Subject: Re: [PATCH, RFC, tip/core/rcu] scalable classic RCU implementation
From: Josh Triplett <josht@linux.vnet.ibm.com>
To: paulmck@linux.vnet.ibm.com
Cc: Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org,
       cl@linux-foundation.org, akpm@linux-foundation.org,
       manfred@colorfullife.com, dipankar@in.ibm.com, schamp@sgi.com,
       niv@us.ibm.com, dvhltc@us.ibm.com, ego@in.ibm.com, laijs@cn.fujitsu.com,
       rostedt@goodmis.org
In-Reply-To: <20080822172221.GA9593@linux.vnet.ibm.com>
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	 <20080822172221.GA9593@linux.vnet.ibm.com>
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Date: Fri, 22 Aug 2008 11:16:38 -0700
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On Fri, 2008-08-22 at 10:22 -0700, Paul E. McKenney wrote:
> On Fri, Aug 22, 2008 at 06:47:20AM -0700, Paul E. McKenney wrote:
> > On Fri, Aug 22, 2008 at 06:37:15AM +0200, Ingo Molnar wrote:
> > > 
> > > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> > > 
> > > > +#define MAX_RCU_LEVELS 3
> > > > +#if NR_CPUS <= CONFIG_RCU_FANOUT
> > > > +#define NUM_RCU_LEVELS 1
> > > > +#define NUM_RCU_LEVEL_1 1
> > > > +#define NUM_RCU_LEVEL_2 NR_CPUS
> > > > +#define NUM_RCU_LEVEL_3 0
> > > > +#define NUM_RCU_LEVEL_4 0
> > > > +#define NUM_RCU_NODES NUM_RCU_LEVEL_1
> > > > +#elif NR_CPUS <= CONFIG_RCU_FANOUT * CONFIG_RCU_FANOUT
> > > > +#define NUM_RCU_LEVELS 2
> > > > +#define NUM_RCU_LEVEL_1 1
> > > > +#define NUM_RCU_LEVEL_2 \
> > > > +	(((NR_CPUS) + (CONFIG_RCU_FANOUT) - 1) / (CONFIG_RCU_FANOUT))
> > > > +#define NUM_RCU_LEVEL_3 NR_CPUS
> > > > +#define NUM_RCU_LEVEL_4 0
> > > > +#define NUM_RCU_NODES \
> > > > +	((NUM_RCU_LEVEL_1) + (NUM_RCU_LEVEL_2))
> > > > +#elif NR_CPUS <= CONFIG_RCU_FANOUT * CONFIG_RCU_FANOUT * CONFIG_RCU_FANOUT
> > > > +#define NUM_RCU_LEVELS 3
> > > > +#define RCU_FANOUT_SQ ((CONFIG_RCU_FANOUT) * (CONFIG_RCU_FANOUT))
> > > > +#define NUM_RCU_LEVEL_1 1
> > > > +#define NUM_RCU_LEVEL_2 \
> > > > +	(((NR_CPUS) + (RCU_FANOUT_SQ) - 1) / (RCU_FANOUT_SQ))
> > > > +#define NUM_RCU_LEVEL_3 \
> > > > +	((NR_CPUS) + (CONFIG_RCU_FANOUT) - 1) / (CONFIG_RCU_FANOUT)
> > > > +#define NUM_RCU_LEVEL_4 NR_CPUS
> > > > +#define NUM_RCU_NODES \
> > > > +	((NUM_RCU_LEVEL_1) + \
> > > > +	 (NUM_RCU_LEVEL_2) + \
> > > > +	 (NUM_RCU_LEVEL_3))
> > > > +#else
> > > > +#error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
> > > > +#endif
> > > 
> > > just a quick stylistic suggestion: if feasible then such sizing ugliness 
> > > should be hidden in a Kconfig file. (if Kconfig is capable enough for 
> > > this that is)
> > 
> > I have no idea if Kconfig can do it, but I will check.
> 
> OK, Kconfig does not currently support arithmetic, based on zconf.y:
> 
> expr:	  symbol				{ $$ = expr_alloc_symbol($1); }
> 	| symbol T_EQUAL symbol			{ $$ = expr_alloc_comp(E_EQUAL, $1, $3); }
> 	| symbol T_UNEQUAL symbol		{ $$ = expr_alloc_comp(E_UNEQUAL, $1, $3); }
> 	| T_OPEN_PAREN expr T_CLOSE_PAREN	{ $$ = $2; }
> 	| T_NOT expr				{ $$ = expr_alloc_one(E_NOT, $2); }
> 	| expr T_OR expr			{ $$ = expr_alloc_two(E_OR, $1, $3); }
> 	| expr T_AND expr			{ $$ = expr_alloc_two(E_AND, $1, $3); }
> ;
> 
> All we currently get is basic comparison and logical operators.  It would
> not be all -that- hard to add general arithmetic (famous last words),
> but when I tried mapping out what the sizing code would look like in
> such an augmented Kconfig, it was even uglier than the above.

Makes sense.

> So I took a hard look at the current mess, and prettied it as shown below.
> Is this a sufficient improvement?

Looks significantly improved.

> Another alternative I am considering is moving this to a separate
> include file.

I personally don't think that would help.  The revised version seems
fine.

> #define MAX_RCU_LEVELS 3
> #define RCU_FANOUT (CONFIG_RCU_FANOUT)
> #define RCU_FANOUT_SQ (RCU_FANOUT * RCU_FANOUT)
> #define RCU_FANOUT_CUBE (RCU_FANOUT_SQ * RCU_FANOUT)
> 
> #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 */
> 
> #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)

- Josh Triplett


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