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Message-Id: <1219428998.5197.4.camel@josh-work.beaverton.ibm.com>
Date: Fri, 22 Aug 2008 11:16:38 -0700
From: Josh Triplett <josht@...ux.vnet.ibm.com>
To: paulmck@...ux.vnet.ibm.com
Cc: Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
cl@...ux-foundation.org, akpm@...ux-foundation.org,
manfred@...orfullife.com, dipankar@...ibm.com, schamp@....com,
niv@...ibm.com, dvhltc@...ibm.com, ego@...ibm.com,
laijs@...fujitsu.com, rostedt@...dmis.org
Subject: Re: [PATCH, RFC, tip/core/rcu] scalable classic RCU implementation
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@...ux.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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