Date:	Fri, 5 Sep 2008 12:33:40 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, cl@...ux-foundation.org,
	mingo@...e.hu, manfred@...orfullife.com, dipankar@...ibm.com,
	josht@...ux.vnet.ibm.com, schamp@....com, niv@...ibm.com,
	dvhltc@...ibm.com, ego@...ibm.com, laijs@...fujitsu.com,
	rostedt@...dmis.org, peterz@...radead.org, penberg@...helsinki.fi,
	andi@...stfloor.org
Subject: Re: [PATCH, RFC] v4 scalable classic RCU implementation

On Fri, 5 Sep 2008 08:29:30 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> Hello!
> 
> Still experimental, not for inclusion.  But ready for serious experimental
> use, in particular, experience on an actual >1000-CPU machine would be
> most welcome.
> 
> Updates from v3:
> 
> o	The hierarchical-RCU implementation has been moved to its own
> 	"rcutree" set of files.  This allows configuring three different
> 	implementations of RCU (CLASSIC_RCU, PREEMPT_RCU, and the new
> 	TREE_RCU).  More importantly, it enables easy application of
> 	this patch to a wide variety of Linux versions.
> 
> 	I hope that this implementation can completely replace Classic
> 	RCU, but in the meantime, this split makes for easier testing
> 	and review.
> 
> o	The stalled-CPU detection is now implemented and working,
> 	enabled by the CONFIG_RCU_CPU_STALL config parameter.  Complaints
> 	are kprint()ed 3 seconds into the stall, and every 30 seconds
> 	thereafter.  It also now attempts to force quiescent states.

The CONFIG_RCU_CPU_STALL identifier seems poorly-chosen to me - it
sounds like it will stall my CPU.  Should it be
CONFIG_RCU_CPU_STALL_DETECTOR?  If it's a debugging option then it
should have _DEBUG in there too.

> o	The algorithm uses pre-fabricated masks rather than shifting
> 	on each access.
> 
> o	Review comments have been applied (thank you all!!!).
> 	For but one example, call_rcu() and call_rcu_bh() are now
> 	one-liners.
> 
> o	The rcu_pending() and rcu_needs_cpu() primitives are now
> 	much more aggressive about permitting CPUs to enter dynticks
> 	idle mode.  Only CPUs that have RCU callbacks are kept out
> 	of dynticks idle mode.
> 
> 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.  It is OK for experimental
> work assuming only modestly brave experimenters (and perhaps even
> cowardly experiementers), but not yet ready 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 a number 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	Entering and leaving dynticks idle mode is a quiescent state,
> 	but the current patch doesn't take advantage of this (noted
> 	by Manfred).  It appears that it should be possible to make
> 	nmi_enter() and nmi_exit() provide an in_nmi(), which would make
> 	it possible for rcu_irq_enter() and rcu_irq_exit() to figure
> 	out whether it is safe to tell RCU about the quiescent state --
> 	and also greatly simplify the code.  However, a first attempt
> 	to hack this into existence failed, so will be taking a more
> 	measured approach.
> 
> o	There are a few places where grace periods are unnecessarily
> 	delayed.
> 
> o	There are probably hangs, rcutorture failures, &c.  In particular,
> 	the case where an interrupt from dynticks idle invokes call_rcu()
> 	requires a bit more thought.  And it requires NMIs to be sorted
> 	as noted above.
> 
> o	There are a few architectures that will sometimes execute irq
> 	handlers on CPUs that are already marked offline.  This is the
> 	subject of separate patches.  (Yes, you do have to have a very
> 	unlikely code construct hitting an unlikely sequence of events
> 	for anything bad to happen, but still needs to be fixed.)
> 
> o	Structure field layout is likely highly suboptimal.  On the other
> 	hand, given that the read-side primitives do not touch any of
> 	this data, this issue is not as pressing as it might otherwise be.
> 
> o	There is not yet a human-readable design document.  Will be fixed.

You forgot

  o	Adds yet another RCU flavour

Having alternative implementations of the same thing is a real cost in
terms of maintainability, supportability, etc, etc.

