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SubjectRe: CFS: some bad numbers with Java/database threading [FIXED]

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

> >> have an impact) Keep CONFIG_SCHED_DEBUG=y to be able to twiddle the
> >> sysctl.
> It looks good now! Updated results here:
> http://devloop.org.uk/documentation/database-performance/Linux-Kernels/Kernels-ManyThreads-CombinedTests5-10msYield-noload.png
> http://devloop.org.uk/documentation/database-performance/Linux-Kernels/Kernels-ManyThreads-CombinedTests5-10msYield.png
> Compared with more kernels here - a bit more cluttered:
> http://devloop.org.uk/documentation/database-performance/Linux-Kernels/Kernels-ManyThreads-CombinedTests4-10msYield-noload.png
>
> Thanks Ingo!
> Does this mean that I'll have to keep doing:
> echo 1 > /proc/sys/kernel/sched_yield_bug_workaround
> Or are you planning on finding a more elegant solution?

just to make sure - can you get it to work fast with the
-rc6+yield-patch solution too? (i.e. not CFS-devel) We need a (tested)
solution for 2.6.23 and the CFS-devel patches are not for 2.6.23. I've
attached below the latest version of the -rc6 yield patch - the switch
is not dependent on SCHED_DEBUG anymore but always available.

Ingo

------------------->
Subject: sched: yield workaround
From: Ingo Molnar <mingo@elte.hu>
sched_yield() is fundamentally broken, and CFS has changed
its behavior. Some apps that mistakenly rely on sched_yield()
want "weak" sched yield (such as desktop apps) - while some
apps want "strong" sched_yield() (for example some JDKs).

There's no way for the scheduler to figure out which of the
two variants the app really wants - because sched_yield() is
all about hiding from the kernel the true structure of the
user-space locking code.

As a solution, provide a workaround, to introduce a more
agressive sched_yield implementation:

# 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
in the future, the use of this sysctl might generate
a deprecation warning, so that apps start moving away
from their reliance on sched_yield().

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
@@ -1402,10 +1402,12 @@ extern void sched_idle_next(void);

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;

Index: linux/kernel/sched_fair.c
===================================================================
--- linux.orig/kernel/sched_fair.c
+++ linux/kernel/sched_fair.c
@@ -42,6 +42,16 @@ unsigned int sysctl_sched_latency __read
*/
unsigned int sysctl_sched_min_granularity __read_mostly = 2000000ULL;

+unsigned int sysctl_sched_yield_granularity __read_mostly = 10000000ULL;
+
+/*
+ * sys_sched_yield workaround switch.
+ *
+ * This option switches the yield implementation of the
+ * old scheduler back on.
+ */
+unsigned int __read_mostly sysctl_sched_yield_bug_workaround;
+
/*
* SCHED_BATCH wake-up granularity.
* (default: 25 msec, units: nanoseconds)
@@ -901,15 +911,66 @@ 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 (curr != first)
+ next = rb_next(curr);
+ else
+ next = first;
+ /*
+ * 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),
@@ -288,6 +299,14 @@ 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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