpu 14:   0   0   ...   0   0   0   2   0   0   1   1   0   0 .... 1   0
cpu 15:   0   0   ...   1   0   0   2   0   0   1   1   0   0 .... 0   0
output with this patch:
-----------------------
cpu 00:   0   0   ...   1   1   2   1   1   1   2   1   1   1 .... 1   3
cpu 01:   0   0   ...   1   1   1   1   1   1   2   1   1   1 .... 1   3
cpu 02:   0   0   ...   2   2   3   2   0   2   1   2   1   1 .... 1   1
cpu 03:   0   0   ...   2   2   3   2   1   2   1   2   1   1 .... 1   1
cpu 04:   0   1   ...   2   0   0   1   0   1   3   1   1   1 .... 1   1
cpu 05:   0   1   ...   2   0   1   1   0   1   2   1   1   1 .... 1   1
cpu 06:   0   0   ...   2   1   1   2   0   1   2   1   1   1 .... 2   1
cpu 07:   0   0   ...   2   1   1   2   0   1   2   1   1   1 .... 2   1
cpu 08:   0   0   ...   1   1   1   1   1   1   1   1   1   1 .... 0   0
cpu 09:   0   0   ...   1   1   1   1   1   1   1   1   1   1 .... 0   0
cpu 10:   0   0   ...   1   1   1   0   0   1   1   1   1   1 .... 0   0
cpu 11:   0   0   ...   1   1   1   0   0   1   1   1   1   2 .... 1   0
cpu 12:   0   0   ...   1   1   1   0   1   1   0   0   0   1 .... 2   1
cpu 13:   0   0   ...   1   1   1   0   1   1   1   0   1   2 .... 2   0
cpu 14:   0   0   ...   2   0   0   0   0   1   1   1   1   1 .... 2   2
cpu 15:   0   0   ...   2   0   0   1   0   1   1   1   1   1 .... 2   2
------------------------------------------------------------------------
Where you can see that CPU is much busier with this patch.
v2: make it stealable at __down_write_trylock as well, pointed by Michel

Reported-by: LKP project <lkp@linux.intel.com>
Suggested-by: Ingo Molnar <mingo@kernel.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Michel Lespinasse <walken@google.com>
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
backported for Grouper Kernel from LKML reference

Signed-off-by: faux123 <reioux@gmail.com>
rwsem: steal writing sem for better performance

Commit 5a50508 changes to rwsem from mutex, caused aim7 fork_test
performance to dropped 50%. Yuanhan liu did an analysis, found it was
caused by strict sequential writing. Ingo suggest stealing sem writing from
front task in wait queue. https://lkml.org/lkml/2013/1/29/84

So does this patch.
In this patch, I just allow write stealing to happen when the first waiter
is also writer. The performance fully is now fully recovered.

Reported-by: lkp@linux.intel.com
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Alex Shi <alex.shi@intel.com>
lib/rwsem.c: fix compatibility issues with Linux 3.1.y

Signed-off-by: faux123 <reioux@gmail.com>
rcu: Fix batch-limit size problem

commit 878d7439d0f45a95869e417576774673d1fa243f upstream.

Commit 29c00b4a1d9e27 (rcu: Add event-tracing for RCU callback
invocation) added a regression in rcu_do_batch()

Under stress, RCU is supposed to allow to process all items in queue,
instead of a batch of 10 items (blimit), but an integer overflow makes
the effective limit being 1.  So, unless there is frequent idle periods
(during which RCU ignores batch limits), RCU can be forced into a
state where it cannot keep up with the callback-generation rate,
eventually resulting in OOM.

This commit therefore converts a few variables in rcu_do_batch() from
int to long to fix this problem, along with the module parameters
controlling the batch limits.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[bwh: Backported to 3.2:
 - Adjust context
 - Module parameters remain hidden from sysfs]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

Conflicts:

	kernel/rcutree.c
CPU hotplug, cpusets, suspend: Don't modify cpusets during suspend/resume

commit d35be8bab9b0ce44bed4b9453f86ebf64062721e upstream.

In the event of CPU hotplug, the kernel modifies the cpusets' cpus_allowed
masks as and when necessary to ensure that the tasks belonging to the cpusets
have some place (online CPUs) to run on. And regular CPU hotplug is
destructive in the sense that the kernel doesn't remember the original cpuset
configurations set by the user, across hotplug operations.

