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Commit c072a388 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: demote SRCU_SYNCHRONIZE_DELAY from kernel-parameter status 



Because the adaptive synchronize_srcu_expedited() approach has
worked very well in testing, remove the kernel parameter and
replace it by a C-preprocessor macro.  If someone finds problems
with this approach, a more complex and aggressively adaptive
approach might be required.

Longer term, SRCU will be merged with the other RCU implementations,
at which point synchronize_srcu_expedited() will be event driven,
just as synchronize_sched_expedited() currently is.  At that point,
there will be no need for this adaptive approach.

Reported-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 394f4528

init/Kconfig

+0 −15
Original line number Diff line number Diff line
@@ -497,21 +497,6 @@ 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

+13 −2
Original line number Diff line number Diff line
@@ -155,6 +155,16 @@ void __srcu_read_unlock(struct srcu_struct *sp, int idx) 
}

EXPORT_SYMBOL_GPL(__srcu_read_unlock);



/*

 * We use an adaptive strategy for synchronize_srcu() and especially for

 * synchronize_srcu_expedited().  We spin for a fixed time period

 * (defined below) to allow SRCU readers to exit their read-side critical

 * sections.  If there are still some readers after 10 microseconds,

 * we repeatedly block for 1-millisecond time periods.  This approach

 * has done well in testing, so there is no need for a config parameter.

 */

#define SYNCHRONIZE_SRCU_READER_DELAY 10



/*

 * Helper function for synchronize_srcu() and synchronize_srcu_expedited().

 */
@@ -207,11 +217,12 @@ static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void)) 
	 * 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.

	 * seconds per iteration.  The 10-microsecond value has done

	 * very well in testing.

	 */



	if (srcu_readers_active_idx(sp, idx))

		udelay(CONFIG_SRCU_SYNCHRONIZE_DELAY);

		udelay(SYNCHRONIZE_SRCU_READER_DELAY);

	while (srcu_readers_active_idx(sp, idx))

		schedule_timeout_interruptible(1);