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SubjectRe: [PATCH v3] sched/rt : return accurate release rq lock info
On Sat, 6 Oct 2018 06:22:11 +0800
Peng Hao <peng.hao2@zte.com.cn> wrote:

> find_lock_lowest_rq may or not releease rq lock when return
> lowest_rq=NULL, but it is fuzzy.
> If not releasing rq lock, it is unnecessary to re-call
> pick_next_pushable_task.
> When CONFIG_PREEMPT=n, not releasing rq lock and return
> lowest_rq=null frequently happens in a simple test case:
> Four different rt priority tasks run on limited two cpus.
> Thanks for Steven Rostedt's advice.
>
> Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
> ---
> kernel/sched/rt.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 2e2955a..be0fc43 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1754,7 +1754,7 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
> !task_on_rq_queued(task))) {
>
> double_unlock_balance(rq, lowest_rq);
> - lowest_rq = NULL;
> + lowest_rq = RETRY_TASK;
> break;
> }
> }
> @@ -1830,7 +1830,9 @@ static int push_rt_task(struct rq *rq)
>
> /* find_lock_lowest_rq locks the rq if found */
> lowest_rq = find_lock_lowest_rq(next_task, rq);
> - if (!lowest_rq) {
> + if (!lowest_rq)
> + goto out;
> + if (lowest_rq == RETRY_TASK) {

This probably makes no difference, and can be blown off as just a
preference, but should this be:

if (!lowest_rq) {
goto out;
} else if (lowest_rq == RETRY_TASK) {

The logic is the same regardless, so it's really just a matter of taste.

That said:

Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

-- Steve


> struct task_struct *task;
> /*
> * find_lock_lowest_rq releases rq->lock

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