Re: [patch] sched: yield debugging

Ingo Molnar
2007-07-31T20:35:44Z

* Tim Chen <tim.c.chen@linux.intel.com> wrote:

> Here's a summary of Volanomark performance numbers:
> Variant 0 is 80% down from 2.6.22
> Variant 1 is 20% down from 2.6.22 (this is indeed helped)
> Variant 2 is 89% down from 2.6.22

ok, good! Could you try the updated debug patch below? I've done two
changes: made '1' the default, and added the
/proc/sys/kernel/sched_yield_granularity_ns tunable. (available if
CONFIG_SCHED_DEBUG=y)

Could you try to change the yield-granularity tunable and see which
value gives the best performance? A value of '100000' should in theory
give the current (80% degraded) volanomark performance, the default
value should give the above '20% down' result. The question is, is '20%
down' the best we can get out of it? Does larger/smaller
yield-granularity help perhaps? You can change it to any value between
100 usecs and 1 second.

Ingo

------------------------------>
Subject: sched: yield debugging
From: Ingo Molnar <mingo@elte.hu>

introduce various sched_yield implementations:

# default one:
echo 0 > /proc/sys/kernel/sched_yield_bug_workaround

# always queues the current task next to the next task:
echo 1 > /proc/sys/kernel/sched_yield_bug_workaround

# NOP:
echo 2 > /proc/sys/kernel/sched_yield_bug_workaround

tunability depends on CONFIG_SCHED_DEBUG=y.

Not-yet-signed-off-by: Ingo Molnar <mingo@elte.hu>
---
include/linux/sched.h | 2 +
kernel/sched_fair.c | 72 +++++++++++++++++++++++++++++++++++++++++++++-----
kernel/sysctl.c | 19 +++++++++++++
3 files changed, 87 insertions(+), 6 deletions(-)

Index: linux/include/linux/sched.h
===================================================================
--- linux.orig/include/linux/sched.h
+++ linux/include/linux/sched.h
@@ -1397,10 +1397,12 @@ static inline void idle_task_exit(void)
extern void sched_idle_next(void);

extern unsigned int sysctl_sched_granularity;
+extern unsigned int sysctl_sched_yield_granularity;
extern unsigned int sysctl_sched_wakeup_granularity;
extern unsigned int sysctl_sched_batch_wakeup_granularity;
extern unsigned int sysctl_sched_stat_granularity;
extern unsigned int sysctl_sched_runtime_limit;
+extern unsigned int sysctl_sched_yield_bug_workaround;
extern unsigned int sysctl_sched_child_runs_first;
extern unsigned int sysctl_sched_features;

Index: linux/kernel/sched_fair.c
===================================================================
--- linux.orig/kernel/sched_fair.c
+++ linux/kernel/sched_fair.c
@@ -32,6 +32,7 @@
* systems, 4x on 8-way systems, 5x on 16-way systems, etc.)
*/
unsigned int sysctl_sched_granularity __read_mostly = 2000000000ULL/HZ;
+unsigned int sysctl_sched_yield_granularity __read_mostly = 2000000000ULL/HZ;

/*
* SCHED_BATCH wake-up granularity.
@@ -62,6 +63,16 @@ unsigned int sysctl_sched_stat_granulari
unsigned int sysctl_sched_runtime_limit __read_mostly;

/*
+ * sys_sched_yield workaround switch.
+ *
+ * This option switches the yield implementation of the
+ * old scheduler back on.
+ */
+unsigned int sysctl_sched_yield_bug_workaround __read_mostly = 1;
+
+EXPORT_SYMBOL_GPL(sysctl_sched_yield_bug_workaround);
+
+/*
* Debugging: various feature bits
*/
enum {
@@ -834,14 +845,63 @@ dequeue_task_fair(struct rq *rq, struct
static void yield_task_fair(struct rq *rq, struct task_struct *p)
{
struct cfs_rq *cfs_rq = task_cfs_rq(p);
+ struct rb_node *curr, *next, *first;
+ struct task_struct *p_next;
u64 now = __rq_clock(rq);
+ s64 yield_key;

- /*
- * Dequeue and enqueue the task to update its
- * position within the tree:
- */
- dequeue_entity(cfs_rq, &p->se, 0, now);
- enqueue_entity(cfs_rq, &p->se, 0, now);
+
+ switch (sysctl_sched_yield_bug_workaround) {
+ default:
+ /*
+ * Dequeue and enqueue the task to update its
+ * position within the tree:
+ */
+ dequeue_entity(cfs_rq, &p->se, 0, now);
+ enqueue_entity(cfs_rq, &p->se, 0, now);
+ break;
+ case 1:
+ curr = &p->se.run_node;
+ first = first_fair(cfs_rq);
+ /*
+ * Move this task to the second place in the tree:
+ */
+ if (unlikely(curr != first)) {
+ next = first;
+ } else {
+ next = rb_next(curr);
+ /*
+ * We were the last one already - nothing to do, return
+ * and reschedule:
+ */
+ if (unlikely(!next))
+ return;
+ }
+
+ p_next = rb_entry(next, struct task_struct, se.run_node);
+ /*
+ * Minimally necessary key value to be the second in the tree:
+ */
+ yield_key = p_next->se.fair_key + (int)sysctl_sched_yield_granularity;
+
+ dequeue_entity(cfs_rq, &p->se, 0, now);
+
+ /*
+ * Only update the key if we need to move more backwards
+ * than the minimally necessary position to be the second:
+ */
+ if (p->se.fair_key < yield_key)
+ p->se.fair_key = yield_key;
+
+ __enqueue_entity(cfs_rq, &p->se);
+ break;
+ case 2:
+ /*
+ * Just reschedule, do nothing else:
+ */
+ resched_task(p);
+ break;
+ }
}

/*
Index: linux/kernel/sysctl.c
===================================================================
--- linux.orig/kernel/sysctl.c
+++ linux/kernel/sysctl.c
@@ -234,6 +234,17 @@ static ctl_table kern_table[] = {
},
{
.ctl_name = CTL_UNNUMBERED,
+ .procname = "sched_yield_granularity_ns",
+ .data = &sysctl_sched_yield_granularity,
+ .maxlen = sizeof(unsigned int),
+ .mode = 0644,
+ .proc_handler = &proc_dointvec_minmax,
+ .strategy = &sysctl_intvec,
+ .extra1 = &min_sched_granularity_ns,
+ .extra2 = &max_sched_granularity_ns,
+ },
+ {
+ .ctl_name = CTL_UNNUMBERED,
.procname = "sched_wakeup_granularity_ns",
.data = &sysctl_sched_wakeup_granularity,
.maxlen = sizeof(unsigned int),
@@ -278,6 +289,14 @@ static ctl_table kern_table[] = {
},
{
.ctl_name = CTL_UNNUMBERED,
+ .procname = "sched_yield_bug_workaround",
+ .data = &sysctl_sched_yield_bug_workaround,
+ .maxlen = sizeof(unsigned int),
+ .mode = 0644,
+ .proc_handler = &proc_dointvec,
+ },
+ {
+ .ctl_name = CTL_UNNUMBERED,
.procname = "sched_child_runs_first",
.data = &sysctl_sched_child_runs_first,
.maxlen = sizeof(unsigned int),