Tim,
* Tim Chen <[email protected]> wrote:
> Ingo,
>
> Volanomark slows by 80% with CFS scheduler on 2.6.23-rc1. Benchmark
> was run on a 2 socket Core2 machine.
thanks for testing and reporting this!
> The change in scheduler treatment of sched_yield could play a part in
> changing Volanomark behavior.
Could you try the patch below? It does not change the default behavior
of yield but introduces 2 other yield strategies which you can activate
runtime (if CONFIG_SCHED_DEBUG=y) via:
# 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
does variant '1' improve Java's VolanoMark performance perhaps?
i'm also wondering, which JDK is this, and where does Java make use of
sys_sched_yield()? It's a voefully badly defined (and thus unreliable)
system call, IMO Java should stop using it ASAP and use a saner locking
model.
thanks,
Ingo
------------------------------->
Subject: sched: yield debugging
From: Ingo Molnar <[email protected]>
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 <[email protected]>
---
include/linux/sched.h | 1
kernel/sched_fair.c | 71 +++++++++++++++++++++++++++++++++++++++++++++-----
kernel/sysctl.c | 8 +++++
3 files changed, 74 insertions(+), 6 deletions(-)
Index: linux/include/linux/sched.h
===================================================================
--- linux.orig/include/linux/sched.h
+++ linux/include/linux/sched.h
@@ -1401,6 +1401,7 @@ extern unsigned int sysctl_sched_wakeup_
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
@@ -62,6 +62,16 @@ 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 = 0;
+
+EXPORT_SYMBOL_GPL(sysctl_sched_yield_bug_workaround);
+
+/*
* Debugging: various feature bits
*/
enum {
@@ -834,14 +844,63 @@ 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_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
@@ -278,6 +278,14 @@ 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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