Re: [PATCH 04/14] sched: SCHED_DEADLINE SMP-related data structures & logic.

[Juri Lelli <juri.lelli@gmail.com>]

On 10/15/2013 12:35 PM, Peter Zijlstra wrote:
> On Tue, Oct 15, 2013 at 11:36:17AM +0200, Juri Lelli wrote:
>> On 10/14/2013 02:03 PM, Peter Zijlstra wrote:
>>> On Mon, Oct 14, 2013 at 12:43:36PM +0200, Juri Lelli wrote:
>>>> +static inline void dl_set_overload(struct rq *rq)
>>>> +{
>>>> +	if (!rq->online)
>>>> +		return;
>>>> +
>>>> +	cpumask_set_cpu(rq->cpu, rq->rd->dlo_mask);
>>>> +	/*
>>>> +	 * Must be visible before the overload count is
>>>> +	 * set (as in sched_rt.c).
>>>> +	 */
>>>> +	wmb();
>>>> +	atomic_inc(&rq->rd->dlo_count);
>>>> +}
>>>
>>> Please, make that smp_wmb() and modify the comment to point to the
>>> matching barrier ; I couldn't find one! Which suggests something is
>>> amiss.
>>>
>>> Ideally we'd have something like smp_wmb__after_set_bit() but alas.
>>>
>>
>> The only user of this function is pull_dl_task (that tries to pull only if at
>> least one runqueue of the root_domain is overloaded). Surely makes sense to
>> ensure that changes in the dlo_mask have to be visible before we check if we
>> should look at that mask. Am I right if I say that the matching barrier is
>> constituted by the spin_lock on this_rq acquired by schedule() before calling
>> pre_schedule()?
>>
>> Same thing in rt_set_overload(), do we need to modify the comment also there?
> 
> So I haven't looked at the dl code, but for the RT code the below is
> required.
> 

Same thing for dl code.

Thanks,

- Juri

> Without that smp_rmb() in there we could actually miss seeing the
> rto_mask bit.
> 
> ---
>  kernel/sched/rt.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index e9304cdc26fe..a848f526b941 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -246,8 +246,10 @@ static inline void rt_set_overload(struct rq *rq)
>  	 * if we should look at the mask. It would be a shame
>  	 * if we looked at the mask, but the mask was not
>  	 * updated yet.
> +	 *
> +	 * Matched by the barrier in pull_rt_task().
>  	 */
> -	wmb();
> +	smp_wmb();
>  	atomic_inc(&rq->rd->rto_count);
>  }
>  
> @@ -1626,6 +1628,12 @@ static int pull_rt_task(struct rq *this_rq)
>  	if (likely(!rt_overloaded(this_rq)))
>  		return 0;
>  
> +	/*
> +	 * Match the barrier from rt_set_overloaded; this guarantees that if we
> +	 * see overloaded we must also see the rto_mask bit.
> +	 */
> +	smp_rmb();
> +
>  	for_each_cpu(cpu, this_rq->rd->rto_mask) {
>  		if (this_cpu == cpu)
>  			continue;
>