From mboxrd@z Thu Jan 1 00:00:00 1970 From: Patrick Bellasi Subject: Re: [PATCH v6 05/16] sched/core: uclamp: Update CPU's refcount on clamp changes Date: Tue, 22 Jan 2019 10:43:05 +0000 Message-ID: <20190122104305.6vjx37muqsxm536t@e110439-lin> References: <20190115101513.2822-1-patrick.bellasi@arm.com> <20190115101513.2822-6-patrick.bellasi@arm.com> <20190121153308.GL27931@hirez.programming.kicks-ass.net> <20190121154412.fak2t2iquj3aixtu@e110439-lin> <20190122093704.GM27931@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20190122093704.GM27931@hirez.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, linux-api@vger.kernel.org, Ingo Molnar , Tejun Heo , "Rafael J . Wysocki" , Vincent Guittot , Viresh Kumar , Paul Turner , Quentin Perret , Dietmar Eggemann , Morten Rasmussen , Juri Lelli , Todd Kjos , Joel Fernandes , Steve Muckle , Suren Baghdasaryan List-Id: linux-api@vger.kernel.org On 22-Jan 10:37, Peter Zijlstra wrote: > On Mon, Jan 21, 2019 at 03:44:12PM +0000, Patrick Bellasi wrote: > > On 21-Jan 16:33, Peter Zijlstra wrote: > > > On Tue, Jan 15, 2019 at 10:15:02AM +0000, Patrick Bellasi wrote: > > > > > > > +static inline void > > > > +uclamp_task_update_active(struct task_struct *p, unsigned int clamp_id) > > > > +{ > > > > + struct rq_flags rf; > > > > + struct rq *rq; > > > > + > > > > + /* > > > > + * Lock the task and the CPU where the task is (or was) queued. > > > > + * > > > > + * We might lock the (previous) rq of a !RUNNABLE task, but that's the > > > > + * price to pay to safely serialize util_{min,max} updates with > > > > + * enqueues, dequeues and migration operations. > > > > + * This is the same locking schema used by __set_cpus_allowed_ptr(). > > > > + */ > > > > + rq = task_rq_lock(p, &rf); > > > > + > > > > + /* > > > > + * Setting the clamp bucket is serialized by task_rq_lock(). > > > > + * If the task is not yet RUNNABLE and its task_struct is not > > > > + * affecting a valid clamp bucket, the next time it's enqueued, > > > > + * it will already see the updated clamp bucket value. > > > > + */ > > > > + if (!p->uclamp[clamp_id].active) > > > > + goto done; > > > > + > > > > + uclamp_cpu_dec_id(p, rq, clamp_id); > > > > + uclamp_cpu_inc_id(p, rq, clamp_id); > > > > + > > > > +done: > > > > + task_rq_unlock(rq, p, &rf); > > > > +} > > > > > > > @@ -1008,11 +1043,11 @@ static int __setscheduler_uclamp(struct task_struct *p, > > > > > > > > mutex_lock(&uclamp_mutex); > > > > if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) { > > > > - uclamp_bucket_inc(&p->uclamp[UCLAMP_MIN], > > > > + uclamp_bucket_inc(p, &p->uclamp[UCLAMP_MIN], > > > > UCLAMP_MIN, lower_bound); > > > > } > > > > if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX) { > > > > - uclamp_bucket_inc(&p->uclamp[UCLAMP_MAX], > > > > + uclamp_bucket_inc(p, &p->uclamp[UCLAMP_MAX], > > > > UCLAMP_MAX, upper_bound); > > > > } > > > > mutex_unlock(&uclamp_mutex); > > > > > > > > > But.... __sched_setscheduler() actually does the whole dequeue + enqueue > > > thing already ?!? See where it does __setscheduler(). > > > > This is slow-path accounting, not fast path. > > Sure; but that's still no reason for duplicate or unneeded code. > > > There are two refcounting going on here: > > > > 1) mapped buckets: > > > > clamp_value <--(M1)--> bucket_id > > > > 2) RUNNABLE tasks: > > > > bucket_id <--(M2)--> RUNNABLE tasks in a bucket > > > > What we fix here is the refcounting for the buckets mapping. If a task > > does not have a task specific clamp value it does not refcount any > > bucket. The moment we assign a task specific clamp value, we need to > > refcount the task in the bucket corresponding to that clamp value. > > > > This will keep the bucket in use at least as long as the task will > > need that clamp value. > > Sure, I get that. What I don't get is why you're adding that (2) here. > Like said, __sched_setscheduler() already does a dequeue/enqueue under > rq->lock, which should already take care of that. Oh, ok... got it what you mean now. With: [PATCH v6 01/16] sched/core: Allow sched_setattr() to use the current policy <20190115101513.2822-2-patrick.bellasi@arm.com> we can call __sched_setscheduler() with: attr->sched_flags & SCHED_FLAG_KEEP_POLICY whenever we want just to change the clamp values of a task without changing its class. Thus, we can end up returning from __sched_setscheduler() without doing an actual dequeue/enqueue. This is likely the most common use-case. I'll better check if I can propagate this info and avoid M2 if we actually did a dequeue/enqueue. Cheers Patrick -- #include Patrick Bellasi