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srcu: Specify auto-expedite holdoff time

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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@linux.vnet.ibm.com>
Tested-by: Mike Galbraith <efault@gmx.de>
This commit is contained in:
Paul E. McKenney 2017-04-25 14:03:11 -07:00
parent 2da4b2a7fd
commit 22607d66bb
3 changed files with 26 additions and 1 deletions
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8
Documentation/admin-guide/kernel-parameters.txt
View File

@ -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.

1
include/linux/srcutree.h
View File

@ -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. */

18
kernel/rcu/srcutree.c
View File

@ -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)))