Re: [PATCH V3 RFC 1/2] sched: Bail out of yield_to when source andtarget runqueue has one task
From: Chegu Vinod
Date: Wed Nov 28 2012 - 21:21:11 EST
On 11/28/2012 5:09 PM, Chegu Vinod wrote:
On 11/27/2012 6:23 AM, Chegu Vinod wrote:
On 11/27/2012 2:30 AM, Raghavendra K T wrote:
On 11/26/2012 07:05 PM, Andrew Jones wrote:
On Mon, Nov 26, 2012 at 05:37:54PM +0530, Raghavendra K T wrote:
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
In case of undercomitted scenarios, especially in large guests
yield_to overhead is significantly high. when run queue length of
source and target is one, take an opportunity to bail out and return
-ESRCH. This return condition can be further exploited to quickly
come
out of PLE handler.
(History: Raghavendra initially worked on break out of kvm ple
handler upon
seeing source runqueue length = 1, but it had to export rq length).
Peter came up with the elegant idea of return -ESRCH in
scheduler core.
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Raghavendra, Checking the rq length of target vcpu condition
added.(thanks Avi)
Reviewed-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
---
kernel/sched/core.c | 25 +++++++++++++++++++------
1 file changed, 19 insertions(+), 6 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2d8927f..fc219a5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4289,7 +4289,10 @@ EXPORT_SYMBOL(yield);
* It's the caller's job to ensure that the target task struct
* can't go away on us before we can do any checks.
*
- * Returns true if we indeed boosted the target task.
+ * Returns:
+ * true (>0) if we indeed boosted the target task.
+ * false (0) if we failed to boost the target.
+ * -ESRCH if there's no task to yield to.
*/
bool __sched yield_to(struct task_struct *p, bool preempt)
{
@@ -4303,6 +4306,15 @@ bool __sched yield_to(struct task_struct
*p, bool preempt)
again:
p_rq = task_rq(p);
+ /*
+ * If we're the only runnable task on the rq and target rq also
+ * has only one task, there's absolutely no point in yielding.
+ */
+ if (rq->nr_running == 1 && p_rq->nr_running == 1) {
+ yielded = -ESRCH;
+ goto out_irq;
+ }
+
double_rq_lock(rq, p_rq);
while (task_rq(p) != p_rq) {
double_rq_unlock(rq, p_rq);
@@ -4310,13 +4322,13 @@ again:
}
if (!curr->sched_class->yield_to_task)
- goto out;
+ goto out_unlock;
if (curr->sched_class != p->sched_class)
- goto out;
+ goto out_unlock;
if (task_running(p_rq, p) || p->state)
- goto out;
+ goto out_unlock;
yielded = curr->sched_class->yield_to_task(rq, p, preempt);
if (yielded) {
@@ -4329,11 +4341,12 @@ again:
resched_task(p_rq->curr);
}
-out:
+out_unlock:
double_rq_unlock(rq, p_rq);
+out_irq:
local_irq_restore(flags);
- if (yielded)
+ if (yielded > 0)
schedule();
return yielded;
Acked-by: Andrew Jones <drjones@xxxxxxxxxx>
Thank you Drew.
Marcelo Gleb.. Please let me know if you have comments / concerns on
the patches..
Andrew, Vinod, IMO, the patch set looks good for undercommit scenarios
especially for large guests where we do have overhead of vcpu iteration
of ple handler..
.
Thanks Raghu. Will try to get this latest patch set evaluated and get
back to you.
Vinod
< Resending as prev. email to the kvm and lkml email aliases bounced
twice... Apologies for any repeats! >
Hi Raghu,
Here is some preliminary data with your latest set ofPLE patches (&
also with Andrew's throttled yield_to() change).
Ran a single guest on a 80 core Westmere platform. [Note: Host and
Guest had the latest kernel from kvm.git and also using the latestqemu
from qemu.git as of yesterday morning].
The guest was running a AIM7 high_systime workload. (Note:
high_systime is a kernel intensive micro-benchmark but in this case it
was run just as a workload in the guest to trigger spinlock etc.
contention in the guest OS and hence PLE (i.e. this is not a real
benchmark run). 'have run this workload with a constant # (i.e. 2000)
users with 100 jobs per user. The numbers below represent the # of
jobs per minute (JPM) -higher the better) .
40VCPU60VCPU80VCPU
a) 3.7.0-rc6+ w/ ple_gap=0~102K~88K~81K
b) 3.7.0-rc6+~53K~25K~18-20K
c) 3.7.0-rc6+ w/ PLE patches~100K~81K~48K-69K<- lot of variation from
run to run.
d) 3.7.0-rc6+ w/throttled
yield_to() change~101K~87K~78K
---
The PLE patches case (c) does show improvements in this non-overcommit
large guest case when compared to the case (b). However at 80way
started to observe quite a bit of variation from run to run and the
JPM was lower when compared with the throttled yield_to() change case (d).
For this 80way in case (c) also noticed that average time spent in the
PLE exit (as reported by a small samplings from perf kvm stat) was
varying quite a bit and was at times much greater when compared with
the case of throttled yield_to() change case (d). More details are
included below.
--
Thanks
Vinod
Case c :PLE patches(80-way)
-------------------------------
Analyze events for all VCPUs:
VM-EXITSamplesSamples%Time%Avg time
PAUSE_INSTRUCTION247814491.97%96.71%88.38us ( +-1.63% )
MSR_WRITE1593845.91%1.05%14.90us ( +-1.07% )
EXTERNAL_INTERRUPT395071.47%1.31%74.91us ( +-25.57% )
PENDING_INTERRUPT136070.50%0.12%20.60us ( +-7.64% )
HLT16730.06%0.73%985.40us ( +-1.30% )
CPUID15080.06%0.01%10.48us ( +-3.64% )
EXCEPTION_NMI5130.02%0.01%50.90us ( +-12.10% )
EPT_MISCONFIG2200.01%0.06%598.15us ( +-23.24% )
MSR_READ600.00%0.00%101.37us ( +-78.48% )
RDPMC220.00%0.00%14.30us ( +- 22.46% )
CR_ACCESS20.00%0.00%18.07us ( +-55.64% )
NMI_WINDOW10.00%0.00%6.81us ( +--nan% )
Total Samples:2694641, Total events handled time:226458587.95us.
Case d:throttled yield_to() change (80-way)
----------------------------------------------
Analyze events for all VCPUs:
VM-EXITSamplesSamples%Time%Avg time
MSR_WRITE133508034.82%0.52%5.70us ( +-0.08% )
HLT94545824.66%98.67%1513.60us ( +-1.04% )
PAUSE_INSTRUCTION79223620.66%0.42%7.66us ( +-0.18% )
EXTERNAL_INTERRUPT44680311.65%0.34%11.01us ( +-0.16% )
CPUID1589864.15%0.02%1.78us ( +-0.25% )
PENDING_INTERRUPT1111642.90%0.02%2.32us ( +-0.22% )
EXCEPTION_NMI417701.09%0.01%3.83us ( +-0.69% )
EPT_MISCONFIG 16520.04%0.00%29.02us ( +-3.56% )
MSR_READ6180.02%0.00%3.30us ( +-4.16% )
RDPMC2280.01%0.00%2.16us ( +-1.38% )
CR_ACCESS90.00%0.00%4.94us ( +-8.58% )
NMI_WINDOW80.00%0.00%1.95us ( +-4.33% )
IO_INSTRUCTION10.00%0.00%15.48us ( +--nan% )
EPT_VIOLATION10.00%0.00%752.38us ( +--nan% )
Total Samples:3834014, Total events handled time:1450387642.32us.
--
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