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

On Mon, Nov 09, 2020 at 06:23:13PM -0500, 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@digitalocean.com>
> > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > > ---
> > > 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().

Since I am going to split this into a new patch, could you please send me a
detailed commit message explaining the issue? I feel the below one liner is
unclear / insufficient as a commit message.

thanks,

- Joel


> -chris
>
>
> PATCH] sched: zero out the core scheduling cookie on clone
>
> From: chris hyser <chris.hyser@oracle.com>
>
> As the cookie is reference counted, even if inherited, zero this and allow
> explicit sharing.
>
> Signed-off-by: Chris Hyser <chris.hyser@oracle.com>
> ---
> 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
> --
> 1.8.3.1
>

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