Messages in this thread | | | Date | Mon, 21 Jun 2021 10:57:18 +0000 | From | Quentin Perret <> | Subject | Re: [PATCH v2 1/3] sched: Fix UCLAMP_FLAG_IDLE setting |
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Hi Dietmar,
On Thursday 17 Jun 2021 at 17:27:56 (+0200), Dietmar Eggemann wrote: > On 11/06/2021 09:25, Quentin Perret wrote: > > On Thursday 10 Jun 2021 at 21:05:12 (+0200), Peter Zijlstra wrote: > >> On Thu, Jun 10, 2021 at 03:13:04PM +0000, Quentin Perret wrote: > >>> The UCLAMP_FLAG_IDLE flag is set on a runqueue when dequeueing the last > >>> active task to maintain the last uclamp.max and prevent blocked util > >>> from suddenly becoming visible. > >>> > >>> However, there is an asymmetry in how the flag is set and cleared which > >>> can lead to having the flag set whilst there are active tasks on the rq. > >>> Specifically, the flag is cleared in the uclamp_rq_inc() path, which is > >>> called at enqueue time, but set in uclamp_rq_dec_id() which is called > >>> both when dequeueing a task _and_ in the update_uclamp_active() path. As > >>> a result, when both uclamp_rq_{dec,ind}_id() are called from > >>> update_uclamp_active(), the flag ends up being set but not cleared, > >>> hence leaving the runqueue in a broken state. > >>> > >>> Fix this by setting the flag in the uclamp_rq_inc_id() path to ensure > >>> things remain symmetrical. > >> > >> The code you moved is neither in uclamp_rq_inc_id(), although > >> uclamp_idle_reset() is called from there > > > > Yep, that is what I was trying to say. > > > >> nor does it _set_ the flag. > > > > Ahem. That I don't have a good excuse for ... > > (A) dequeue -> set > > (1) dequeue_task() -> uclamp_rq_dec() -> > > (2) cpu_util_update_eff() -> ... -> uclamp_update_active() -> > > uclamp_rq_dec_id() > > uclamp_rq_max_value() > > /* No tasks -- default clamp values */ > uclamp_idle_value() { > > if (clamp_id == UCLAMP_MAX) > rq->uclamp_flags |= UCLAMP_FLAG_IDLE; <-- set > } > > --- > > (B) enqueue -> clear > > (1) enqueue_task() -> > > uclamp_rq_inc() { > > (2) cpu_util_update_eff() -> ... -> uclamp_update_active() -> > > uclamp_rq_inc_id() { > > uclamp_idle_reset() { > <-- new clear > } ^ > } | > | > if (rq->uclamp_flags & UCLAMP_FLAG_IDLE) | > rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE; <-- old clear > } > > --- > > uclamp_update_active() > > if (p->uclamp[clamp_id].active) { > uclamp_rq_dec_id() <-- (A2) > uclamp_rq_inc_id() <-- (B2) > } > > Is this existing asymmetry in setting the flag but not clearing it in > uclamp_update_active() the only issue this patch fixes?
I think this is the root of the problem, but it can have odd symptoms. In a bad case that can lead to hitting the WARN in uclamp_rq_dec_id (which is how we've found the bug in the first place).
I'll try and repost this with a correct commit message soon -- still fighting with my inbox right now.
Thanks, Quentin
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