Re: [patch] RCU for low latency [1/2]

[Dipankar Sarma]

On Fri, Jan 09, 2004 at 02:12:00AM +1100, Nick Piggin wrote:
> Dipankar Sarma wrote:
> 
> >Provide a rq_has_rt_task() interface to detect runqueues with
> >real time priority tasks. Useful for RCU optimizations.
> >
> 
> Can you make rq_has_rt_task the slow path? Adding things like this
> can actually be noticable on microbenchmarks (eg. pipe based ctx
> switching). Its probably cache artifacts that I see, but it wouldn't
> hurt to keep the scheduler as tight as possible.
> 
> I think this should cover it.
> 
> int rq_has_rt_task(int cpu)
> {
> 	runqueue_t *rq = cpu_rq(cpu);
> 	return (sched_find_first_bit(rq->active) < MAX_RT_PRIO);
> }
> 
> Any good?

Much better, except, well, rq->active->bitmap. Here is the new
rq_has_rt_task() patch.

Thanks
Dipankar



Provide a rq_has_rt_task() interface to detect runqueues with
real time priority tasks. Useful for RCU optimizations.


 include/linux/sched.h |    1 +
 kernel/sched.c        |    6 ++++++
 2 files changed, 7 insertions(+)

diff -puN kernel/sched.c~rq-has-rt-task kernel/sched.c
--- linux-2.6.0-smprcu/kernel/sched.c~rq-has-rt-task	2004-01-08 23:16:02.000000000 +0530
+++ linux-2.6.0-smprcu-dipankar/kernel/sched.c	2004-01-08 23:36:03.000000000 +0530
@@ -338,6 +338,12 @@ static inline void enqueue_task(struct t
 	p->array = array;
 }
 
+int rq_has_rt_task(int cpu)
+{
+        runqueue_t *rq = cpu_rq(cpu);
+        return (sched_find_first_bit(rq->active->bitmap) < MAX_RT_PRIO);
+}
+
 /*
  * effective_prio - return the priority that is based on the static
  * priority but is modified by bonuses/penalties.
diff -puN include/linux/sched.h~rq-has-rt-task include/linux/sched.h
--- linux-2.6.0-smprcu/include/linux/sched.h~rq-has-rt-task	2004-01-08 23:36:27.000000000 +0530
+++ linux-2.6.0-smprcu-dipankar/include/linux/sched.h	2004-01-08 23:36:52.000000000 +0530
@@ -525,6 +525,7 @@ extern int task_prio(task_t *p);
 extern int task_nice(task_t *p);
 extern int task_curr(task_t *p);
 extern int idle_cpu(int cpu);
+extern int rq_has_rt_task(int cpu);
 
 void yield(void);
 

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