Commit-ID: 22607d66bbc3e81140d3bcf08894f4378eb36428
Gitweb: http://git.kernel.org/tip/22607d66bbc3e81140d3bcf08894f4378eb36428
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
AuthorDate: Tue, 25 Apr 2017 14:03:11 -0700
Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CommitDate: Wed, 26 Apr 2017 16:32:17 -0700
srcu: Specify auto-expedite holdoff time
On small systems, in the absence of readers, expedited SRCU grace
periods can complete in less than a microsecond. This means that an
eight-CPU system can have all CPUs doing synchronize_srcu() in a tight
loop and almost always expedite. This might actually be desirable in
some situations, but in general it is a good way to needlessly burn
CPU cycles. And in those situations where it is desirable, your friend
is the function synchronize_srcu_expedited().
For other situations, this commit adds a kernel parameter that specifies
a holdoff between completing the last SRCU grace period and auto-expediting
the next. If the next grace period starts before the holdoff expires,
auto-expediting is disabled. The holdoff is 50 microseconds by default,
and can be tuned to the desired number of nanoseconds. A value of zero
disables auto-expediting.
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Tested-by: Mike Galbraith <efault@xxxxxx>
---
Documentation/admin-guide/kernel-parameters.txt | 8 ++++++++
include/linux/srcutree.h | 1 +
kernel/rcu/srcutree.c | 18 +++++++++++++++++-
3 files changed, 26 insertions(+), 1 deletion(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index facc20a..4a4b926 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3779,6 +3779,14 @@
spia_pedr=
spia_peddr=
+ srcutree.exp_holdoff [KNL]
+ Specifies how many nanoseconds must elapse
+ since the end of the last SRCU grace period for
+ a given srcu_struct until the next normal SRCU
+ grace period will be considered for automatic
+ expediting. Set to zero to disable automatic
+ expediting.
+
stacktrace [FTRACE]
Enabled the stack tracer on boot up.
diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
index 86df48d..32e86d8 100644
--- a/include/linux/srcutree.h
+++ b/include/linux/srcutree.h
@@ -84,6 +84,7 @@ struct srcu_struct {
unsigned long srcu_gp_seq; /* Grace-period seq #. */
unsigned long srcu_gp_seq_needed; /* Latest gp_seq needed. */
unsigned long srcu_gp_seq_needed_exp; /* Furthest future exp GP. */
+ unsigned long srcu_last_gp_end; /* Last GP end timestamp (ns) */
struct srcu_data __percpu *sda; /* Per-CPU srcu_data array. */
unsigned long srcu_barrier_seq; /* srcu_barrier seq #. */
struct mutex srcu_barrier_mutex; /* Serialize barrier ops. */
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 2286e06..74c283f 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -34,10 +34,14 @@
#include <linux/sched.h>
#include <linux/smp.h>
#include <linux/delay.h>
+#include <linux/module.h>
#include <linux/srcu.h>
#include "rcu.h"
+ulong exp_holdoff = 50 * 1000; /* Holdoff (ns) for auto-expediting. */
+module_param(exp_holdoff, ulong, 0444);
+
static void srcu_invoke_callbacks(struct work_struct *work);
static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay);
@@ -145,6 +149,7 @@ static int init_srcu_struct_fields(struct srcu_struct *sp, bool is_static)
sp->sda = alloc_percpu(struct srcu_data);
init_srcu_struct_nodes(sp, is_static);
sp->srcu_gp_seq_needed_exp = 0;
+ sp->srcu_last_gp_end = ktime_get_mono_fast_ns();
smp_store_release(&sp->srcu_gp_seq_needed, 0); /* Init done. */
return sp->sda ? 0 : -ENOMEM;
}
@@ -498,6 +503,7 @@ static void srcu_gp_end(struct srcu_struct *sp)
idx = rcu_seq_state(sp->srcu_gp_seq);
WARN_ON_ONCE(idx != SRCU_STATE_SCAN2);
cbdelay = srcu_get_delay(sp);
+ sp->srcu_last_gp_end = ktime_get_mono_fast_ns();
rcu_seq_end(&sp->srcu_gp_seq);
gpseq = rcu_seq_current(&sp->srcu_gp_seq);
if (ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, gpseq))
@@ -700,9 +706,10 @@ static void srcu_flip(struct srcu_struct *sp)
*/
static bool srcu_might_be_idle(struct srcu_struct *sp)
{
+ unsigned long curseq;
unsigned long flags;
struct srcu_data *sdp;
- unsigned long curseq;
+ unsigned long t;
/* If the local srcu_data structure has callbacks, not idle. */
local_irq_save(flags);
@@ -718,6 +725,15 @@ static bool srcu_might_be_idle(struct srcu_struct *sp)
* Exact information would require acquiring locks, which would
* kill scalability, hence the probabalistic nature of the probe.
*/
+
+ /* First, see if enough time has passed since the last GP. */
+ t = ktime_get_mono_fast_ns();
+ if (exp_holdoff == 0 ||
+ time_in_range_open(t, sp->srcu_last_gp_end,
+ sp->srcu_last_gp_end + exp_holdoff))
+ return false; /* Too soon after last GP. */
+
+ /* Next, check for probable idleness. */
curseq = rcu_seq_current(&sp->srcu_gp_seq);
smp_mb(); /* Order ->srcu_gp_seq with ->srcu_gp_seq_needed. */
if (ULONG_CMP_LT(curseq, READ_ONCE(sp->srcu_gp_seq_needed)))
--
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