Messages in this thread Patch in this message | | | Subject | Re: commit e9e9250b: sync wakeup bustage when waker is an RT task | From | Mike Galbraith <> | Date | Sun, 16 May 2010 09:21:50 +0200 |
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On Sat, 2010-05-15 at 19:07 +0200, Mike Galbraith wrote: > On Sat, 2010-05-15 at 14:04 +0200, Peter Zijlstra wrote: > > On Sat, 2010-05-15 at 13:57 +0200, Mike Galbraith wrote: > > > Hi Peter, > > > > > > This commit excluded RT tasks from rq->load, was that intentional? The > > > comment in struct rq states that load reflects *all* tasks, but since > > > this commit, that's no longer true. > > > > Right, because a static load value does not accurately reflect a RT task > > which can run as long as it pretty well pleases. So instead we measure > > the time spend running !fair tasks and scale down the cpu_power > > proportionally. > > > > > Looking at lmbench lat_udp in a PREEMPT_RT kernel, I noticed that > > > wake_affine() is failing for sync wakeups when it should not. It's > > > doing so because the waker in this case is an RT kernel thread > > > (sirq-net-rx) - we subtract the sync waker's weight, when it was never > > > added in the first place, resulting in this_load going gaga. End result > > > is quite high latency numbers due to tasks jabbering cross-cache. > > > > > > If the exclusion was intentional, I suppose I can do a waker class check > > > in wake_affine() to fix it. > > > > So basically make all RT wakeups sync? > > I was going to just skip subtracting waker's weight ala > > /* > * If sync wakeup then subtract the (maximum possible) > * effect of the currently running task from the load > * of the current CPU: > */ > if (sync && !task_has_rt_policy(curr))
One-liner doesn't work. We have one task on the cfs_rq, the one who is the waker in !PREEMPT_RT, which is a fail case for wake_affine() if you don't do the weight subtraction. I did the below instead.
sched: RT waker sync wakeup bugfix
An RT waker's weight is not on the runqueue, but we try to subrtact it anyway in the sync wakeup case, sending this_load negative. This leads to affine wakeup failure in cases where it should succeed. This was found while testing an PREEMPT_RT kernel with lmbench's lat_udp. In a PREEMPT_RT kernel, softirq threads act as a ~proxy for the !RT buddy. Approximate !PREEMPT_RT sync wakeup behavior by looking at the buddy instead, and subtracting the maximum task weight that will not send this_load negative.
Signed-off-by: Mike Galbraith <efault@gmx.de> Cc: Ingo Molnar <mingo@elte.hu> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Thomas Gleixner <tglx@linutronix.de> LKML-Reference: <new-submission>
kernel/sched_fair.c | 9 +++++++++ 1 files changed, 9 insertions(+), 0 deletions(-)
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c index 5240469..cc40849 100644 --- a/kernel/sched_fair.c +++ b/kernel/sched_fair.c @@ -1280,6 +1280,15 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync) tg = task_group(current); weight = current->se.load.weight; + /* + * An RT waker's weight is not on the runqueue. Subtract the + * maximum task weight that will not send this_load negative. + */ + if (task_has_rt_policy(current)) { + weight = max_t(unsigned long, NICE_0_LOAD, p->se.load.weight); + weight = min(weight, this_load); + } + this_load += effective_load(tg, this_cpu, -weight, -weight); load += effective_load(tg, prev_cpu, 0, -weight); }
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