Commit 026ad283 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

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: default avatarPaul Mackerras <paulus@au1.ibm.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 9a5739d7

kernel/rcutree.c

+16 −2
Original line number Diff line number Diff line
@@ -218,8 +218,8 @@ module_param(blimit, long, 0444); 
module_param(qhimark, long, 0444);

module_param(qlowmark, long, 0444);



static ulong jiffies_till_first_fqs = RCU_JIFFIES_TILL_FORCE_QS;

static ulong jiffies_till_next_fqs = RCU_JIFFIES_TILL_FORCE_QS;

static ulong jiffies_till_first_fqs = ULONG_MAX;

static ulong jiffies_till_next_fqs = ULONG_MAX;



module_param(jiffies_till_first_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)

{

	ulong d;

	int i;

	int j;

	int n = nr_cpu_ids;

	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 (rcu_fanout_leaf == CONFIG_RCU_FANOUT_LEAF &&

	    nr_cpu_ids == NR_CPUS)

kernel/rcutree.h

+10 −5
Original line number Diff line number Diff line
@@ -343,12 +343,17 @@ struct rcu_data { 
#define RCU_FORCE_QS		3	/* Need to force quiescent state. */

#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 */

						/*  to take at least one */

						/*  scheduling clock irq */

						/*  before ratting on them. */

#define RCU_JIFFIES_FQS_DIV	256	/* Very large systems need more */

					/*  delay between bouts of */

					/*  quiescent-state forcing. */



#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)							\

do {									\

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