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

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Date:   Mon, 16 Nov 2020 19:45:44 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     chris hyser <chris.hyser@...cle.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [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@...italocean.com>
> > > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.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@...cle.com>
> 
> As the cookie is reference counted, even if inherited, zero this and allow
> explicit sharing.
> 
> Signed-off-by: Chris Hyser <chris.hyser@...cle.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
>