However, suspend/resume (which uses CPU hotplug) is a special case in which
the kernel has the responsibility to restore the system (during resume), to
exactly the same state it was in before suspend.

In order to achieve that, do the following:

1. Don't modify cpusets during suspend/resume. At all.
   In particular, don't move the tasks from one cpuset to another, and
   don't modify any cpuset's cpus_allowed mask. So, simply ignore cpusets
   during the CPU hotplug operations that are carried out in the
   suspend/resume path.

2. However, cpusets and sched domains are related. We just want to avoid
   altering cpusets alone. So, to keep the sched domains updated, build
   a single sched domain (containing all active cpus) during each of the
   CPU hotplug operations carried out in s/r path, effectively ignoring
   the cpusets' cpus_allowed masks.

   (Since userspace is frozen while doing all this, it will go unnoticed.)

3. During the last CPU online operation during resume, build the sched
   domains by looking up the (unaltered) cpusets' cpus_allowed masks.
   That will bring back the system to the same original state as it was in
   before suspend.

Ultimately, this will not only solve the cpuset problem related to suspend
resume (ie., restores the cpusets to exactly what it was before suspend, by
not touching it at all) but also speeds up suspend/resume because we avoid
running cpuset update code for every CPU being offlined/onlined.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20120524141611.3692.20155.stgit@srivatsabhat.in.ibm.com
[Preeti U Murthy: Please apply this patch to the stable tree 3.0.y]
Signed-off-by: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
workqueue: reimplement work_on_cpu() using system_wq

commit ed48ece27cd3d5ee0354c32bbaec0f3e1d4715c3 upstream.

The existing work_on_cpu() implementation is hugely inefficient.  It
creates a new kthread, execute that single function and then let the
kthread die on each invocation.

Now that system_wq can handle concurrent executions, there's no
advantage of doing this.  Reimplement work_on_cpu() using system_wq
which makes it simpler and way more efficient.

stable: While this isn't a fix in itself, it's needed to fix a
        workqueue related bug in cpufreq/powernow-k8.  AFAICS, this
        shouldn't break other existing users.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Len Brown <lenb@kernel.org>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
workqueue: add missing smp_wmb() in process_one_work()

commit 959d1af8cffc8fd38ed53e8be1cf4ab8782f9c00 upstream.

WORK_STRUCT_PENDING is used to claim ownership of a work item and
process_one_work() releases it before starting execution.  When
someone else grabs PENDING, all pre-release updates to the work item
should be visible and all updates made by the new owner should happen
afterwards.

Grabbing PENDING uses test_and_set_bit() and thus has a full barrier;
however, clearing doesn't have a matching wmb.  Given the preceding
spin_unlock and use of clear_bit, I don't believe this can be a
problem on an actual machine and there hasn't been any related report
but it still is theretically possible for clear_pending to permeate
upwards and happen before work->entry update.

Add an explicit smp_wmb() before work_clear_pending().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
workqueue: UNBOUND -> REBIND morphing in rebind_workers() should be atomic

commit 96e65306b81351b656835c15931d1d237b252f27 upstream.

The compiler may compile the following code into TWO write/modify
instructions.

	worker->flags &= ~WORKER_UNBOUND;
	worker->flags |= WORKER_REBIND;

so the other CPU may temporarily see worker->flags which doesn't have
either WORKER_UNBOUND or WORKER_REBIND set and perform local wakeup
prematurely.

Fix it by using single explicit assignment via ACCESS_ONCE().

Because idle workers have another WORKER_NOT_RUNNING flag, this bug
doesn't exist for them; however, update it to use the same pattern for
consistency.

tj: Applied the change to idle workers too and updated comments and
    patch description a bit.

stable: Idle worker rebinding doesn't apply for -stable and
        WORKER_UNBOUND used to be WORKER_ROGUE.  Updated accordingly.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
workqueue: fix possible stall on try_to_grab_pending() of a delayed work item

commit 3aa62497594430ea522050b75c033f71f2c60ee6 upstream.

