[PATCH] Re: [PATCH] sched: Buggy comparison in check_preempt_tick

Mike Galbraith
2011-01-05T04:41:29Z

Going through my mailbox, I see this remains unaddressed. I chose the
keep it option, but whack it and revisit later is also viable.

On Tue, 2010-12-28 at 06:48 +0100, Mike Galbraith wrote:
> 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.

sched: fix signed unsigned comparison in check_preempt_tick()

signed unsigned comparison may lead to superfluous resched if leftmost
is right of the current task, wasting a few cycles, and inadvertently
_lengthening_ the current task's slice.

Reported-by: Venkatesh Pallipadi <venki@google.com>
Signed-off-by: Mike Galbraith <efault@gmx.de>

---
kernel/sched_fair.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-2.6.37.git/kernel/sched_fair.c
===================================================================
--- linux-2.6.37.git.orig/kernel/sched_fair.c
+++ linux-2.6.37.git/kernel/sched_fair.c
@@ -872,7 +872,7 @@ check_preempt_tick(struct cfs_rq *cfs_rq
struct sched_entity *se = __pick_next_entity(cfs_rq);
s64 delta = curr->vruntime - se->vruntime;

- if (delta > ideal_runtime)