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Commit 46fdb093 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Make synchronize_srcu_expedited() fast if running readers 



The synchronize_srcu_expedited() function is currently quick if there
are no active readers, but will delay a full jiffy if there are any.
If these readers leave their SRCU read-side critical sections quickly,
this is way too long to wait.  So this commit first waits ten microseconds,
and only then falls back to jiffy-at-a-time waiting.

Reported-by: default avatarAvi Kivity <avi@redhat.com>
Reported-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
Tested-by: default avatarTakuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent db3a8920

init/Kconfig

+15 −0
Original line number Diff line number Diff line
@@ -488,6 +488,21 @@ config RCU_BOOST_DELAY 


	  Accept the default if unsure.



config SRCU_SYNCHRONIZE_DELAY

	int "Microseconds to delay before waiting for readers"

	range 0 20

	default 10

	help

	  This option controls how long SRCU delays before entering its

	  loop waiting on SRCU readers.  The purpose of this loop is

	  to avoid the unconditional context-switch penalty that would

	  otherwise be incurred if there was an active SRCU reader,

	  in a manner similar to adaptive locking schemes.  This should

	  be set to be a bit longer than the common-case SRCU read-side

	  critical-section overhead.



	  Accept the default if unsure.



endmenu # "RCU Subsystem"



config IKCONFIG

kernel/srcu.c

+7 −1
Original line number Diff line number Diff line
@@ -31,6 +31,7 @@ 
#include <linux/rcupdate.h>

#include <linux/sched.h>

#include <linux/smp.h>

#include <linux/delay.h>

#include <linux/srcu.h>



static int init_srcu_struct_fields(struct srcu_struct *sp)
@@ -203,9 +204,14 @@ static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void)) 
	 * all srcu_read_lock() calls using the old counters have completed.

	 * Their corresponding critical sections might well be still

	 * executing, but the srcu_read_lock() primitives themselves

	 * will have finished executing.

	 * will have finished executing.  We initially give readers

	 * an arbitrarily chosen 10 microseconds to get out of their

	 * SRCU read-side critical sections, then loop waiting 1/HZ

	 * seconds per iteration.

	 */



	if (srcu_readers_active_idx(sp, idx))

		udelay(CONFIG_SRCU_SYNCHRONIZE_DELAY);

	while (srcu_readers_active_idx(sp, idx))

		schedule_timeout_interruptible(1);