Re: [RFC][PATCH 03/16] sched: Wrap rq::lock access

[Subhra Mazumdar]

On 3/29/19 6:35 AM, Julien Desfossez wrote:
> On Fri, Mar 22, 2019 at 8:09 PM Subhra Mazumdar <subhra.mazumdar@xxxxxxxxxx>
> wrote:
> > Is the core wide lock primarily responsible for the regression? I ran
> > upto patch
> > 12 which also has the core wide lock for tagged cgroups and also calls
> > newidle_balance() from pick_next_task(). I don't see any regression.  Of
> > course
> > the core sched version of pick_next_task() may be doing more but
> > comparing with
> > the __pick_next_task() it doesn't look too horrible.
> On further testing and investigation, we also agree that spinlock contention
> is not the major cause for the regression, but we feel that it should be one
> of the major contributing factors to this performance loss.
I finally did some code bisection and found the following lines are
basically responsible for the regression. Commenting them out I don't see
the regressions. Can you confirm? I am yet to figure if this is needed for
the correctness of core scheduling and if so can we do this better?

-------->8-------------

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index fe3918c..3b3388a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3741,8 +3741,8 @@ pick_next_task(struct rq *rq, struct task_struct 
*prev, struct rq_flags *rf)
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * If there weren't no cookies; we 
don't need
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ * to bother with the other siblings.
*/
-ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ if (i == cpu && !rq->core->core_cookie)
-ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ goto next_class;
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ //if (i == cpu && !rq->core->core_cookie)
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ //goto next_class;

continue;
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ }