Re: [RFC][PATCH v4] sched/rt: Use IPI to trigger RT task push migration instead of pulling
From: Peter Zijlstra <peterz@infradead.org>
Date: 2015-02-27 09:08:51
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lkml
On Thu, Feb 26, 2015 at 11:26:35AM -0500, Steven Rostedt wrote:
quoted hunk ↗ jump to hunk
Index: linux-rt.git/kernel/sched/rt.c ===================================================================--- linux-rt.git.orig/kernel/sched/rt.c 2015-02-26 10:55:26.107945935 -0500 +++ linux-rt.git/kernel/sched/rt.c 2015-02-26 10:55:38.277777892 -0500
+/* Called from hardirq context */
+static void try_to_push_tasks(void *arg)
+{
+ struct rt_rq *rt_rq = arg;
+ struct rq *rq, *src_rq;
+ int this_cpu;
+ int cpu;
+
+ this_cpu = rt_rq->push_cpu;
+
+ /* Paranoid check */
+ BUG_ON(this_cpu != smp_processor_id());
+
+ rq = cpu_rq(this_cpu);
+ src_rq = rq_of_rt_rq(rt_rq);
+
+ again:Superfluous space there!
+ if (has_pushable_tasks(rq)) {
+ raw_spin_lock(&rq->lock);
+ push_rt_task(rq);
+ raw_spin_unlock(&rq->lock);
+ }So push_rt_task() has a return value; should we use it? That is, currently we iterate the entire rto mask and migrate everything we come across, is there an argument to be had to only migrate 1 task and then call it quits?
quoted hunk ↗ jump to hunk
Index: linux-rt.git/kernel/sched/sched.h ===================================================================--- linux-rt.git.orig/kernel/sched/sched.h 2015-02-26 10:55:26.107945935 -0500 +++ linux-rt.git/kernel/sched/sched.h 2015-02-26 10:55:28.082918664 -0500@@ -6,6 +6,7 @@ #include <linux/mutex.h> #include <linux/spinlock.h> #include <linux/stop_machine.h> +#include <linux/irq_work.h> #include <linux/tick.h> #include <linux/slab.h>@@ -435,6 +436,11 @@ struct rt_rq { unsigned long rt_nr_total; int overloaded; struct plist_head pushable_tasks; + struct call_single_data push_csd;
You waaztin' maa spaaz!
+ int push_flags; + int push_cpu; + struct irq_work push_work; + raw_spinlock_t push_lock; #endif int rt_queued;
One could make an argument for using a separate per-cpu variable and cacheline align the thing...
quoted hunk ↗ jump to hunk
Index: linux-rt.git/kernel/sched/features.h ===================================================================--- linux-rt.git.orig/kernel/sched/features.h 2015-02-26 10:55:26.107945935 -0500 +++ linux-rt.git/kernel/sched/features.h 2015-02-26 10:55:28.083918650 -0500@@ -56,6 +56,17 @@ SCHED_FEAT(NONTASK_CAPACITY, true) */ SCHED_FEAT(TTWU_QUEUE, true) +/* + * In order to avoid a thundering herd attack of CPUS that are
I would suggest you remap your caps-lock to some useful key :-)
+ * lowering their priorities at the same time, and there being + * a single CPU that has an RT task that can migrate and is waiting + * to run, where the other CPUs will try to take that CPUs + * rq lock and possibly create a large contention, sending an + * IPI to that CPU and let that CPU push the RT task to where + * it should go may be a better scenario. + */ +SCHED_FEAT(RT_PUSH_IPI, true) + SCHED_FEAT(FORCE_SD_OVERLAP, false) SCHED_FEAT(RT_RUNTIME_SHARE, true) SCHED_FEAT(LB_MIN, false)