Re: [PATCH] sched: Buggy comparison in check_preempt_tick

Mike Galbraith
2010-12-28T05:48:28Z

On Sun, 2010-12-26 at 08:23 +0100, Mike Galbraith wrote:

> But anyway..
>
> echo NO_WAKEUP_PREEMPT > sched_features
> echo NO_TESTME > sched_features
> two hogs running on isolcpu 3, pid 6890 at nice -2
>
> while sleep 1; do grep 'pert.*6890' /proc/sched_debug; done
>
> runnable tasks:
> task PID tree-key switches prio
> -------------------------------------------------------
> R pert 6890 50201.071851 7453 118
> R pert 6890 50596.171290 7513 118 +60
> R pert 6890 50991.265264 7572 118 +59
> R pert 6890 51383.781965 7631 118 +59
> pert 6890 51781.463129 7691 118 +60
>
> echo TESTME > sched_features
> pert 6890 126881.306733 18977 118
> R pert 6890 127273.825719 19036 118 +59
> R pert 6890 127668.928218 19095 118 +59
> R pert 6890 128064.031372 19154 118 +59
> R pert 6890 128459.134339 19213 118 +59
>
> ...with a compute load, the thing should be a noop, and appears to be so
> (with busted compare fixed anyway;). You have to be well overloaded for
> buddies to kick in these days, so it's probably pretty hard to get
> enough spread for the thing to fire.

I did a bit more testing yesterday with wakeup loads. There's enough
spread for the test to nudge things a few [0..4] times per second/core.

I'd either fix the comparison, and let it keep on nudging once in a
while, or whack the whole thing.

-Mike