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SubjectRe: [PATCH v2] sched: add a tuning knob to allow changing RR tmeslice
On Fri, 25 Jan 2013 09:19:16 +0100
Ingo Molnar <mingo@kernel.org> wrote:

>
> * Clark Williams <williams@redhat.com> wrote:
>
> > On Thu, 24 Jan 2013 17:59:55 +0100
> > Ingo Molnar <mingo@kernel.org> wrote:
> >
> > >
> > > * Clark Williams <williams@redhat.com> wrote:
> > >
> > > > This version stores the user-input value in a separate
> > > > location from the jiffies values used by the scheduler, to
> > > > prevent a race condition.
> > > >
> > > > Subject: [PATCH v2] sched: add a tuning knob to allow changing
> > > > RR timeslice
> > >
> > > looks useful.
> > >
> > > > @@ -2010,7 +2010,7 @@ static void task_tick_rt(struct rq *rq, struct
> > > > task_struct *p, int queued) if (--p->rt.time_slice)
> > > > return;
> > > >
> > > > - p->rt.time_slice = RR_TIMESLICE;
> > > > + p->rt.time_slice = sched_rr_timeslice;
> > > >
> > > > /*
> > > > * Requeue to the end of queue if we (and all of our
> > > > ancestors) are the @@ -2041,7 +2041,7 @@ static unsigned int
> > > > get_rr_interval_rt(struct rq *rq, struct task_struct *task)
> > > > * Time slice is 0 for SCHED_FIFO tasks
> > >
> > > Patch wont apply due to patch corruption, alas.
> > >
> > >
> >
> > Easily fixed. Modified for 3.8-rc4:
>
> Thanks. Some more substantial review feedback this time around:
>
> >
> > commit 0e2d40c5c84d06670f85cc212591f27f69f59c62
> > Author: Clark Williams <williams@redhat.com>
> > Date: Thu Jan 24 13:51:01 2013 -0600
> >
> > [kernel] sched: add a tuning knob to allow changing RR timeslice
> >
> > Add a /proc/sys/kernel scheduler knob named sched_rr_timeslice_ms
>
> s/Add a /proc/sys/kernel/sched_rr_timeslice_ms scheduler knob
>
> > that allows global changing of the SCHED_RR timeslice value. User
> > visable value is in milliseconds but is stored as jiffies. Setting
>
> s/visible
>
> > to 0 (zero) resets to the default (currently 100ms).
> >
> > Signed-off-by: Clark Williams <williams@redhat.com>
> >
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 6fc8f45..d803690 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -2082,6 +2082,11 @@ int sched_rt_handler(struct ctl_table *table, int write,
> > void __user *buffer, size_t *lenp,
> > loff_t *ppos);
> >
> > +extern int sched_rr_timeslice;
> > +extern int sched_rr_handler(struct ctl_table *table, int write,
> > + void __user *buffer, size_t *lenp,
> > + loff_t *ppos);
> > +
>
> Shouldn't this be in kernel/sched/sched.h instead of the
> (already too large) linux/sched.h?
>

I don't think that will work be cause kernel/sysctl.c needs the
externs and I doubt we want to include kernel/sched/sched.h from there.

> > #ifdef CONFIG_SCHED_AUTOGROUP
> > extern unsigned int sysctl_sched_autogroup_enabled;
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 257002c..5675074 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -7507,6 +7507,24 @@ static int sched_rt_global_constraints(void)
> > }
> > #endif /* CONFIG_RT_GROUP_SCHED */
> >
> > +int sched_rr_handler(struct ctl_table *table, int write,
> > + void __user *buffer, size_t *lenp,
> > + loff_t *ppos)
> > +{
> > + int ret;
> > + static DEFINE_MUTEX(mutex);
>
> This mutex should be outside the function (not in local scope),
> and named like this, with an explanation:
>
> +/*
> + * Since it's an int the CPU will always read a full word
> + * of the RR timeslice interval - no need for locking.
> + *
> + * But in the RR handler we read the value multiple times
> + * before setting it, which should be protected - hence
> + * this mutex:
> + */
> +static DEFINE_MUTEX(rr_timeslice_mutex);
>

Done.

>
> > +
> > + mutex_lock(&mutex);
> > + ret = proc_dointvec(table, write, buffer, lenp, ppos);
> > + /* make sure that internally we keep jiffies */
> > + /* also, writing zero resets timeslice to default */
> > + if (!ret && write)
> > + sched_rr_timeslice = sched_rr_timeslice <= 0 ?
> > + RR_TIMESLICE : msecs_to_jiffies(sched_rr_timeslice);
> > + mutex_unlock(&mutex);
> > + return ret;
>
> A couple of stray spaces at end of lines. Also, please put curly
> braces around multi-line statements.
>

I didn't put curly braces there because "technically" that's a single
line statement. But since in fact it is multi-line, I've added them :).

> Multi-line comments should be like this:
>
> /*
> * Comment .....
> * ...... goes here.
> */
>

Done.

>
> > +}
> > +
> > int sched_rt_handler(struct ctl_table *table, int write,
> > void __user *buffer, size_t *lenp,
> > loff_t *ppos)
> > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> > index 418feb0..6c54e83 100644
> > --- a/kernel/sched/rt.c
> > +++ b/kernel/sched/rt.c
> > @@ -11,6 +11,8 @@ static int do_sched_rt_period_timer(struct rt_bandwidth *rt_b, int overrun);
> >
> > struct rt_bandwidth def_rt_bandwidth;
> >
> > +int sched_rr_timeslice = RR_TIMESLICE;
> > +
>
> I think this could go into core.c as well, together with the
> mutex. That way it's easier to see why the mutex is needed as
> well.
>
> It should also be __read_mostly.

