Messages in this thread Patch in this message |  | | | Subject | [PATCH] Re: [PATCH] sched: Buggy comparison in check_preempt_tick | | From | Mike Galbraith <> | | Date | Wed, 05 Jan 2011 05:41:17 +0100 |
| |
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) + if (delta > (s64)ideal_runtime) resched_task(rq_of(cfs_rq)->curr); } }
|  |