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SubjectRe: scheduler oddity [bug?]
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On Sun, 2009-03-08 at 10:58 +0100, Mike Galbraith wrote:
> On Sun, 2009-03-08 at 10:42 +0100, Mike Galbraith wrote:
> > On Sat, 2009-03-07 at 18:47 +0100, Balazs Scheidler wrote:
> > > Hi,
> > >
> > > I'm experiencing an odd behaviour from the Linux scheduler. I have an
> > > application that feeds data to another process using a pipe. Both
> > > processes use a fair amount of CPU time apart from writing to/reading
> > > from this pipe.
> > >
> > > The machine I'm running on is an Opteron Quad-Core CPU:
> > > model name : Quad-Core AMD Opteron(tm) Processor 2347 HE
> > > stepping : 3
> > >
> > > What I see is that only one of the cores is used, the other three is
> > > idling without doing any work. If I explicitly set the CPU affinity of
> > > the processes to use distinct CPUs the performance goes up
> > > significantly. (e.g. it starts to use the other cores and the load
> > > scales linearly).
> > >
> > > I've tried to reproduce the problem by writing a small test program,
> > > which you can find attached. The program creates two processes, one
> > > feeds the other using a pipe and each does a series of memset() calls to
> > > simulate CPU load. I've also added capability to the program to set its
> > > own CPU affinity. The results (the more the better):
> > >
> > > Without enabling CPU affinity:
> > > $ ./a.out
> > > Check: 0 loops/sec, sum: 1
> > > Check: 12 loops/sec, sum: 13
> > > Check: 41 loops/sec, sum: 54
> > > Check: 41 loops/sec, sum: 95
> > > Check: 41 loops/sec, sum: 136
> > > Check: 41 loops/sec, sum: 177
> > > Check: 41 loops/sec, sum: 218
> > > Check: 40 loops/sec, sum: 258
> > > Check: 41 loops/sec, sum: 299
> > > Check: 41 loops/sec, sum: 340
> > > Check: 41 loops/sec, sum: 381
> > > Check: 41 loops/sec, sum: 422
> > > Check: 41 loops/sec, sum: 463
> > > Check: 41 loops/sec, sum: 504
> > > Check: 41 loops/sec, sum: 545
> > > Check: 40 loops/sec, sum: 585
> > > Check: 41 loops/sec, sum: 626
> > > Check: 41 loops/sec, sum: 667
> > > Check: 41 loops/sec, sum: 708
> > > Check: 41 loops/sec, sum: 749
> > > Check: 41 loops/sec, sum: 790
> > > Check: 41 loops/sec, sum: 831
> > > Final: 39 loops/sec, sum: 831
> > >
> > >
> > > With CPU affinity:
> > > # ./a.out 1
> > > Check: 0 loops/sec, sum: 1
> > > Check: 41 loops/sec, sum: 42
> > > Check: 49 loops/sec, sum: 91
> > > Check: 49 loops/sec, sum: 140
> > > Check: 49 loops/sec, sum: 189
> > > Check: 49 loops/sec, sum: 238
> > > Check: 49 loops/sec, sum: 287
> > > Check: 50 loops/sec, sum: 337
> > > Check: 49 loops/sec, sum: 386
> > > Check: 49 loops/sec, sum: 435
> > > Check: 49 loops/sec, sum: 484
> > > Check: 49 loops/sec, sum: 533
> > > Check: 49 loops/sec, sum: 582
> > > Check: 49 loops/sec, sum: 631
> > > Check: 49 loops/sec, sum: 680
> > > Check: 49 loops/sec, sum: 729
> > > Check: 49 loops/sec, sum: 778
> > > Check: 49 loops/sec, sum: 827
> > > Check: 49 loops/sec, sum: 876
> > > Check: 49 loops/sec, sum: 925
> > > Check: 50 loops/sec, sum: 975
> > > Check: 49 loops/sec, sum: 1024
> > > Final: 48 loops/sec, sum: 1024
> > >
> > > The difference is about 20%, which is about the same work performed by
> > > the slave process. If the two processes race for the same CPU this 20%
> > > of performance is lost.
> > >
> > > I've tested this on 3 computers and each showed the same symptoms:
> > > * quad core Opteron, running Ubuntu kernel 2.6.27-13.29
> > > * Core 2 Duo, running Ubuntu kernel 2.6.27-11.27
> > > * Dual Core Opteron, Debian backports.org kernel 2.6.26-13~bpo40+1
> > >
> > > Is this a bug, or a feature?
> >
> > Both. Affine wakeups are cache friendly, and generally a feature, but
> > can lead to underutilized CPUs in some cases, thus turning feature into
> > bug as your testcase demonstrates. The metric we for the affinity hint
> > works well, but clearly wants some refinement.
> >
> > You can turn this scheduler hint off via:
> > echo NO_SYNC_WAKEUPS > /sys/kernel/debug/sched_features
> >
>

(reply got munged)

> The problem with your particular testcase is that while one half has an
> avg_overlap (what we use as affinity hint for synchronous wakeups) which
> triggers the affinity hint, the other half has avg_overlap of zero, what
> it was born with, so despite significant execution overlap, the
> scheduler treats them as if they were truly synchronous tasks.
>
> The below cures it, but is only a demo hack.

diff --git a/kernel/sched.c b/kernel/sched.c
index 8e2558c..85f9ced 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1712,11 +1712,15 @@ static void enqueue_task(struct rq *rq, struct task_struct *p, int wakeup)

static void dequeue_task(struct rq *rq, struct task_struct *p, int sleep)
{
+ u64 limit = sysctl_sched_migration_cost;
+ u64 runtime = p->se.sum_exec_runtime - p->se.prev_sum_exec_runtime;
+
if (sleep && p->se.last_wakeup) {
update_avg(&p->se.avg_overlap,
p->se.sum_exec_runtime - p->se.last_wakeup);
p->se.last_wakeup = 0;
- }
+ } else if (p->se.avg_overlap < limit && runtime >= limit)
+ update_avg(&p->se.avg_overlap, runtime);

sched_info_dequeued(p);
p->sched_class->dequeue_task(rq, p, sleep);
pipetest (6701, #threads: 1)
---------------------------------------------------------
se.exec_start : 5607096.896687
se.vruntime : 274158.274352
se.sum_exec_runtime : 139434.783417
se.avg_overlap : 6.477067 <== was zero
nr_switches : 2246
nr_voluntary_switches : 1
nr_involuntary_switches : 2245
se.load.weight : 1024
policy : 0
prio : 120
clock-delta : 102
pipetest (6702, #threads: 1)
---------------------------------------------------------
se.exec_start : 5607096.896687
se.vruntime : 274098.273516
se.sum_exec_runtime : 32987.899515
se.avg_overlap : 0.502174 <== was always < migration cost
nr_switches : 13631
nr_voluntary_switches : 11639
nr_involuntary_switches : 1992
se.load.weight : 1024
policy : 0
prio : 120
clock-delta : 117



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