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Date: Mon, 27 Aug 2007 12:53:17 +0200
From: Ingo Molnar <mingo@...e.hu>
To: Al Boldi <a1426z@...ab.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
Mike Galbraith <efault@....de>,
Andrew Morton <akpm@...ux-foundation.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
linux-kernel@...r.kernel.org
Subject: Re: CFS review
* Al Boldi <a1426z@...ab.com> wrote:
> > could you send the exact patch that shows what you did?
>
> On 2.6.22.5-v20.3 (not v20.4):
>
> 340- curr->delta_exec += delta_exec;
> 341-
> 342- if (unlikely(curr->delta_exec > sysctl_sched_stat_granularity)) {
> 343:// __update_curr(cfs_rq, curr);
> 344- curr->delta_exec = 0;
> 345- }
> 346- curr->exec_start = rq_of(cfs_rq)->clock;
ouch - this produces a really broken scheduler - with this we dont do
any run-time accounting (!).
Could you try the patch below instead, does this make 3x glxgears smooth
again? (if yes, could you send me your Signed-off-by line as well.)
Ingo
------------------------>
Subject: sched: make the scheduler converge to the ideal latency
From: Ingo Molnar <mingo@...e.hu>
de-HZ-ification of the granularity defaults unearthed a pre-existing
property of CFS: while it correctly converges to the granularity goal,
it does not prevent run-time fluctuations in the range of [-gran ...
+gran].
With the increase of the granularity due to the removal of HZ
dependencies, this becomes visible in chew-max output (with 5 tasks
running):
out: 28 . 27. 32 | flu: 0 . 0 | ran: 9 . 13 | per: 37 . 40
out: 27 . 27. 32 | flu: 0 . 0 | ran: 17 . 13 | per: 44 . 40
out: 27 . 27. 32 | flu: 0 . 0 | ran: 9 . 13 | per: 36 . 40
out: 29 . 27. 32 | flu: 2 . 0 | ran: 17 . 13 | per: 46 . 40
out: 28 . 27. 32 | flu: 0 . 0 | ran: 9 . 13 | per: 37 . 40
out: 29 . 27. 32 | flu: 0 . 0 | ran: 18 . 13 | per: 47 . 40
out: 28 . 27. 32 | flu: 0 . 0 | ran: 9 . 13 | per: 37 . 40
average slice is the ideal 13 msecs and the period is picture-perfect 40
msecs. But the 'ran' field fluctuates around 13.33 msecs and there's no
mechanism in CFS to keep that from happening: it's a perfectly valid
solution that CFS finds.
the solution is to add a granularity/preemption rule that knows about
the "target latency", which makes tasks that run longer than the ideal
latency run a bit less. The simplest approach is to simply decrease the
preemption granularity when a task overruns its ideal latency. For this
we have to track how much the task executed since its last preemption.
( this adds a new field to task_struct, but we can eliminate that
overhead in 2.6.24 by putting all the scheduler timestamps into an
anonymous union. )
with this change in place, chew-max output is fluctuation-less all
around:
out: 28 . 27. 39 | flu: 0 . 2 | ran: 13 . 13 | per: 41 . 40
out: 28 . 27. 39 | flu: 0 . 2 | ran: 13 . 13 | per: 41 . 40
out: 28 . 27. 39 | flu: 0 . 2 | ran: 13 . 13 | per: 41 . 40
out: 28 . 27. 39 | flu: 0 . 2 | ran: 13 . 13 | per: 41 . 40
out: 28 . 27. 39 | flu: 0 . 1 | ran: 13 . 13 | per: 41 . 40
out: 28 . 27. 39 | flu: 0 . 1 | ran: 13 . 13 | per: 41 . 40
this patch has no impact on any fastpath or on any globally observable
scheduling property. (unless you have sharp enough eyes to see
millisecond-level ruckles in glxgears smoothness :-)
Also, with this mechanism in place the formula for adaptive granularity
can be simplified down to the obvious "granularity = latency/nr_running"
calculation.
