Re: [PATCH 2/4] sched: Document Program-Order guarantees

Boqun Feng
2015-11-20T14:09:25Z

[-- Attachment #1: Type: text/plain, Size: 2205 bytes --]

Hi Peter,

On Fri, Nov 20, 2015 at 11:02:30AM +0100, Peter Zijlstra wrote:
[snip]
> + * BLOCKING -- aka. SLEEP + WAKEUP
> + *
> + * For blocking we (obviously) need to provide the same guarantee as for
> + * migration. However the means are completely different as there is no lock
> + * chain to provide order. Instead we do:
> + *
> + * 1) smp_store_release(X->on_cpu, 0)
> + * 2) smp_cond_acquire(!X->on_cpu)
> + *
> + * Example:
> + *
> + * CPU0 (schedule) CPU1 (try_to_wake_up) CPU2 (schedule)
> + *
> + * LOCK rq(0)->lock LOCK X->pi_lock
> + * dequeue X
> + * sched-out X
> + * smp_store_release(X->on_cpu, 0);
> + *
> + * smp_cond_acquire(!X->on_cpu);
> + * X->state = WAKING
> + * set_task_cpu(X,2)
> + *
> + * LOCK rq(2)->lock
> + * enqueue X
> + * X->state = RUNNING
> + * UNLOCK rq(2)->lock
> + *
> + * LOCK rq(2)->lock // orders against CPU1
> + * sched-out Z
> + * sched-in X
> + * UNLOCK rq(1)->lock
> + *
> + * UNLOCK X->pi_lock
> + * UNLOCK rq(0)->lock
> + *
> + *
> + * However; for wakeups there is a second guarantee we must provide, namely we
> + * must observe the state that lead to our wakeup. That is, not only must our
> + * task observe its own prior state, it must also observe the stores prior to
> + * its wakeup.
> + *
> + * This means that any means of doing remote wakeups must order the CPU doing
> + * the wakeup against the CPU the task is going to end up running on. This,
> + * however, is already required for the regular Program-Order guarantee above,
> + * since the waking CPU is the one issueing the ACQUIRE (2).
> + *

Hope I'm the only one who got confused about the "2" in "ACQUIRE (2)",
what does that refer? "2) smp_cond_acquire(!X->on_cpu)"?

The comments are great, just try to understand your meaning here ;-)

Regards,
Boqun

> + */
> +
> /**
> * try_to_wake_up - wake up a thread
> * @p: the thread to be awakened

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 473 bytes --]