Merge branch 'rcu/next' of...

	using RCU rather than SRCU, because RCU is almost always faster

	and easier to use than is SRCU.

	If you need to enter your read-side critical section in a

	hardirq or exception handler, and then exit that same read-side

	critical section in the task that was interrupted, then you need

	to srcu_read_lock_raw() and srcu_read_unlock_raw(), which avoid

	the lockdep checking that would otherwise this practice illegal.

	Also unlike other forms of RCU, explicit initialization

	and cleanup is required via init_srcu_struct() and

	cleanup_srcu_struct().	These are passed a "struct srcu_struct"

		"srcu_expedited": srcu_read_lock(), srcu_read_unlock() and

			synchronize_srcu_expedited().

		"srcu_raw": srcu_read_lock_raw(), srcu_read_unlock_raw(),

			and call_srcu().

		"srcu_raw_sync": srcu_read_lock_raw(), srcu_read_unlock_raw(),

			and synchronize_srcu().

		"sched": preempt_disable(), preempt_enable(), and

			call_rcu_sched().

	reasons, e.g., the grace period ended first.

CONFIG_TINY_RCU and CONFIG_TINY_PREEMPT_RCU debugfs Files and Formats

CONFIG_TINY_RCU debugfs Files and Formats

These implementations of RCU provides a single debugfs file under the

top-level directory RCU, namely rcu/rcudata, which displays fields in

rcu_bh_ctrlblk, rcu_sched_ctrlblk and, for CONFIG_TINY_PREEMPT_RCU,

rcu_preempt_ctrlblk.

rcu_bh_ctrlblk and rcu_sched_ctrlblk.

The output of "cat rcu/rcudata" is as follows:

rcu_preempt: qlen=24 gp=1097669 g197/p197/c197 tasks=...

             ttb=. btg=no ntb=184 neb=0 nnb=183 j=01f7 bt=0274

             normal balk: nt=1097669 gt=0 bt=371 b=0 ny=25073378 nos=0

             exp balk: bt=0 nos=0

rcu_sched: qlen: 0

rcu_bh: qlen: 0

This is split into rcu_preempt, rcu_sched, and rcu_bh sections, with the

rcu_preempt section appearing only in CONFIG_TINY_PREEMPT_RCU builds.

The last three lines of the rcu_preempt section appear only in

CONFIG_RCU_BOOST kernel builds.  The fields are as follows:

This is split into rcu_sched and rcu_bh sections.  The field is as

follows:

o	"qlen" is the number of RCU callbacks currently waiting either

	for an RCU grace period or waiting to be invoked.  This is the

	only field present for rcu_sched and rcu_bh, due to the

	short-circuiting of grace period in those two cases.

o	"gp" is the number of grace periods that have completed.

o	"g197/p197/c197" displays the grace-period state, with the

	"g" number being the number of grace periods that have started

	(mod 256), the "p" number being the number of grace periods

	that the CPU has responded to (also mod 256), and the "c"

	number being the number of grace periods that have completed

	(once again mode 256).

	Why have both "gp" and "g"?  Because the data flowing into

	"gp" is only present in a CONFIG_RCU_TRACE kernel.

o	"tasks" is a set of bits.  The first bit is "T" if there are

	currently tasks that have recently blocked within an RCU

	read-side critical section, the second bit is "N" if any of the

	aforementioned tasks are blocking the current RCU grace period,

	and the third bit is "E" if any of the aforementioned tasks are

	blocking the current expedited grace period.  Each bit is "."

	if the corresponding condition does not hold.

o	"ttb" is a single bit.  It is "B" if any of the blocked tasks

	need to be priority boosted and "." otherwise.

o	"btg" indicates whether boosting has been carried out during

	the current grace period, with "exp" indicating that boosting

	is in progress for an expedited grace period, "no" indicating

	that boosting has not yet started for a normal grace period,

	"begun" indicating that boosting has bebug for a normal grace

	period, and "done" indicating that boosting has completed for

	a normal grace period.

o	"ntb" is the total number of tasks subjected to RCU priority boosting

	periods since boot.

o	"neb" is the number of expedited grace periods that have had

	to resort to RCU priority boosting since boot.

o	"nnb" is the number of normal grace periods that have had

	to resort to RCU priority boosting since boot.

o	"j" is the low-order 16 bits of the jiffies counter in hexadecimal.

o	"bt" is the low-order 16 bits of the value that the jiffies counter

	will have at the next time that boosting is scheduled to begin.

o	In the line beginning with "normal balk", the fields are as follows:

	o	"nt" is the number of times that the system balked from

		boosting because there were no blocked tasks to boost.

		Note that the system will balk from boosting even if the

		grace period is overdue when the currently running task

		is looping within an RCU read-side critical section.

