On 05/26/2012 08:44 AM, Paul Turner wrote:
On 04/09/2012 03:25 PM, Glauber Costa wrote:
In the interest of providing a per-cgroup figure of common statistics,
this patch adds a nr_switches counter to each group runqueue (both cfs
and rt).
To avoid impact on schedule(), we don't walk the tree at stat gather
time. This is because schedule() is called much more frequently than
the tick functions, in which we do walk the tree.
When this figure needs to be read (different patch), we will
aggregate them at read time.
Paul,
How about the following patch instead?
It is still using the cfs_rq and rt_rq's structures, (this code actually
only touches fair.c as a PoC, rt would be similar).
Tasks in the root cgroup (without an se->parent), will do a branch and
exit. For the others, we accumulate here, and simplify the reader.
My reasoning for this, is based on the fact that all the se->parent
relations should be cached by our recent call to put_prev_task (well,
unless of course we have a really big chain)
This would incur a slightly higher context switch time for tasks inside
a cgroup.
The reader (in a different patch) would then be the same as the others:
+static u64 tg_nr_switches(struct task_group *tg, int cpu)
+{
+ if (tg != &root_task_group)
+ return rt_rq(rt_nr_switches, tg, cpu)
+fair_rq(nr_switches, tg, cpu);
+
+ return cpu_rq(cpu)->nr_switches;
+}
I plan to measure this today, but an extra branch cost for the common
case of a task in the root cgroup + O(depth) for tasks inside cgroups
may be acceptable, given the simplification it brings.
Let me know what you think.
Index: linux/kernel/sched/fair.c
===================================================================
--- linux.orig/kernel/sched/fair.c
+++ linux/kernel/sched/fair.c
@@ -2990,6 +2990,22 @@ static struct task_struct *pick_next_tas
if (hrtick_enabled(rq))
hrtick_start_fair(rq, p);
+#ifdef CONFIG_FAIR_GROUP_SCHED
+ if (!se->parent)
+ goto out;
+ cfs_rq = group_cfs_rq(se->parent);
+ if (cfs_rq->prev == se)
+ goto out;
+ cfs_rq->prev = se;
+
+ while (se->parent) {
+ se = se->parent;
+ cfs_rq = group_cfs_rq(se);
+ cfs_rq->nr_switches++;
+ }
+out:
+#endif
+
return p;
}
Index: linux/kernel/sched/sched.h
===================================================================
--- linux.orig/kernel/sched/sched.h
+++ linux/kernel/sched/sched.h
@@ -237,6 +237,8 @@ struct cfs_rq {
struct list_head leaf_cfs_rq_list;
struct task_group *tg; /* group that "owns" this runqueue */
+ u64 nr_switches;
+ struct sched_entity *prev;
#ifdef CONFIG_SMP
/*
* h_load = weight * f(tg)
@@ -307,6 +309,9 @@ struct rt_rq {
struct rq *rq;
struct list_head leaf_rt_rq_list;
struct task_group *tg;
+
+ u64 rt_nr_switches;
+ struct sched_rt_entity *prev;
#endif
};
