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rcu: Drive quiescent-state-forcing delay from HZ 

Systems with HZ=100 can have slow bootup times due to the default
three-jiffy delays between quiescent-state forcing attempts.  This
commit therefore auto-tunes the RCU_JIFFIES_TILL_FORCE_QS value based
on the value of HZ.  However, this would break very large systems that
require more time between quiescent-state forcing attempts.  This
commit therefore also ups the default delay by one jiffy for each
256 CPUs that might be on the system (based off of nr_cpu_ids at
runtime, -not- NR_CPUS at build time).

Updated to collapse #ifdefs for RCU_JIFFIES_TILL_FORCE_QS into a
step-function definition as suggested by Josh Triplett.

Reported-by: Paul Mackerras <paulus@au1.ibm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2013-04-03 22:14:11 -07:00
parent 9a5739d73f
commit 026ad2835c
2 changed files with 26 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

18
kernel/rcutree.c
View File

@ -218,8 +218,8 @@ module_param(blimit, long, 0444);
module_param(qhimark, long, 0444); module_param(qhimark, long, 0444);
module_param(qlowmark, long, 0444); module_param(qlowmark, long, 0444);
static ulong jiffies_till_first_fqs = RCU_JIFFIES_TILL_FORCE_QS; static ulong jiffies_till_first_fqs = ULONG_MAX;
static ulong jiffies_till_next_fqs = RCU_JIFFIES_TILL_FORCE_QS; static ulong jiffies_till_next_fqs = ULONG_MAX;
module_param(jiffies_till_first_fqs, ulong, 0644); module_param(jiffies_till_first_fqs, ulong, 0644);
module_param(jiffies_till_next_fqs, ulong, 0644); module_param(jiffies_till_next_fqs, ulong, 0644);
@ -3265,11 +3265,25 @@ static void __init rcu_init_one(struct rcu_state *rsp,
*/ */
static void __init rcu_init_geometry(void) static void __init rcu_init_geometry(void)
{ {
ulong d;
int i; int i;
int j; int j;
int n = nr_cpu_ids; int n = nr_cpu_ids;
int rcu_capacity[MAX_RCU_LVLS + 1]; int rcu_capacity[MAX_RCU_LVLS + 1];
/*
* Initialize any unspecified boot parameters.
* The default values of jiffies_till_first_fqs and
* jiffies_till_next_fqs are set to the RCU_JIFFIES_TILL_FORCE_QS
* value, which is a function of HZ, then adding one for each
* RCU_JIFFIES_FQS_DIV CPUs that might be on the system.
*/
d = RCU_JIFFIES_TILL_FORCE_QS + nr_cpu_ids / RCU_JIFFIES_FQS_DIV;
if (jiffies_till_first_fqs == ULONG_MAX)
jiffies_till_first_fqs = d;
if (jiffies_till_next_fqs == ULONG_MAX)
jiffies_till_next_fqs = d;
/* If the compile-time values are accurate, just leave. */ /* If the compile-time values are accurate, just leave. */
if (rcu_fanout_leaf == CONFIG_RCU_FANOUT_LEAF && if (rcu_fanout_leaf == CONFIG_RCU_FANOUT_LEAF &&
nr_cpu_ids == NR_CPUS) nr_cpu_ids == NR_CPUS)

15
kernel/rcutree.h
View File

@ -343,12 +343,17 @@ struct rcu_data {
#define RCU_FORCE_QS 3 /* Need to force quiescent state. */ #define RCU_FORCE_QS 3 /* Need to force quiescent state. */
#define RCU_SIGNAL_INIT RCU_SAVE_DYNTICK #define RCU_SIGNAL_INIT RCU_SAVE_DYNTICK
#define RCU_JIFFIES_TILL_FORCE_QS 3 /* for rsp->jiffies_force_qs */ #define RCU_JIFFIES_TILL_FORCE_QS (1 + (HZ > 250) + (HZ > 500))
/* For jiffies_till_first_fqs and */
/* and jiffies_till_next_fqs. */
#define RCU_STALL_RAT_DELAY 2 /* Allow other CPUs time */ #define RCU_JIFFIES_FQS_DIV 256 /* Very large systems need more */
/* to take at least one */ /* delay between bouts of */
/* scheduling clock irq */ /* quiescent-state forcing. */
/* before ratting on them. */
#define RCU_STALL_RAT_DELAY 2 /* Allow other CPUs time to take */
/* at least one scheduling clock */
/* irq before ratting on them. */
#define rcu_wait(cond) \ #define rcu_wait(cond) \
do { \ do { \