Commit-ID: 46fdb0937f26124700fc9fc80da4776330cc00d3 Gitweb: http://git.kernel.org/tip/46fdb0937f26124700fc9fc80da4776330cc00d3 Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> AuthorDate: Tue, 26 Oct 2010 02:11:40 -0700 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> CommitDate: Mon, 29 Nov 2010 22:02:40 -0800 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: Avi Kivity <avi@xxxxxxxxxx> Reported-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx> Tested-by: Takuya Yoshikawa <yoshikawa.takuya@xxxxxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> --- init/Kconfig | 15 +++++++++++++++ kernel/srcu.c | 8 +++++++- 2 files changed, 22 insertions(+), 1 deletions(-) diff --git a/init/Kconfig b/init/Kconfig index 929adf6..3551824 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -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 diff --git a/kernel/srcu.c b/kernel/srcu.c index c71e075..98d8c1e 100644 --- a/kernel/srcu.c +++ b/kernel/srcu.c @@ -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); -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
|