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SubjectRe: [PATCH v2] schedutil: Allow cpufreq requests to be made even when kthread kicked
On Mon, May 21, 2018 at 11:50:55AM +0100, Patrick Bellasi wrote:
> On 18-May 11:55, Joel Fernandes (Google.) wrote:
> > From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
> >
> > Currently there is a chance of a schedutil cpufreq update request to be
> > dropped if there is a pending update request. This pending request can
> > be delayed if there is a scheduling delay of the irq_work and the wake
> > up of the schedutil governor kthread.
> >
> > A very bad scenario is when a schedutil request was already just made,
> > such as to reduce the CPU frequency, then a newer request to increase
> > CPU frequency (even sched deadline urgent frequency increase requests)
> > can be dropped, even though the rate limits suggest that its Ok to
> > process a request. This is because of the way the work_in_progress flag
> > is used.
> >
> > This patch improves the situation by allowing new requests to happen
> > even though the old one is still being processed. Note that in this
> > approach, if an irq_work was already issued, we just update next_freq
> > and don't bother to queue another request so there's no extra work being
> > done to make this happen.
>
> Maybe I'm missing something but... is not this patch just a partial
> mitigation of the issue you descrive above?
>
> If a DL freq increase is queued, with this patch we store the request
> but we don't actually increase the frequency until the next schedutil
> update, which can be one tick away... isn't it?
>
> If that's the case, maybe something like the following can complete
> the cure?
>
> ---8<---
> #define SUGOV_FREQ_NONE 0
>
> static unsigned int sugov_work_update(struct sugov_policy *sg_policy,
> unsigned int prev_freq)
> {
> unsigned long irq_flags;
> bool update_freq = true;
> unsigned int next_freq;
>
> /*
> * Hold sg_policy->update_lock shortly to handle the case where:
> * incase sg_policy->next_freq is read here, and then updated by
> * sugov_update_shared just before work_in_progress is set to false
> * here, we may miss queueing the new update.
> *
> * Note: If a work was queued after the update_lock is released,
> * sugov_work will just be called again by kthread_work code; and the
> * request will be proceed before the sugov thread sleeps.
> */
> raw_spin_lock_irqsave(&sg_policy->update_lock, irq_flags);
> next_freq = sg_policy->next_freq;
> sg_policy->work_in_progress = false;
> if (prev_freq == next_freq)
> update_freq = false;

About this patch on top of mine, I believe this check is already being done
by sugov_update_commit? :

static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time,
unsigned int next_freq)
{
struct cpufreq_policy *policy = sg_policy->policy;

if (sg_policy->next_freq == next_freq)
return;

sg_policy->next_freq = next_freq;
sg_policy->last_freq_update_time = time;
----

thanks,

- Joel

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