>
> ...
>
> +#if (NR_CPUS) <= RCU_FANOUT
> +#  define NUM_RCU_LVLS	      1
> +#  define NUM_RCU_LVL_0	      1
> +#  define NUM_RCU_LVL_1	      (NR_CPUS)
> +#  define NUM_RCU_LVL_2	      0
> +#  define NUM_RCU_LVL_3	      0
> +#elif (NR_CPUS) <= RCU_FANOUT_SQ
> +#  define NUM_RCU_LVLS	      2
> +#  define NUM_RCU_LVL_0	      1
> +#  define NUM_RCU_LVL_1	      (((NR_CPUS) + RCU_FANOUT - 1) / RCU_FANOUT)
> +#  define NUM_RCU_LVL_2	      (NR_CPUS)
> +#  define NUM_RCU_LVL_3	      0
> +#elif (NR_CPUS) <= RCU_FANOUT_CUBE
> +#  define NUM_RCU_LVLS	      3
> +#  define NUM_RCU_LVL_0	      1
> +#  define NUM_RCU_LVL_1	      (((NR_CPUS) + RCU_FANOUT_SQ - 1) / RCU_FANOUT_SQ)
> +#  define NUM_RCU_LVL_2	      (((NR_CPUS) + (RCU_FANOUT) - 1) / (RCU_FANOUT))
> +#  define NUM_RCU_LVL_3	      NR_CPUS
> +#else
> +# error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
> +#endif /* #if (NR_CPUS) <= RCU_FANOUT */

Using NR_CPUS for anything at all is grossly, grossly inaccurate. 
Vendors can and will ship kernels with NR_CPUS=1024 and their customers
can and will run those kernels on 4-cpu machines.  Lots of customers.

That's a two-and-a-half-order-of-magnitude inaccuracy.  It makes all
your above work meaningless.

To be useful, these decisions should be made at runtime.

> +#define RCU_SUM (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3)
> +#define NUM_RCU_NODES (RCU_SUM - NR_CPUS)
> +
> +/*
> + * Definition for node within the RCU grace-period-detection hierarchy.
> + */
> +struct rcu_node {
> +	spinlock_t lock;
> +	unsigned long qsmask;	/* CPUs or groups that need to switch in */
> +				/*  order for current grace period to proceed.*/
> +	unsigned long qsmaskinit;
> +				/* Per-GP initialization for qsmask. */
> +	unsigned long grpmask;	/* Mask to apply to parent qsmask. */
> +	int	grplo;		/* lowest-numbered CPU or group here. */
> +	int	grphi;		/* highest-numbered CPU or group here. */
> +	u8	grpnum;		/* CPU/group number for next level up. */
> +	u8	level;		/* root is at level 0. */
> +	struct rcu_node *parent;
> +} ____cacheline_internodealigned_in_smp;

So this is a 4096-byte structure on some setups.

How many of them do we expect to be concurrently instantiated?

>
> ...
>
> +#define __rcu_read_lock() \
> +	do { \
> +		preempt_disable(); \
> +		__acquire(RCU); \
> +		rcu_read_acquire(); \
> +	} while (0)
> +#define __rcu_read_unlock() \
> +	do { \
> +		rcu_read_release(); \
> +		__release(RCU); \
> +		preempt_enable(); \
> +	} while (0)
> +#define __rcu_read_lock_bh() \
> +	do { \
> +		local_bh_disable(); \
> +		__acquire(RCU_BH); \
> +		rcu_read_acquire(); \
> +	} while (0)
> +#define __rcu_read_unlock_bh() \
> +	do { \
> +		rcu_read_release(); \
> +		__release(RCU_BH); \
> +		local_bh_enable(); \
> +	} while (0)

did they have to be implemented in macros?  (it's generally best to use
C where poss)

> +#define __synchronize_sched() synchronize_rcu()
> +
> +#define call_rcu_sched(head, func) call_rcu(head, func)
> +
> +extern void __rcu_init(void);
> +#define rcu_init_sched()	do { } while (0)

static inline void rcu_init_sched(void)
{
}

, IMO

>
> ...
>
> +/*
> + * Enter nohz mode, in other words, -leave- the mode in which RCU
> + * read-side critical sections can occur.  (Though RCU read-side
> + * critical sections can occur in irq handlers in nohz mode, a possibility
> + * handled by rcu_irq_enter() and rcu_irq_exit()).
> + *
> + * @@@ note quiescent state???
> + */
> +static inline void rcu_enter_nohz(void)
> +{
> +	static DEFINE_RATELIMIT_STATE(rs, 10 * HZ, 1);
> +
> +	smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
> +	__get_cpu_var(rcu_data).dynticks++;
> +	WARN_ON_RATELIMIT(__get_cpu_var(rcu_data).dynticks & 0x1, &rs);
> +	__get_cpu_var(rcu_bh_data).dynticks++;
> +	WARN_ON_RATELIMIT(__get_cpu_var(rcu_bh_data).dynticks & 0x1, &rs);
> +}
> +
> +/*
> + * Exit nohz mode.
> + */
> +static inline void rcu_exit_nohz(void)
> +{
> +	static DEFINE_RATELIMIT_STATE(rs, 10 * HZ, 1);
> +
> +	__get_cpu_var(rcu_data).dynticks++;
> +	WARN_ON_RATELIMIT(!(__get_cpu_var(rcu_data).dynticks & 0x1), &rs);
> +	__get_cpu_var(rcu_bh_data).dynticks++;
> +	WARN_ON_RATELIMIT(!(__get_cpu_var(rcu_bh_data).dynticks & 0x1), &rs);
> +	smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
> +}

These are massive.  But it seems they'll only ever be used once, in
tick-sched.c so whatever.

