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SubjectRe: [RFC][PATCH 12/13] sched/deadline: Introduce deadline servers
Hi Dietmar,

On 07/08/19 18:31, Dietmar Eggemann wrote:
> On 7/26/19 4:54 PM, Peter Zijlstra wrote:
> >
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>
> [...]
>
> > @@ -889,6 +891,8 @@ static void update_curr(struct cfs_rq *c
> > trace_sched_stat_runtime(curtask, delta_exec, curr->vruntime);
> > cgroup_account_cputime(curtask, delta_exec);
> > account_group_exec_runtime(curtask, delta_exec);
> > + if (curtask->server)
> > + dl_server_update(curtask->server, delta_exec);
> > }
>
> I get a lockdep_assert_held(&rq->lock) related warning in start_dl_timer()
> when running the full stack.
>
> ...
> [ 0.530216] root domain span: 0-5 (max cpu_capacity = 1024)
> [ 0.538655] devtmpfs: initialized
> [ 0.556485] update_curr: rq mismatch rq[0] != rq[4]
> [ 0.561519] update_curr: rq mismatch rq[0] != rq[4]
> [ 0.566497] update_curr: rq mismatch rq[0] != rq[4]
> [ 0.571443] update_curr: rq mismatch rq[0] != rq[4]
> [ 0.576762] update_curr: rq mismatch rq[2] != rq[4]
> [ 0.581674] update_curr: rq mismatch rq[2] != rq[4]
> [ 0.586569] ------------[ cut here ]------------
> [ 0.591220] WARNING: CPU: 2 PID: 2 at kernel/sched/deadline.c:916 start_dl_timer+0x160/0x178
> [ 0.599686] Modules linked in:
> [ 0.602756] CPU: 2 PID: 2 Comm: kthreadd Tainted: G W 5.3.0-rc3-00013-ga33cf033cc99-dirty #64
> [ 0.612620] Hardware name: ARM Juno development board (r0) (DT)
> [ 0.618560] pstate: 60000085 (nZCv daIf -PAN -UAO)
> [ 0.623369] pc : start_dl_timer+0x160/0x178
> [ 0.627572] lr : start_dl_timer+0x160/0x178
> [ 0.631768] sp : ffff000010013cb0
> ...
> [ 0.715075] Call trace:
> [ 0.717531] start_dl_timer+0x160/0x178
> [ 0.721382] update_curr_dl_se+0x108/0x208
> [ 0.725494] dl_server_update+0x2c/0x38
> [ 0.729348] update_curr+0x1b4/0x3b8
> [ 0.732934] task_tick_fair+0x74/0xa88
> [ 0.736698] scheduler_tick+0x94/0x110
> [ 0.740461] update_process_times+0x48/0x60
> ...
>
> Seems to be related to the fact that the rq can change:
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e4c14851a34c..5e3130a200ec 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -891,8 +891,17 @@ static void update_curr(struct cfs_rq *cfs_rq)
> trace_sched_stat_runtime(curtask, delta_exec, curr->vruntime);
> cgroup_account_cputime(curtask, delta_exec);
> account_group_exec_runtime(curtask, delta_exec);
> - if (curtask->server)
> + if (curtask->server) {
> + struct rq *rq = rq_of(cfs_rq);
> + struct rq *rq2 = curtask->server->rq;
> +
> + if (rq != rq2) {
> + printk("update_curr: rq mismatch rq[%d] != rq[%d]\n",
> + cpu_of(rq), cpu_of(rq2));
> + }
> +
> dl_server_update(curtask->server, delta_exec);
> + }
> }
>
> ...

Yeah, I actually noticed the same. Some debugging seems to point to
early boot spawning of kthreads. I can reliably for example attribute
this mismatch to ksoftirqd(s). It looks like they can avoid the
dl_server assignment in pick_next_task_dl() and this breaks things.
Still need to figure out why this happens and how to fix it, though.

Best,

Juri

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