• Paul E. McKenney's avatar
    "Tree RCU": scalable classic RCU implementation · 64db4cff
    Paul E. McKenney authored
    This patch fixes a long-standing performance bug in classic RCU that
    results in massive internal-to-RCU lock contention on systems with
    more than a few hundred CPUs.  Although this patch creates a separate
    flavor of RCU for ease of review and patch maintenance, it is intended
    to replace classic RCU.
    This patch still handles stress better than does mainline, so I am still
    calling it ready for inclusion.  This patch is against the -tip tree.
    Nevertheless, experience on an actual 1000+ CPU machine would still be
    most welcome.
    Most of the changes noted below were found while creating an rcutiny
    (which should permit ejecting the current rcuclassic) and while doing
    detailed line-by-line documentation.
    Updates from v9 (http://lkml.org/lkml/2008/12/2/334):
    o	Fixes from remainder of line-by-line code walkthrough,
    	including comment spelling, initialization, undesirable
    	narrowing due to type conversion, removing redundant memory
    	barriers, removing redundant local-variable initialization,
    	and removing redundant local variables.
    	I do not believe that any of these fixes address the CPU-hotplug
    	issues that Andi Kleen was seeing, but please do give it a whirl
    	in case the machine is smarter than I am.
    	A writeup from the walkthrough may be found at the following
    	URL, in case you are suffering from terminal insomnia or
    o	Made rcutree tracing use seq_file, as suggested some time
    	ago by Lai Jiangshan.
    o	Added a .csv variant of the rcudata debugfs trace file, to allow
    	people having thousands of CPUs to drop the data into
    	a spreadsheet.	Tested with oocalc and gnumeric.  Updated
    	documentation to suit.
    Updates from v8 (http://lkml.org/lkml/2008/11/15/139):
    o	Fix a theoretical race between grace-period initialization and
    	force_quiescent_state() that could occur if more than three
    	jiffies were required to carry out the grace-period
    	initialization.  Which it might, if you had enough CPUs.
    o	Apply Ingo's printk-standardization patch.
    o	Substitute local variables for repeated accesses to global
    o	Fix comment misspellings and redundant (but harmless) increments
    	of ->n_rcu_pending (this latter after having explicitly added it).
    o	Apply checkpatch fixes.
    Updates from v7 (http://lkml.org/lkml/2008/10/10/291):
    o	Fixed a number of problems noted by Gautham Shenoy, including
    	the cpu-stall-detection bug that he was having difficulty
    	convincing me was real.  ;-)
    o	Changed cpu-stall detection to wait for ten seconds rather than
    	three in order to reduce false positive, as suggested by Ingo
    o	Produced a design document (http://lwn.net/Articles/305782/).
    	The act of writing this document uncovered a number of both
    	theoretical and "here and now" bugs as noted below.
    o	Fix dynticks_nesting accounting confusion, simplify WARN_ON()
    	condition, fix kerneldoc comments, and add memory barriers
    	in dynticks interface functions.
    o	Add more data to tracing.
    o	Remove unused "rcu_barrier" field from rcu_data structure.
    o	Count calls to rcu_pending() from scheduling-clock interrupt
    	to use as a surrogate timebase should jiffies stop counting.
    o	Fix a theoretical race between force_quiescent_state() and
    	grace-period initialization.  Yes, initialization does have to
    	go on for some jiffies for this race to occur, but given enough
    Updates from v6 (http://lkml.org/lkml/2008/9/23/448):
    o	Fix a number of checkpatch.pl complaints.
    o	Apply review comments from Ingo Molnar and Lai Jiangshan
    	on the stall-detection code.
    o	Fix several bugs in !CONFIG_SMP builds.
    o	Fix a misspelled config-parameter name so that RCU now announces
    	at boot time if stall detection is configured.
    o	Run tests on numerous combinations of configurations parameters,
    	which after the fixes above, now build and run correctly.
    Updates from v5 (http://lkml.org/lkml/2008/9/15/92, bad subject line):
    o	Fix a compiler error in the !CONFIG_FANOUT_EXACT case (blew a
    	changeset some time ago, and finally got around to retesting
    	this option).
    o	Fix some tracing bugs in rcupreempt that caused incorrect
    	totals to be printed.
    o	I now test with a more brutal random-selection online/offline
    	script (attached).  Probably more brutal than it needs to be
    	on the people reading it as well, but so it goes.
    o	A number of optimizations and usability improvements:
    	o	Make rcu_pending() ignore the grace-period timeout when
    		there is no grace period in progress.
    	o	Make force_quiescent_state() avoid going for a global
    		lock in the case where there is no grace period in
    	o	Rearrange struct fields to improve struct layout.
    	o	Make call_rcu() initiate a grace period if RCU was
    		idle, rather than waiting for the next scheduling
    		clock interrupt.
    	o	Invoke rcu_irq_enter() and rcu_irq_exit() only when
    		idle, as suggested by Andi Kleen.  I still don't
    		completely trust this change, and might back it out.
    	o	Make CONFIG_RCU_TRACE be the single config variable
    		manipulated for all forms of RCU, instead of the prior
    	o	Document tracing files and formats for both rcupreempt
    		and rcutree.
