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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@redhat.com>
Reported-by: Marcelo Tosatti <mtosatti@redhat.com>
Tested-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2010-10-26 02:11:40 -07:00
parent db3a892099
commit 46fdb0937f
2 changed files with 22 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

15
init/Kconfig
View File

@ -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

8
kernel/srcu.c
View File

@ -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);