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Date: Tue, 5 May 2020 15:27:58 +0100
From: Qais Yousef <qais.yousef@....com>
To: Patrick Bellasi <derkling@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Jonathan Corbet <corbet@....net>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Luis Chamberlain <mcgrof@...nel.org>,
Kees Cook <keescook@...omium.org>,
Iurii Zaikin <yzaikin@...gle.com>,
Quentin Perret <qperret@...gle.com>,
Valentin Schneider <valentin.schneider@....com>,
Pavan Kondeti <pkondeti@...eaurora.org>,
Randy Dunlap <rdunlap@...radead.org>,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v4 1/2] sched/uclamp: Add a new sysctl to control RT
default boost value
On 05/03/20 19:37, Patrick Bellasi wrote:
[...]
> > +static inline void uclamp_sync_util_min_rt_default(struct task_struct *p,
> > + enum uclamp_id clamp_id)
> > +{
> > + struct uclamp_se *uc_se;
> > +
> > + /* Only sync for UCLAMP_MIN and RT tasks */
> > + if (clamp_id != UCLAMP_MIN || likely(!rt_task(p)))
> ^^^^^^
> Are we sure that likely makes any difference when used like that?
>
> I believe you should either use:
>
> if (likely(clamp_id != UCLAMP_MIN || !rt_task(p)))
>
> or completely drop it.
I agree all these likely/unlikely better dropped.
>
> > + return;
> > +
> > + uc_se = &p->uclamp_req[UCLAMP_MIN];
>
> nit-pick: you can probably move this at declaration time.
>
> The compiler will be smart enough to either post-pone the init or, given
> the likely() above, "pre-fetch" the value.
>
> Anyway, the compiler is likely smarter then us. :)
I'll fling this question to the reviewers who voiced concerns about the
overhead. Personally I see the v3 implementation is the best fit :)
>
> > +
> > + /*
> > + * Only sync if user didn't override the default request and the sysctl
> > + * knob has changed.
> > + */
> > + if (unlikely(uc_se->user_defined) ||
> > + likely(uc_se->value == sysctl_sched_uclamp_util_min_rt_default))
> > + return;
>
> Same here, I believe likely/unlikely work only if wrapping a full if()
> condition. Thus, you should probably better split the above in two
> separate checks, which also makes for a better inline doc.
>
> > +
> > + uclamp_se_set(uc_se, sysctl_sched_uclamp_util_min_rt_default, false);
>
> Nit-pick: perhaps we can also improve a bit readability by defining at
> the beginning an alias variable with a shorter name, e.g.
>
> unsigned int uclamp_min = sysctl_sched_uclamp_util_min_rt_default;
>
> ?
Could do. I used default_util_min as a name though.
>
> > +}
> > +
> > static inline struct uclamp_se
> > uclamp_tg_restrict(struct task_struct *p, enum uclamp_id clamp_id)
> > {
> > @@ -907,8 +949,15 @@ uclamp_tg_restrict(struct task_struct *p, enum uclamp_id clamp_id)
> > static inline struct uclamp_se
> > uclamp_eff_get(struct task_struct *p, enum uclamp_id clamp_id)
> > {
> > - struct uclamp_se uc_req = uclamp_tg_restrict(p, clamp_id);
> > - struct uclamp_se uc_max = uclamp_default[clamp_id];
> > + struct uclamp_se uc_req, uc_max;
> > +
> > + /*
> > + * Sync up any change to sysctl_sched_uclamp_util_min_rt_default value.
> ^^^^^
> > + */
>
> nit-pick: we can use a single line comment if you drop the (useless)
> 'value' at the end.
Okay.
>
> > + uclamp_sync_util_min_rt_default(p, clamp_id);
> > +
> > + uc_req = uclamp_tg_restrict(p, clamp_id);
> > + uc_max = uclamp_default[clamp_id];
> >
> > /* System default restrictions always apply */
> > if (unlikely(uc_req.value > uc_max.value))
> > @@ -1114,12 +1163,13 @@ int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
> > loff_t *ppos)
> > {
> > bool update_root_tg = false;
> > - int old_min, old_max;
> > + int old_min, old_max, old_min_rt;
> > int result;
> >
> > mutex_lock(&uclamp_mutex);
> > old_min = sysctl_sched_uclamp_util_min;
> > old_max = sysctl_sched_uclamp_util_max;
> > + old_min_rt = sysctl_sched_uclamp_util_min_rt_default;
> >
> > result = proc_dointvec(table, write, buffer, lenp, ppos);
> > if (result)
> > @@ -1133,6 +1183,18 @@ int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
> > goto undo;
> > }
> >
> > + /*
> > + * The new value will be applied to RT tasks the next time the
> > + * scheduler needs to calculate the effective uclamp.min for that task,
> > + * assuming the task is using the system default and not a user
> > + * specified value. In the latter we shall leave the value as the user
> > + * requested.
>
> IMO it does not make sense to explain here what you will do with this
> value. This will make even more complicated to maintain the comment
> above if the code using it should change in the future.
>
> So, if the code where we use the knob is not clear enough, maybe we can
> move this comment to the description of:
> uclamp_sync_util_min_rt_default()
> or to be part of the documentation of:
> sysctl_sched_uclamp_util_min_rt_default
>
> By doing that you can also just add this if condition with the previous ones.
Okay.
>
> > + */
> > + if (sysctl_sched_uclamp_util_min_rt_default > SCHED_CAPACITY_SCALE) {
> > + result = -EINVAL;
> > + goto undo;
> > + }
> > +
> > if (old_min != sysctl_sched_uclamp_util_min) {
> > uclamp_se_set(&uclamp_default[UCLAMP_MIN],
> > sysctl_sched_uclamp_util_min, false);
> > @@ -1158,6 +1220,7 @@ int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
> > undo:
> > sysctl_sched_uclamp_util_min = old_min;
> > sysctl_sched_uclamp_util_max = old_max;
> > + sysctl_sched_uclamp_util_min_rt_default = old_min_rt;
> > done:
> > mutex_unlock(&uclamp_mutex);
> >
> > @@ -1200,9 +1263,13 @@ static void __setscheduler_uclamp(struct task_struct *p,
> > if (uc_se->user_defined)
> > continue;
> >
> > - /* By default, RT tasks always get 100% boost */
> > + /*
> > + * By default, RT tasks always get 100% boost, which the admins
> > + * are allowed to change via
> > + * sysctl_sched_uclamp_util_min_rt_default knob.
> > + */
> > if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
> > - clamp_value = uclamp_none(UCLAMP_MAX);
> > + clamp_value = sysctl_sched_uclamp_util_min_rt_default;
>
> Mmm... I suspect we don't need this anymore.
>
> If the task has a user_defined value, we skip this anyway.
> If the task has not a user_defined value, we will do set this anyway at
> each enqueue time.
>
> No?
Indeed.
Thanks
--
Qais Yousef
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