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SubjectRe: [PATCH -tip 22/32] sched: Split the cookie and setup per-task cookie on fork
On Wed, Nov 25, 2020 at 01:54:47PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 17, 2020 at 06:19:52PM -0500, Joel Fernandes (Google) wrote:
> > +/* Per-task interface */
> > +static unsigned long sched_core_alloc_task_cookie(void)
> > +{
> > + struct sched_core_cookie *ptr =
> > + kmalloc(sizeof(struct sched_core_cookie), GFP_KERNEL);
> > +
> > + if (!ptr)
> > + return 0;
> > + refcount_set(&ptr->refcnt, 1);
> > +
> > + /*
> > + * NOTE: sched_core_put() is not done by put_task_cookie(). Instead, it
> > + * is done after the stopper runs.
> > + */
> > + sched_core_get();
> > + return (unsigned long)ptr;
> > +}
> > +
> > +static bool sched_core_get_task_cookie(unsigned long cookie)
> > +{
> > + struct sched_core_cookie *ptr = (struct sched_core_cookie *)cookie;
> > +
> > + /*
> > + * NOTE: sched_core_put() is not done by put_task_cookie(). Instead, it
> > + * is done after the stopper runs.
> > + */
> > + sched_core_get();
> > + return refcount_inc_not_zero(&ptr->refcnt);
> > +}
> > +
> > +static void sched_core_put_task_cookie(unsigned long cookie)
> > +{
> > + struct sched_core_cookie *ptr = (struct sched_core_cookie *)cookie;
> > +
> > + if (refcount_dec_and_test(&ptr->refcnt))
> > + kfree(ptr);
> > +}
>
> > + /*
> > + * NOTE: sched_core_get() is done by sched_core_alloc_task_cookie() or
> > + * sched_core_put_task_cookie(). However, sched_core_put() is done
> > + * by this function *after* the stopper removes the tasks from the
> > + * core queue, and not before. This is just to play it safe.
> > + */
>
> So for no reason what so ever you've made the code more difficult?

You're right, I could just do sched_core_get() in the caller. I changed it as in
the diff below:

---8<-----------------------

diff --git a/kernel/sched/coretag.c b/kernel/sched/coretag.c
index 800c0f8bacfc..75e2edb53a48 100644
--- a/kernel/sched/coretag.c
+++ b/kernel/sched/coretag.c
@@ -274,6 +274,7 @@ void sched_core_change_group(struct task_struct *p, struct task_group *new_tg)
/* Per-task interface: Used by fork(2) and prctl(2). */
static void sched_core_put_cookie_work(struct work_struct *ws);

+/* Caller has to call sched_core_get() if non-zero value is returned. */
static unsigned long sched_core_alloc_task_cookie(void)
{
struct sched_core_task_cookie *ck =
@@ -284,11 +285,6 @@ static unsigned long sched_core_alloc_task_cookie(void)
refcount_set(&ck->refcnt, 1);
INIT_WORK(&ck->work, sched_core_put_cookie_work);

- /*
- * NOTE: sched_core_put() is not done by put_task_cookie(). Instead, it
- * is done after the stopper runs.
- */
- sched_core_get();
return (unsigned long)ck;
}

@@ -354,12 +350,6 @@ int sched_core_share_tasks(struct task_struct *t1, struct task_struct *t2)

mutex_lock(&sched_core_tasks_mutex);

- /*
- * NOTE: sched_core_get() is done by sched_core_alloc_task_cookie() or
- * sched_core_put_task_cookie(). However, sched_core_put() is done
- * by this function *after* the stopper removes the tasks from the
- * core queue, and not before. This is just to play it safe.
- */
if (!t2) {
if (t1->core_task_cookie) {
sched_core_put_task_cookie(t1->core_task_cookie);
@@ -370,7 +360,9 @@ int sched_core_share_tasks(struct task_struct *t1, struct task_struct *t2)
/* Assign a unique per-task cookie solely for t1. */

cookie = sched_core_alloc_task_cookie();
- if (!cookie)
+ if (cookie)
+ sched_core_get();
+ else
goto out_unlock;

if (t1->core_task_cookie) {
@@ -401,7 +393,9 @@ int sched_core_share_tasks(struct task_struct *t1, struct task_struct *t2)

/* CASE 1. */
cookie = sched_core_alloc_task_cookie();
- if (!cookie)
+ if (cookie)
+ sched_core_get();
+ else
goto out_unlock;

/* Add another reference for the other task. */
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