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Subject: Re: [PATCH v8 -tip 17/26] sched: Split the cookie and setup per-task
 cookie on fork
References: <4ff7f030-b797-6711-fb6a-fe39bb02075a@oracle.com>
From:   chris hyser <chris.hyser@oracle.com>
To:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        linux-kernel@vger.kernel.org
Message-ID: <6e864ca1-d02b-dc56-7ec2-6224eeb0b32d@oracle.com>
Date:   Mon, 9 Nov 2020 18:23:13 -0500
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> 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().

-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