    Updates from v4 for those missing v5 given its bad subject line:
    o	Separated dynticks interface so that NMIs and irqs call separate
    	functions, greatly simplifying it.  In particular, this code
    	no longer requires a proof of correctness.  ;-)
    o	Separated dynticks state out into its own per-CPU structure,
    	avoiding the duplicated accounting.
    o	The case where a dynticks-idle CPU runs an irq handler that
    	invokes call_rcu() is now correctly handled, forcing that CPU
    	out of dynticks-idle mode.
    o	Review comments have been applied (thank you all!!!).
    	For but one example, fixed the dynticks-ordering issue that
    	Manfred pointed out, saving me much debugging.  ;-)
    o	Adjusted rcuclassic and rcupreempt to handle dynticks changes.
    Attached is an updated patch to Classic RCU that applies a hierarchy,
    greatly reducing the contention on the top-level lock for large machines.
    This passes 10-hour concurrent rcutorture and online-offline testing on
    128-CPU ppc64 without dynticks enabled, and exposes some timekeeping
    bugs in presence of dynticks (exciting working on a system where
    "sleep 1" hangs until interrupted...), which were fixed in the
    2.6.27 kernel.  It is getting more reliable than mainline by some
    measures, so the next version will be against -tip for inclusion.
    See also Manfred Spraul's recent patches (or his earlier work from
    2004 at http://marc.info/?l=linux-kernel&m=108546384711797&w=2).
    We will converge onto a common patch in the fullness of time, but are
    currently exploring different regions of the design space.  That said,
    I have already gratefully stolen quite a few of Manfred's ideas.
    This patch provides CONFIG_RCU_FANOUT, which controls the bushiness
    of the RCU hierarchy.  Defaults to 32 on 32-bit machines and 64 on
    64-bit machines.  If CONFIG_NR_CPUS is less than CONFIG_RCU_FANOUT,
    there is no hierarchy.  By default, the RCU initialization code will
    adjust CONFIG_RCU_FANOUT to balance the hierarchy, so strongly NUMA
    architectures may choose to set CONFIG_RCU_FANOUT_EXACT to disable
    this balancing, allowing the hierarchy to be exactly aligned to the
    underlying hardware.  Up to two levels of hierarchy are permitted
    (in addition to the root node), allowing up to 16,384 CPUs on 32-bit
    systems and up to 262,144 CPUs on 64-bit systems.  I just know that I
    am going to regret saying this, but this seems more than sufficient
    for the foreseeable future.  (Some architectures might wish to set
    CONFIG_RCU_FANOUT=4, which would limit such architectures to 64 CPUs.
    If this becomes a real problem, additional levels can be added, but I
    doubt that it will make a significant difference on real hardware.)
    In the common case, a given CPU will manipulate its private rcu_data
    structure and the rcu_node structure that it shares with its immediate
    neighbors.  This can reduce both lock and memory contention by multiple
    orders of magnitude, which should eliminate the need for the strange
    manipulations that are reported to be required when running Linux on
    very large systems.
    Some shortcomings:
    o	More bugs will probably surface as a result of an ongoing
    	line-by-line code inspection.
    	Patches will be provided as required.
    o	There are probably hangs, rcutorture failures, &c.  Seems
    	quite stable on a 128-CPU machine, but that is kind of small
    	compared to 4096 CPUs.  However, seems to do better than
    	Patches will be provided as required.
    o	The memory footprint of this version is several KB larger
    	than rcuclassic.
    	A separate UP-only rcutiny patch will be provided, which will
    	reduce the memory footprint significantly, even compared
    	to the old rcuclassic.  One such patch passes light testing,
    	and has a memory footprint smaller even than rcuclassic.
    	Initial reaction from various embedded guys was "it is not
    	worth it", so am putting it aside.
    o	Manfred Spraul for ideas, review comments, and bugs spotted,
    	as well as some good friendly competition.  ;-)
    o	Josh Triplett, Ingo Molnar, Peter Zijlstra, Mathieu Desnoyers,
    	Lai Jiangshan, Andi Kleen, Andy Whitcroft, and Andrew Morton
    	for reviews and comments.
    o	Thomas Gleixner for much-needed help with some timer issues
    	(see patches below).
    o	Jon M. Tollefson, Tim Pepper, Andrew Theurer, Jose R. Santos,
    	Andy Whitcroft, Darrick Wong, Nishanth Aravamudan, Anton
    	Blanchard, Dave Kleikamp, and Nathan Lynch for keeping machines
    	alive despite my heavy abuse^Wtesting.
    Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
rcutree.h 11.5 KB