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Commit 0209f649 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: limit rcu_node leaf-level fanout 



Some recent benchmarks have indicated possible lock contention on the
leaf-level rcu_node locks.  This commit therefore limits the number of
CPUs per leaf-level rcu_node structure to 16, in other words, there
can be at most 16 rcu_data structures fanning into a given rcu_node
structure.  Prior to this, the limit was 32 on 32-bit systems and 64 on
64-bit systems.

Note that the fanout of non-leaf rcu_node structures is unchanged.  The
organization of accesses to the rcu_node tree is such that references
to non-leaf rcu_node structures are much less frequent than to the
leaf structures.

Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 121dfc4b

kernel/rcutree.c

+2 −1
Original line number Diff line number Diff line
@@ -1869,8 +1869,9 @@ static void __init rcu_init_levelspread(struct rcu_state *rsp) 
{

	int i;



	for (i = NUM_RCU_LVLS - 1; i >= 0; i--)

	for (i = NUM_RCU_LVLS - 1; i > 0; i--)

		rsp->levelspread[i] = CONFIG_RCU_FANOUT;

	rsp->levelspread[0] = RCU_FANOUT_LEAF;

}

#else /* #ifdef CONFIG_RCU_FANOUT_EXACT */

static void __init rcu_init_levelspread(struct rcu_state *rsp)

kernel/rcutree.h

+25 −20
Original line number Diff line number Diff line
@@ -31,46 +31,51 @@ 
/*

 * Define shape of hierarchy based on NR_CPUS and CONFIG_RCU_FANOUT.

 * In theory, it should be possible to add more levels straightforwardly.

 * In practice, this has not been tested, so there is probably some

 * bug somewhere.

 * In practice, this did work well going from three levels to four.

 * Of course, your mileage may vary.

 */

#define MAX_RCU_LVLS 4

#define RCU_FANOUT	      (CONFIG_RCU_FANOUT)

#define RCU_FANOUT_SQ	      (RCU_FANOUT * RCU_FANOUT)

#define RCU_FANOUT_CUBE	      (RCU_FANOUT_SQ * RCU_FANOUT)

#define RCU_FANOUT_FOURTH     (RCU_FANOUT_CUBE * RCU_FANOUT)



#if NR_CPUS <= RCU_FANOUT

#if CONFIG_RCU_FANOUT > 16

#define RCU_FANOUT_LEAF       16

#else /* #if CONFIG_RCU_FANOUT > 16 */

#define RCU_FANOUT_LEAF       (CONFIG_RCU_FANOUT)

#endif /* #else #if CONFIG_RCU_FANOUT > 16 */

#define RCU_FANOUT_1	      (RCU_FANOUT_LEAF)

#define RCU_FANOUT_2	      (RCU_FANOUT_1 * CONFIG_RCU_FANOUT)

#define RCU_FANOUT_3	      (RCU_FANOUT_2 * CONFIG_RCU_FANOUT)

#define RCU_FANOUT_4	      (RCU_FANOUT_3 * CONFIG_RCU_FANOUT)



#if NR_CPUS <= RCU_FANOUT_1

#  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

#  define NUM_RCU_LVL_4	      0

#elif NR_CPUS <= RCU_FANOUT_SQ

#elif NR_CPUS <= RCU_FANOUT_2

#  define NUM_RCU_LVLS	      2

#  define NUM_RCU_LVL_0	      1

#  define NUM_RCU_LVL_1	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT)

#  define NUM_RCU_LVL_1	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)

#  define NUM_RCU_LVL_2	      (NR_CPUS)

#  define NUM_RCU_LVL_3	      0

#  define NUM_RCU_LVL_4	      0

#elif NR_CPUS <= RCU_FANOUT_CUBE

#elif NR_CPUS <= RCU_FANOUT_3

#  define NUM_RCU_LVLS	      3

#  define NUM_RCU_LVL_0	      1

#  define NUM_RCU_LVL_1	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_SQ)

#  define NUM_RCU_LVL_2	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT)

#  define NUM_RCU_LVL_3	      NR_CPUS

#  define NUM_RCU_LVL_1	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)

#  define NUM_RCU_LVL_2	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)

#  define NUM_RCU_LVL_3	      (NR_CPUS)

#  define NUM_RCU_LVL_4	      0

#elif NR_CPUS <= RCU_FANOUT_FOURTH

#elif NR_CPUS <= RCU_FANOUT_4

#  define NUM_RCU_LVLS	      4

#  define NUM_RCU_LVL_0	      1

#  define NUM_RCU_LVL_1	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_CUBE)

#  define NUM_RCU_LVL_2	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_SQ)

#  define NUM_RCU_LVL_3	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT)

#  define NUM_RCU_LVL_4	      NR_CPUS

#  define NUM_RCU_LVL_1	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_3)

#  define NUM_RCU_LVL_2	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)

#  define NUM_RCU_LVL_3	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)

#  define NUM_RCU_LVL_4	      (NR_CPUS)

#else

# error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"

#endif /* #if (NR_CPUS) <= RCU_FANOUT */

#endif /* #if (NR_CPUS) <= RCU_FANOUT_1 */



#define RCU_SUM (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3 + NUM_RCU_LVL_4)

#define NUM_RCU_NODES (RCU_SUM - NR_CPUS)