Re: [PATCH] [RFC] sched: restrict iowait boost for boosted task only

[Wei Wang]

On Fri, Jan 24, 2020 at 3:01 AM Valentin Schneider
<valentin.schneider@xxxxxxx> wrote:
>
> On 24/01/2020 09:51, Quentin Perret wrote:
> >>> +static inline bool iowait_boosted(struct task_struct *p)
> >>> +{
> >>> +   return p->in_iowait && uclamp_eff_value(p, UCLAMP_MIN) > 0;
> >>
> >> I think this is overloading the usage of util clamp. You're basically using
> >> cpu.uclamp.min to temporarily switch iowait boost on/off.
> >>
> >> Isn't it better to add a new cgroup attribute to toggle this feature?
> >>
> >> The problem does seem generic enough and could benefit other battery-powered
> >> devices outside of the Android world. I don't think the dependency on uclamp &&
> >> energy model are necessary to solve this.
> >
> > I think using uclamp is not a bad idea here, but perhaps we could do
> > things differently. As of today the iowait boost escapes the clamping
> > mechanism, so one option would be to change that. That would let us set
> > a low max clamp in the 'background' cgroup, which in turns would limit
> > the frequency request for those tasks even if they're IO-intensive.
> >

Something we see e.g. is f2fs's gc thread, and my thought on this is
instead of chasing everything down, it is a lot easier for us to only
boost what we know in foreground (and on Android we sort of have that
information on hand from framework).
I was hesitant to introduce a new switch ( e.g. Android's older EAS
kernel prefer_idle toggle ) but would be happy to do that if someone
supports that idea.

>
>
>
> So I'm pretty sure we *do* want tasks with the default clamps to get iowait
> boost'd. What we don't want are background tasks driving up the frequency,
> and that should be via uclamp.max (as Quentin is suggesting) rather than
> uclamp.min (as is suggested in the patch).
>
> Now, whether that is overloading the usage of uclamp... I'm not sure.
> One of the argument for uclamp was actually frequency selection, so if
> we just make iowait boost respect that, IOW not boost further than
> uclamp.max (which is a bit better than a simple on/off switch), that
> wouldn't be too crazy I think.
>
>
> > That'll have to be done at the RQ level, but figuring out what's the
> > current max clamp on the rq should be doable from sugov I think.
> >
> > Wei: would that work for your use case ?
> >
> > Also, the iowait boost really should be per-task and not per-cpu, so it
> > can be taken into account during wake-up balance on big.LITTLE. That
> > might also help on SMP if a task doing a lot of IO migrates, as the
> > boost would migrate with it. But that's perhaps for later ...
> >
> > Thanks,
> > Quentin
> >