Currently, when try_to_grab_pending() grabs a delayed work item, it
leaves its linked work items alone on the delayed_works.  The linked
work items are always NO_COLOR and will cause future
cwq_activate_first_delayed() increase cwq->nr_active incorrectly, and
may cause the whole cwq to stall.  For example,

state: cwq->max_active = 1, cwq->nr_active = 1
       one work in cwq->pool, many in cwq->delayed_works.

step1: try_to_grab_pending() removes a work item from delayed_works
       but leaves its NO_COLOR linked work items on it.

step2: Later on, cwq_activate_first_delayed() activates the linked
       work item increasing ->nr_active.

step3: cwq->nr_active = 1, but all activated work items of the cwq are
       NO_COLOR.  When they finish, cwq->nr_active will not be
       decreased due to NO_COLOR, and no further work items will be
       activated from cwq->delayed_works. the cwq stalls.

Fix it by ensuring the target work item is activated before stealing
PENDING in try_to_grab_pending().  This ensures that all the linked
work items are activated without incorrectly bumping cwq->nr_active.

tj: Updated comment and description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Block: use a freezable workqueue for disk-event polling

commit 62d3c5439c534b0e6c653fc63e6d8c67be3a57b1 upstream.

This patch (as1519) fixes a bug in the block layer's disk-events
polling.  The polling is done by a work routine queued on the
system_nrt_wq workqueue.  Since that workqueue isn't freezable, the
polling continues even in the middle of a system sleep transition.

Obviously, polling a suspended drive for media changes and such isn't
a good thing to do; in the case of USB mass-storage devices it can
lead to real problems requiring device resets and even re-enumeration.

The patch fixes things by creating a new system-wide, non-reentrant,
freezable workqueue and using it for disk-events polling.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Revert "sched: Folding nohz load accounting more accurate"

This reverts commit 807cae75e15b7be8470237f967d6809c21970725.
nohz: Fix update_ts_time_stat idle accounting

update_ts_time_stat currently updates idle time even if we are in
iowait loop at the moment. The only real users of the idle counter
(via get_cpu_idle_time_us) are CPU governors and they expect to get
cumulative time for both idle and iowait times.
The value (idle_sleeptime) is also printed to userspace by print_cpu
but it prints both idle and iowait times so the idle part is misleading.

Let's clean this up and fix update_ts_time_stat to account both counters
properly and update consumers of idle to consider iowait time as well.
If we do this we might use get_cpu_{idle,iowait}_time_us from other
contexts as well and we will get expected values.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: Dave Jones <davej@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Link: http://lkml.kernel.org/r/e9c909c221a8da402c4da07e4cd968c3218f8eb1.1314172057.git.mhocko@suse.cz
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
nohz: Make idle/iowait counter update conditional

get_cpu_{idle,iowait}_time_us update idle/iowait counters
unconditionally if the given CPU is in the idle loop.

This doesn't work well outside of CPU governors which are singletons
so nobody (except for IRQ) can race with them.

We will need to use both functions from /proc/stat handler to properly
handle nohz idle/iowait times.

Make the update depend on a non NULL last_update_time argument.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: Dave Jones <davej@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Link: http://lkml.kernel.org/r/11f23179472635ce52e78921d47a20216b872f23.1314172057.git.mhocko@suse.cz
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
nohz: Remove "Switched to NOHz mode" debugging messages

When performing cpu hotplug tests the kernel printk log buffer gets flooded
with pointless "Switched to NOHz mode..." messages. Especially when afterwards
analyzing a dump this might have removed more interesting stuff out of the
buffer.
Assuming that switching to NOHz mode simply works just remove the printk.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Link: http://lkml.kernel.org/r/20110823112046.GB2540@osiris.boeblingen.de.ibm.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Conflicts:

	kernel/time/tick-sched.c

Conflicts:
	kernel/time/tick-sched.c
nohz: Remove nohz_cpu_mask

RCU no longer uses this global variable, nor does anyone else.  This
commit therefore removes this variable.  This reduces memory footprint
and also removes some atomic instructions and memory barriers from
the dyntick-idle path.

Signed-off-by: Alex Shi <alex.shi@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Conflicts:
	kernel/time/tick-sched.c
rcu: Eliminate in_irq() checks in rcu_enter_nohz()

The in_irq() check in rcu_enter_nohz() is redundant because if we really
are in an interrupt, the attempt to re-enter dyntick-idle mode will invoke
rcu_needs_cpu() in any case, which will force the check for RCU callbacks.
So this commit removes the check along with the set_need_resched().

