Re: sched_yield() call on Linux Kernel 2.6.39 is not behaving correct
From: Yogesh Ahire
Date: Thu Jan 08 2015 - 10:00:53 EST
Thank you Mike. But I can see there are tasks with same priority and
are runnable waiting for CPU, following is the output of
/proc/sched_debug where you can see that the task "symphonyapp"
continuously calls sched_yield() but there are other tasks which are
ready to run are not getting CPU and are queued back.
cpu#4, 2660.325 MHz
.nr_running : 133
.load : 135278
.nr_switches : 3032774521
.nr_load_updates : 4859055152
.nr_uninterruptible : 0
.next_balance : 9153.935253
.curr->pid : 2392
.clock : 1373656869.013078
.cpu_load[0] : 135278
.cpu_load[1] : 116170
.cpu_load[2] : 104329
.cpu_load[3] : 97683
.cpu_load[4] : 93432
.yld_count : 503461840
.sched_switch : 0
.sched_count : -917907006
.sched_goidle : 4735
.avg_idle : 672352
.ttwu_count : 1516392423
.ttwu_local : 1516377141
.bkl_count : 0
cfs_rq[4]:
.exec_clock : 1373608176.832115
.MIN_vruntime : 18008522836.670893
.min_vruntime : 18008522848.670893
.max_vruntime : 18008522959.874760
.spread : 123.203867
.spread0 : 17010031797.944969
.nr_spread_over : -676561100
.nr_running : 133
.load : 135278
rt_rq[4]:
.rt_nr_running : 0
.rt_throttled : 0
.rt_time : 0.000000
.rt_runtime : 950.000000
runnable tasks:
task PID tree-key switches prio
exec-runtime sum-exec sum-sleep
----------------------------------------------------------------------------------------------------------
kworker/4:1 224 18008522836.670893 1516377074 120
18008522836.670893 5565718.111520 18008565537.968010
taskmonitor 10602 18008522860.665039 0 120
18008522860.665039 0.000000 0.000000
taskmonitor 11695 18008522860.665039 0 120
18008522860.665039 0.000000 0.000000
taskmonitor 19704 18008522860.665039 0 120
18008522860.665039 0.000000 0.000000
taskmonitor 25757 18008522860.665039 0 120
18008522860.665039 0.000000 0.000000
taskmonitor 31287 18008522860.665039 0 120
18008522860.665039 0.000000 0.000000
taskmonitor 1510 18008522860.665039 0 120
18008522860.665039 0.000000 0.000000
taskmonitor 14921 18008522860.665039 0 120
18008522860.665039 0.000000 0.000000
taskmonitor 15377 18008522860.665039 0 120
18008522860.665039 0.000000 0.000000
taskmonitor 15707 18008522860.665039 0 120
18008522860.665039 0.000000 0.000000
taskmonitor 16032 18008522860.665039 0 120
18008522860.665039 0.000000 0.000000
taskmonitor 16629 18008522860.665039 0 120
18008522860.665039 0.000000 0.000000
taskmonitor 16695 18008522860.665039 0 120
18008522860.665039 0.000000 0.000000
R symphonyapp 2392 1368041892.701540 1516377057 120
1368041892.702799 1368041445.291337 31.536059
ntpd 9806 18008522856.668288 0 120
18008522856.668288 0.000000 0.000000
crond 11577 18008522856.668288 0 120
18008522856.668288 0.000000 0.000000
crond 15837 18008522856.668288 0 120
18008522856.668288 0.000000 0.000000
crond 20093 18008522856.668288 0 120
18008522856.668288 0.000000 0.000000
ice_ai_pca 22944 18008522856.668288 0 120
18008522856.668288 0.000000 0.000000
crond 24137 18008522856.668288 0 120
18008522856.668288 0.000000 0.000000
crond 27732 18008522856.668288 0 120
18008522856.668288 0.000000 0.000000
crond 31330 18008522856.668288 0 120
18008522856.668288 0.000000 0.000000
crond 2557 18008522856.668288 0 120
18008522856.668288 0.000000 0.000000
crond 6175 18008522856.668288 0 120
18008522856.668288 0.000000 0.000000
crond 9775 18008522856.668288 0 120
18008522856.668288 0.000000 0.000000
crond 13374 18008522856.668288 0 120
18008522856.668288 0.000000 0.000000
crond 16975 18008522856.668288 0 120
18008522856.668288 0.000000 0.000000
Thanks
Yogesh
On Thu, Jan 8, 2015 at 12:26 AM, Mike Galbraith
<umgwanakikbuti@xxxxxxxxx> wrote:
> On Wed, 2015-01-07 at 16:30 -0500, Yogesh Ahire wrote:
>> Hi All,
>>
>> I have a system with multiple CPU cores. I have multiple threads
>> assigned to particular CPU. Among these threads the main thread calls
>> sched_yield() if it has nothing to do, I am hoping that doing so will
>> give chance to other threads to run. But the strange behavior of
>> sched_yield() is , even if there are ready-to-runs tasks on this CPU
>> waiting for their turn, the task which calls sched_yield() is always
>> running ( get scheduled) and not giving chance to any other task to
>> run. It is consuming 100% of CPU. Is sched_yield() is broken on 2.6
>> Kernel?
>
> Nope, your expectation is likely busted. sched_yield() for a fair class
> task is merely a resched check in CFS. IFF there's a runnable task
> that's more deserving by the CFS definition thereof, it'll initiate a
> context switch, otherwise it's a non-free noop. For realtime class
> tasks, behavior is predictable, the scheduler WILL initiate a context
> switch IFF there is a runnable task of >= yielding tasks priority on the
> same CPU. If you depend on a context switch happening in any other
> circumstance, you're gonna be seriously disappointed.
>
> -Mike
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/