>
> ...
>
> +
> +config RCU_FANOUT
> +	int "Tree-based Hierarchical RCU fanout value"

s/H/h/ ?

>
> ...
>
> +#include <linux/types.h>
> +#include <linux/kernel.h>
> +#include <linux/init.h>
> +#include <linux/spinlock.h>
> +#include <linux/smp.h>
> +#include <linux/rcupdate.h>
> +#include <linux/interrupt.h>
> +#include <linux/sched.h>
> +#include <asm/atomic.h>

It still surprises me that we don't have include/linux/atomic.h.

>
> ...
>
> +
> +struct rcu_state rcu_state = RCU_STATE_INITIALIZER(rcu_state);
> +DEFINE_PER_CPU(struct rcu_data, rcu_data) = { 0L };
> +
> +struct rcu_state rcu_bh_state = RCU_STATE_INITIALIZER(rcu_bh_state);
> +DEFINE_PER_CPU(struct rcu_data, rcu_bh_data) = { 0L };

I believe we don't need the explicit initialsiation any more.

>
> ...
>
> +/**
> + * rcu_irq_exit - Called from exiting Hard irq context.

"called when"?

>
> ...
>
> +static void dyntick_record_completed(struct rcu_state *rsp, int comp) { }

I don't personally think such fugliness gains us enough.

static void dyntick_record_completed(struct rcu_state *rsp, int comp)
{
}

looks nicer, is consistent and doesn't break vi's ]] command.

>
> ...
>
> +static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
> +{
> +	long delta;
> +	struct rcu_node *rnp;
> +
> +	delta = get_seconds() - rsp->seconds_stall;
> +	rnp = rdp->mynode;
> +	if ((rnp->qsmask & rdp->grpmask) && delta >= 0L) {
> +
> +		/* We haven't checked in, so go dump stack. */
> +		print_cpu_stall(rsp);
> +
> +	} else if (rsp->gpnum != rsp->completed && delta >= 2L) {

I dislike the L's here.  They don't do anything and they have an
encapsulation-violation feeling about them.  Do we really want code
sprinkled all over the palce whcih "knows" spefically which type was
used to implement these fields?  Or do we want to stick a plain old "2"
in there and have it Just Work.

> +
> +		/* They had two seconds to dump stack, so complain. */
> +		print_other_cpu_stall(rsp);
> +	}
> +}
> +
> +#else /* #ifdef CONFIG_RCU_CPU_STALL */
> +static void record_gp_stall_check_time(struct rcu_state *rsp) { }
> +static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp) { }

]]
]]

>
> ...
>
> +/*
> + * Start a new RCU grace period if warranted, re-initializing the hierarchy
> + * in preparation for detecting the next grace period.  The caller must hold
> + * the root node's ->lock, which is released before return.  Hard irqs must
> + * be disabled.
> + */
> +static void
> +rcu_start_gp(struct rcu_state *rsp, unsigned long iflg)
> +	__releases(rsp->rda[smp_processor_id()]->lock)

hm, does that work?

akpm:/usr/src/25> grep -r __releases Documentation 
akpm:/usr/src/25> 

lolwesuck.

>
> ...
>
> +/*
> + * Remove the specified CPU from the RCU hierarchy and move any pending
> + * callbacks that it might have to the current CPU.  This code assumes
> + * that at least one CPU in the system will remain running at all times.
> + * Any attempt to offline -all- CPUs is likely to strand RCU callbacks.
> + */
> +static void rcu_offline_cpu(int cpu)
> +{
> +	__rcu_offline_cpu(cpu, &rcu_state);
> +	__rcu_offline_cpu(cpu, &rcu_bh_state);
> +}
> +
> +#else /* #ifdef CONFIG_HOTPLUG_CPU */
> +
> +static void
> +rcu_offline_cpu(int cpu)

unneeded \n there

> +{
> +}
> +
> +#endif /* #else #ifdef CONFIG_HOTPLUG_CPU */
> +
>
> ...
>

Looks great!  I don't understand a line of it!

It's a pretty big pill to swallow.  Nice performance testing results
will help it to slide down.
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