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SubjectRe: CFS: some bad numbers with Java/database threading

* Antoine Martin <antoine@nagafix.co.uk> wrote:

> Basically, all the previous kernels are pretty close (2.6.16 through
> to 2.6.20 performed almost identically to 2.6.22 and are not shown
> here to avoid cluttering the graphs)
>
> All the 2.6.23-rc kernels performed poorly (except -rc3!): much more
> erratically and with a sharp performance drop above 800 threads. The
> load starts to go up and the performance takes a nosedive.

hm, could you try the patch below ontop of 2.6.23-rc6 and do:

echo 1 > /proc/sys/kernel/sched_yield_bug_workaround
does this improve the numbers?

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 | 73 +++++++++++++++++++++++++++++++++++++++++++++-----
kernel/sysctl.c | 19 +++++++++++++
3 files changed, 88 insertions(+), 6 deletions(-)
Index: linux/include/linux/sched.h
===================================================================
--- linux.orig/include/linux/sched.h
+++ linux/include/linux/sched.h
@@ -1392,10 +1392,12 @@ extern void sched_idle_next(void);
#ifdef CONFIG_SCHED_DEBUG
extern unsigned int sysctl_sched_latency;
extern unsigned int sysctl_sched_min_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;
#endif
Index: linux/kernel/sched_fair.c
===================================================================
--- linux.orig/kernel/sched_fair.c
+++ linux/kernel/sched_fair.c
@@ -42,6 +42,16 @@ const_debug unsigned int sysctl_sched_la
*/
const_debug unsigned int sysctl_sched_child_runs_first = 1;

+const_debug unsigned int sysctl_sched_yield_granularity = 10000000ULL;
+
+/*
+ * sys_sched_yield workaround switch.
+ *
+ * This option switches the yield implementation of the
+ * old scheduler back on.
+ */
+const_debug unsigned int sysctl_sched_yield_bug_workaround;
+
/*
* Minimal preemption granularity for CPU-bound tasks:
* (default: 2 msec, units: nanoseconds)
@@ -675,15 +685,66 @@ static void dequeue_task_fair(struct rq
*/
static void yield_task_fair(struct rq *rq, struct task_struct *p)
{
- struct cfs_rq *cfs_rq = task_cfs_rq(p);
+ if (!sysctl_sched_yield_bug_workaround) {
+ struct cfs_rq *cfs_rq = task_cfs_rq(p);
+ __update_rq_clock(rq);
+
+ /*
+ * Dequeue and enqueue the task to update its
+ * position within the tree:
+ */
+ dequeue_entity(cfs_rq, &p->se, 0);
+ enqueue_entity(cfs_rq, &p->se, 0);
+ return;
+ }
+
+ if (sysctl_sched_yield_bug_workaround == 1) {
+ struct cfs_rq *cfs_rq = task_cfs_rq(p);
+ struct rb_node *curr, *next, *first;
+ struct task_struct *p_next;
+ s64 yield_key;
+
+ __update_rq_clock(rq);
+ 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);
+
+ /*
+ * 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);
+ return;
+ }

- __update_rq_clock(rq);
/*
- * Dequeue and enqueue the task to update its
- * position within the tree:
+ * Just reschedule, do nothing else:
*/
- dequeue_entity(cfs_rq, &p->se, 0);
- enqueue_entity(cfs_rq, &p->se, 0);
+ resched_task(p);
}

/*
Index: linux/kernel/sysctl.c
===================================================================
--- linux.orig/kernel/sysctl.c
+++ linux/kernel/sysctl.c
@@ -244,6 +244,17 @@ 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),
@@ -266,6 +277,14 @@ 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),
-
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