Re: [PATCH v8 -tip 17/26] sched: Split the cookie and setup per-task cookie on fork

[chris hyser]

On 11/4/20 5:30 PM, chris hyser wrote:
> On 10/19/20 9:43 PM, Joel Fernandes (Google) wrote:
> > In order to prevent interference and clearly support both per-task and CGroup
> > APIs, split the cookie into 2 and allow it to be set from either per-task, or
> > CGroup API. The final cookie is the combined value of both and is computed when
> > the stop-machine executes during a change of cookie.
> >
> > Also, for the per-task cookie, it will get weird if we use pointers of any
> > emphemeral objects. For this reason, introduce a refcounted object who's sole
> > purpose is to assign unique cookie value by way of the object's pointer.
> >
> > While at it, refactor the CGroup code a bit. Future patches will introduce more
> > APIs and support.
> >
> > Tested-by: Julien Desfossez <jdesfossez@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > ---
> >   include/linux/sched.h |   2 +
> >   kernel/sched/core.c   | 241 ++++++++++++++++++++++++++++++++++++++++--
> >   kernel/sched/debug.c  |   4 +
> >   3 files changed, 236 insertions(+), 11 deletions(-)
> >
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index fe6f225bfbf9..c6034c00846a 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -688,6 +688,8 @@ struct task_struct {
> >   #ifdef CONFIG_SCHED_CORE
> >       struct rb_node            core_node;
> >       unsigned long            core_cookie;
> > +    unsigned long            core_task_cookie;
> > +    unsigned long            core_group_cookie;
> >       unsigned int            core_occupation;
> >   #endif
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index bab4ea2f5cd8..30a9e4cb5ce1 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -346,11 +346,14 @@ void sched_core_put(void)
> >       mutex_unlock(&sched_core_mutex);
> >   }
> > +static int sched_core_share_tasks(struct task_struct *t1, struct task_struct *t2);
> > +
> >   #else /* !CONFIG_SCHED_CORE */
> >   static inline void sched_core_enqueue(struct rq *rq, struct task_struct *p) { }
> >   static inline void sched_core_dequeue(struct rq *rq, struct task_struct *p) { }
> >   static bool sched_core_enqueued(struct task_struct *task) { return false; }
> > +static int sched_core_share_tasks(struct task_struct *t1, struct task_struct *t2) { }
> >   #endif /* CONFIG_SCHED_CORE */
> > @@ -3583,6 +3586,20 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
> >   #endif
> >   #ifdef CONFIG_SCHED_CORE
> >       RB_CLEAR_NODE(&p->core_node);
> > +
> > +    /*
> > +     * Tag child via per-task cookie only if parent is tagged via per-task
> > +     * cookie. This is independent of, but can be additive to the CGroup tagging.
> > +     */
> > +    if (current->core_task_cookie) {
> > +
> > +        /* If it is not CLONE_THREAD fork, assign a unique per-task tag. */
> > +        if (!(clone_flags & CLONE_THREAD)) {
> > +            return sched_core_share_tasks(p, p);
> > +               }
> > +        /* Otherwise share the parent's per-task tag. */
> > +        return sched_core_share_tasks(p, current);
> > +    }
> >   #endif
> >       return 0;
> >   }

sched_core_share_tasks() looks at the value of the new tasks core_task_cookie which is non-zero on a
process or thread clone and causes underflow for both the enable flag itself and for cookie ref counts.

So just zero it in __sched_fork().

-chrish

------

sched: zero out the core scheduling cookie on clone

As the cookie is reference counted, even if inherited, zero this and allow
explicit sharing.

Signed-off-by: Chris Hyser <chris.hyser@xxxxxxxxxx>
---
 kernel/sched/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index fd3cc03..2af0ea6 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3378,6 +3378,9 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
     p->capture_control = NULL;
 #endif
     init_numa_balancing(clone_flags, p);
+#ifdef CONFIG_SCHED_CORE
+    p->core_task_cookie = 0;
+#endif
 #ifdef CONFIG_SMP
     p->wake_entry.u_flags = CSD_TYPE_TTWU;
 #endif