Signed-off-by: Ingo Molnar <mingo@...e.hu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
---
include/linux/sched.h | 1 +
kernel/sched_fair.c | 43 ++++++++++++++-----------------------------
2 files changed, 15 insertions(+), 29 deletions(-)
Index: linux/include/linux/sched.h
===================================================================
--- linux.orig/include/linux/sched.h
+++ linux/include/linux/sched.h
@@ -904,6 +904,7 @@ struct sched_entity {
u64 exec_start;
u64 sum_exec_runtime;
+ u64 prev_sum_exec_runtime;
u64 wait_start_fair;
u64 sleep_start_fair;
Index: linux/kernel/sched_fair.c
===================================================================
--- linux.orig/kernel/sched_fair.c
+++ linux/kernel/sched_fair.c
@@ -225,30 +225,6 @@ static struct sched_entity *__pick_next_
* Calculate the preemption granularity needed to schedule every
* runnable task once per sysctl_sched_latency amount of time.
* (down to a sensible low limit on granularity)
- *
- * For example, if there are 2 tasks running and latency is 10 msecs,
- * we switch tasks every 5 msecs. If we have 3 tasks running, we have
- * to switch tasks every 3.33 msecs to get a 10 msecs observed latency
- * for each task. We do finer and finer scheduling up to until we
- * reach the minimum granularity value.
- *
- * To achieve this we use the following dynamic-granularity rule:
- *
- * gran = lat/nr - lat/nr/nr
- *
- * This comes out of the following equations:
- *
- * kA1 + gran = kB1
- * kB2 + gran = kA2
- * kA2 = kA1
- * kB2 = kB1 - d + d/nr
- * lat = d * nr
- *
- * Where 'k' is key, 'A' is task A (waiting), 'B' is task B (running),
- * '1' is start of time, '2' is end of time, 'd' is delay between
- * 1 and 2 (during which task B was running), 'nr' is number of tasks
- * running, 'lat' is the the period of each task. ('lat' is the
- * sched_latency that we aim for.)
*/
static long
sched_granularity(struct cfs_rq *cfs_rq)
@@ -257,7 +233,7 @@ sched_granularity(struct cfs_rq *cfs_rq)
unsigned int nr = cfs_rq->nr_running;
if (nr > 1) {
- gran = gran/nr - gran/nr/nr;
+ gran = gran/nr;
gran = max(gran, sysctl_sched_min_granularity);
}
@@ -668,7 +644,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, st
/*
* Preempt the current task with a newly woken task if needed:
*/
-static void
+static int
__check_preempt_curr_fair(struct cfs_rq *cfs_rq, struct sched_entity *se,
struct sched_entity *curr, unsigned long granularity)
{
@@ -679,8 +655,11 @@ __check_preempt_curr_fair(struct cfs_rq
* preempt the current task unless the best task has
* a larger than sched_granularity fairness advantage:
*/
- if (__delta > niced_granularity(curr, granularity))
+ if (__delta > niced_granularity(curr, granularity)) {
resched_task(rq_of(cfs_rq)->curr);
+ return 1;
+ }
+ return 0;
}
static inline void
@@ -725,6 +704,7 @@ static void put_prev_entity(struct cfs_r
static void entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
{
+ unsigned long gran, delta_exec;
struct sched_entity *next;
/*
@@ -741,8 +721,13 @@ static void entity_tick(struct cfs_rq *c
if (next == curr)
return;
- __check_preempt_curr_fair(cfs_rq, next, curr,
- sched_granularity(cfs_rq));
+ gran = sched_granularity(cfs_rq);
+ delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
+ if (delta_exec > gran)
+ gran = 0;
+
+ if (__check_preempt_curr_fair(cfs_rq, next, curr, gran))
+ curr->prev_sum_exec_runtime = curr->sum_exec_runtime;
}
/**************************************************
-
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