Suggested-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
sched: Convert to struct llist

Use the generic llist primitives.

We had a private lockless list implementation in the scheduler in the wake-list
code, now that we have a generic llist implementation that provides all required
operations, switch to it.

This patch is not expected to change any behavior.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/1315836353.26517.42.camel@twins
Signed-off-by: Ingo Molnar <mingo@elte.hu>
sched: Fix idle_cpu()

On -rt we observed hackbench waking all 400 tasks to a single cpu.
This is because of select_idle_sibling()'s interaction with the new
ipi based wakeup scheme.

The existing idle_cpu() test only checks to see if the current task on
that cpu is the idle task, it does not take already queued tasks into
account, nor does it take queued to be woken tasks into account.

If the remote wakeup IPIs come hard enough, there won't be time to
schedule away from the idle task, and would thus keep thinking the cpu
was in fact idle, regardless of the fact that there were already
several hundred tasks runnable.

We couldn't reproduce on mainline, but there's no reason it couldn't
happen.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-3o30p18b2paswpc9ohy2gltp@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>
sched: Use resched IPI to kick off the nohz idle balance

Current use of smp call function to kick the nohz idle balance can deadlock
in this scenario.

1. cpu-A did a generic_exec_single() to cpu-B and after queuing its call single
data (csd) to the call single queue, cpu-A took a timer interrupt.  Actual IPI
to cpu-B to process the call single queue is not yet sent.

2. As part of the timer interrupt handler, cpu-A decided to kick cpu-B
for the idle load balancing (sets cpu-B's rq->nohz_balance_kick to 1)
and __smp_call_function_single() with nowait will queue the csd to the
cpu-B's queue. But the generic_exec_single() won't send an IPI to cpu-B
as the call single queue was not empty.

3. cpu-A is busy with lot of interrupts

4. Meanwhile cpu-B is entering and exiting idle and noticed that it has
it's rq->nohz_balance_kick set to '1'. So it will go ahead and do the
idle load balancer and clear its rq->nohz_balance_kick.

5. At this point, csd queued as part of the step-2 above is still locked
and waiting to be serviced on cpu-B.

6. cpu-A is still busy with interrupt load and now it got another timer
interrupt and as part of it decided to kick cpu-B for another idle load
balancing (as it finds cpu-B's rq->nohz_balance_kick cleared in step-4
above) and does __smp_call_function_single() with the same csd that is
still locked.

7. And we get a deadlock waiting for the csd_lock() in the
__smp_call_function_single().

Main issue here is that cpu-B can service the idle load balancer kick
request from cpu-A even with out receiving the IPI and this lead to
doing multiple __smp_call_function_single() on the same csd leading to
deadlock.

To kick a cpu, scheduler already has the reschedule vector reserved. Use
that mechanism (kick_process()) instead of using the generic smp call function
mechanism to kick off the nohz idle load balancing and avoid the deadlock.

   [ This issue is present from 2.6.35+ kernels, but marking it -stable
     only from v3.0+ as the proposed fix depends on the scheduler_ipi()
     that is introduced recently. ]

Reported-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: stable@kernel.org # v3.0+
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20111003220934.834943260@sbsiddha-desk.sc.intel.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>
sched: Request for idle balance during nohz idle load balance

rq's idle_at_tick is set to idle/busy during the timer tick
depending on the cpu was idle or not. This will be used later in the load
balance that will be done in the softirq context (which is a process
context in -RT kernels).

For nohz kernels, for the cpu doing nohz idle load balance on behalf of
all the idle cpu's, its rq->idle_at_tick might have a stale value (which is
recorded when it got the timer tick presumably when it is busy).

As the nohz idle load balancing is also being done at the same place
as the regular load balancing, nohz idle load balancing was bailing out
when it sees rq's idle_at_tick not set.

Thus leading to poor system utilization.

Rename rq's idle_at_tick to idle_balance and set it when someone requests
for nohz idle balance on an idle cpu.

Reported-by: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20111003220934.892350549@sbsiddha-desk.sc.intel.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>