Done and done.

>
> > static enum hrtimer_restart sched_rt_period_timer(struct hrtimer *timer)
> > {
> > struct rt_bandwidth *rt_b =
> > @@ -2010,7 +2012,7 @@ static void task_tick_rt(struct rq *rq, struct task_struct *p, int queued)
> > if (--p->rt.time_slice)
> > return;
> >
> > - p->rt.time_slice = RR_TIMESLICE;
> > + p->rt.time_slice = sched_rr_timeslice;
> >
> > /*
> > * Requeue to the end of queue if we (and all of our ancestors) are the
> > @@ -2041,7 +2043,7 @@ static unsigned int get_rr_interval_rt(struct rq *rq, struct task_struct *task)
> > * Time slice is 0 for SCHED_FIFO tasks
> > */
> > if (task->policy == SCHED_RR)
> > - return RR_TIMESLICE;
> > + return sched_rr_timeslice;
> > else
> > return 0;
> > }
> > diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> > index c88878d..1eabf86 100644
> > --- a/kernel/sysctl.c
> > +++ b/kernel/sysctl.c
> > @@ -403,6 +403,14 @@ static struct ctl_table kern_table[] = {
> > .mode = 0644,
> > .proc_handler = sched_rt_handler,
> > },
> > + {
> > + .procname = "sched_rr_timeslice_ms",
> > + .data = &sched_rr_timeslice,
> > + .maxlen = sizeof(int),
> > + .mode = 0644,
> > + .proc_handler = sched_rr_handler,
>
> Does this allow negative values? Wouldn't it be better to make
> it unsigned int all around?

Good point. Changed to unsigned int.

Updated patch:

commit 93dfbf6326cc4ba85c917f9440203f9fc19e9bcc
Author: Clark Williams <williams@redhat.com>
Date: Thu Jan 24 13:51:01 2013 -0600

[kernel] sched: add a tuning knob to allow changing RR timeslice

Add a /proc/sys/kernel/sched_rr_timeslice_ms tuning knob
that allows global changing of the SCHED_RR timeslice value. User
visible value is in milliseconds but is stored as jiffies. Setting
to 0 (zero) resets to the default (currently 100ms).

Signed-off-by: Clark Williams <williams@redhat.com>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 6fc8f45..5d0a7cf 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1226,6 +1226,11 @@ struct sched_rt_entity {
*/
#define RR_TIMESLICE (100 * HZ / 1000)

+extern __read_mostly unsigned int sched_rr_timeslice;
+
+extern int sched_rr_handler(struct ctl_table *table, int write,
+ void __user *buffer, size_t *lenp,
+ loff_t *ppos);
struct rcu_node;

enum perf_event_task_context {
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 257002c..0f7e6a2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7507,6 +7507,43 @@ static int sched_rt_global_constraints(void)
}
#endif /* CONFIG_RT_GROUP_SCHED */

+/*
+ * Since it's an int the CPU will always read a full word
+ * of the RR timeslice interval - no need for locking.
+ *
+ * But in the RR handler we read the value multiple times
+ * before setting it, which should be protected - hence
+ * this mutex:
+ */
+static DEFINE_MUTEX(rr_timeslice_mutex);
+
+unsigned int __read_mostly sched_rr_timeslice = RR_TIMESLICE;
+
+/*
+ * manage /proc/sys/kernel/sched_rr_timeslice_ms entry
+ * for changing SCHED_RR quantum interval
+ */
+
+int sched_rr_handler(struct ctl_table *table, int write,
+ void __user *buffer, size_t *lenp,
+ loff_t *ppos)
+{
+ int ret;
+
+ mutex_lock(&rr_timeslice_mutex);
+ ret = proc_dointvec(table, write, buffer, lenp, ppos);
+ /*
+ * make sure that internally we keep jiffies
+ * also, writing zero resets timeslice to default
+ */
+ if (!ret && write) {
+ sched_rr_timeslice = sched_rr_timeslice == 0 ?
+ RR_TIMESLICE : msecs_to_jiffies(sched_rr_timeslice);
+ }
+ mutex_unlock(&rr_timeslice_mutex);
+ return ret;
+}
+
int sched_rt_handler(struct ctl_table *table, int write,
void __user *buffer, size_t *lenp,
loff_t *ppos)
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 418feb0..71aa6d0 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2010,7 +2010,7 @@ static void task_tick_rt(struct rq *rq, struct task_struct *p, int queued)
if (--p->rt.time_slice)
return;

- p->rt.time_slice = RR_TIMESLICE;
+ p->rt.time_slice = sched_rr_timeslice;

/*
* Requeue to the end of queue if we (and all of our ancestors) are the
@@ -2041,7 +2041,7 @@ static unsigned int get_rr_interval_rt(struct rq *rq, struct task_struct *task)
* Time slice is 0 for SCHED_FIFO tasks
*/
if (task->policy == SCHED_RR)
- return RR_TIMESLICE;
+ return sched_rr_timeslice;
else
return 0;
}
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index c88878d..6770fc8 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -403,6 +403,14 @@ static struct ctl_table kern_table[] = {
.mode = 0644,
.proc_handler = sched_rt_handler,
},
+ {
+ .procname = "sched_rr_timeslice_ms",
+ .data = &sched_rr_timeslice,
+ .maxlen = sizeof(unsigned int),
+ .mode = 0644,
+ .proc_handler = sched_rr_handler,
+ },
+
#ifdef CONFIG_SCHED_AUTOGROUP
{
.procname = "sched_autogroup_enabled",


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