		There is no point in boosting in this case, because

		boosting a running task won't make it run any faster.

	o	"gt" is the number of times that the system balked

		from boosting because, although there were blocked tasks,

		none of them were preventing the current grace period

		from completing.

	o	"bt" is the number of times that the system balked

		from boosting because boosting was already in progress.

	o	"b" is the number of times that the system balked from

		boosting because boosting had already completed for

		the grace period in question.

	o	"ny" is the number of times that the system balked from

		boosting because it was not yet time to start boosting

		the grace period in question.

	o	"nos" is the number of times that the system balked from

		boosting for inexplicable ("not otherwise specified")

		reasons.  This can actually happen due to races involving

		increments of the jiffies counter.

o	In the line beginning with "exp balk", the fields are as follows:

	o	"bt" is the number of times that the system balked from

		boosting because there were no blocked tasks to boost.

	o	"nos" is the number of times that the system balked from

		 boosting for inexplicable ("not otherwise specified")

		 reasons.

	srcu_read_lock		synchronize_srcu	srcu_barrier

	srcu_read_unlock	call_srcu

	srcu_read_lock_raw	synchronize_srcu_expedited

	srcu_read_unlock_raw

	srcu_dereference

	srcu_dereference	synchronize_srcu_expedited

SRCU:	Initialization/cleanup

	init_srcu_struct

a.	Will readers need to block?  If so, you need SRCU.

b.	Is it necessary to start a read-side critical section in a

	hardirq handler or exception handler, and then to complete

	this read-side critical section in the task that was

	interrupted?  If so, you need SRCU's srcu_read_lock_raw() and

	srcu_read_unlock_raw() primitives.

c.	What about the -rt patchset?  If readers would need to block

b.	What about the -rt patchset?  If readers would need to block

	in an non-rt kernel, you need SRCU.  If readers would block

	in a -rt kernel, but not in a non-rt kernel, SRCU is not

	necessary.

d.	Do you need to treat NMI handlers, hardirq handlers,

c.	Do you need to treat NMI handlers, hardirq handlers,

	and code segments with preemption disabled (whether

	via preempt_disable(), local_irq_save(), local_bh_disable(),

	or some other mechanism) as if they were explicit RCU readers?

	If so, RCU-sched is the only choice that will work for you.

e.	Do you need RCU grace periods to complete even in the face

d.	Do you need RCU grace periods to complete even in the face

	of softirq monopolization of one or more of the CPUs?  For

	example, is your code subject to network-based denial-of-service

	attacks?  If so, you need RCU-bh.

f.	Is your workload too update-intensive for normal use of

e.	Is your workload too update-intensive for normal use of

	RCU, but inappropriate for other synchronization mechanisms?

	If so, consider SLAB_DESTROY_BY_RCU.  But please be careful!

g.	Do you need read-side critical sections that are respected

f.	Do you need read-side critical sections that are respected

	even though they are in the middle of the idle loop, during

	user-mode execution, or on an offlined CPU?  If so, SRCU is the

	only choice that will work for you.

h.	Otherwise, use RCU.

g.	Otherwise, use RCU.

Of course, this all assumes that you have determined that RCU is in fact

the right tool for your job.

		calls and by forcing both kernel threads and interrupts

		to execute elsewhere.

Name: kworker/%u:%d%s (cpu, id, priority)

Purpose: Execute workqueue requests

To reduce its OS jitter, do any of the following:

1.	Run your workload at a real-time priority, which will allow

	preempting the kworker daemons.

2.	Do any of the following needed to avoid jitter that your

	application cannot tolerate:

	a.	Build your kernel with CONFIG_SLUB=y rather than

		CONFIG_SLAB=y, thus avoiding the slab allocator's periodic

		use of each CPU's workqueues to run its cache_reap()

		function.

	b.	Avoid using oprofile, thus avoiding OS jitter from

		wq_sync_buffer().

	c.	Limit your CPU frequency so that a CPU-frequency

		governor is not required, possibly enlisting the aid of

		special heatsinks or other cooling technologies.  If done

		correctly, and if you CPU architecture permits, you should

		be able to build your kernel with CONFIG_CPU_FREQ=n to

		avoid the CPU-frequency governor periodically running

		on each CPU, including cs_dbs_timer() and od_dbs_timer().

		WARNING:  Please check your CPU specifications to

		make sure that this is safe on your particular system.

	d.	It is not possible to entirely get rid of OS jitter

		from vmstat_update() on CONFIG_SMP=y systems, but you

		can decrease its frequency by writing a large value to

		/proc/sys/vm/stat_interval.  The default value is HZ,

		for an interval of one second.  Of course, larger values

		will make your virtual-memory statistics update more

		slowly.  Of course, you can also run your workload at

		a real-time priority, thus preempting vmstat_update().

	e.	If running on high-end powerpc servers, build with

		CONFIG_PPC_RTAS_DAEMON=n.  This prevents the RTAS

		daemon from running on each CPU every second or so.

		(This will require editing Kconfig files and will defeat

		this platform's RAS functionality.)  This avoids jitter

		due to the rtas_event_scan() function.

		WARNING:  Please check your CPU specifications to

		make sure that this is safe on your particular system.

	f.	If running on Cell Processor, build your kernel with

		CBE_CPUFREQ_SPU_GOVERNOR=n to avoid OS jitter from

		spu_gov_work().

		WARNING:  Please check your CPU specifications to

		make sure that this is safe on your particular system.

	g.	If running on PowerMAC, build your kernel with

		CONFIG_PMAC_RACKMETER=n to disable the CPU-meter,

		avoiding OS jitter from rackmeter_do_timer().

Name: rcuc/%u

Purpose: Execute RCU callbacks in CONFIG_RCU_BOOST=y kernels.

To reduce its OS jitter, do at least one of the following: