Re: iowait boost is broken

[Joel Fernandes <joel@joelfernandes.org>]

Hi Beata,

On Mon, Jun 07, 2021 at 08:10:32PM +0100, Beata Michalska wrote:
> Hi Joel,
> 
> Thanks for sending this out.

Np, thanks for replying.

> On Mon, Jun 07, 2021 at 12:19:01PM -0400, Joel Fernandes wrote:
> > Hi all,
> > Looks like iowait boost is completely broken upstream. Just
> > documenting my findings of iowait boost issues:
> > 
> I wouldn't go as far to state that it is completely broken. Rather that
> the current sugov implementation for iowait boosting is not meeting
> the expectations and I believe this should be clarified first. More on those
> expectations below.
> > 1. If a CPU requests iowait boost in a cluster, another CPU can go
> > ahead and reset very quickly it since it thinks there's no new request
> > for the iowait boosting CPU
> So the 'boosting' value is being tracked per CPU, so each core in a cluster
> will have it's own variant of that. When calculating the shared freq for
> the cluster, sugov will use max utilization reported on each core, including
> I/O boost. Now, if there is no pending request for boosting on a given core
> at the time of calling sugov_iowait_apply, the current 'boost' will be
> reduced, but only this one and that will not affect boost values on remaining
> CPUs. It means that there was no task waking up on that particular CPU after
> waiting on I/O request. So I would say it's fine. Unless I am misunderstanding
> your case ?

Yes, but consider the case where the I/O is slow on one CPU (call it X) so
say IO wait takes 2 milliseconds. Now another CPU (call it Y) is
continuiously making cpufreq requests much faster than that. Also consider
that the slow CPU X is doing back to back I/O request and has consecutive
I/O sleep time (no other sleep, just I/O sleep). What you'll see is the
CPU X's boost always stays at _MIN boost when it wakes up because Y reset it
to 0 in the meanwhile. So the boost never accumulates. Does that make sense?
I would say that the I/O CPU should have a 'doubling' of boost. Probably the
issue can be solved by making rate_limit_us longer than the iowait time. But
that seems like a hack and would likely cause other issues.

> > 2. If the iowait is longer than a tick, then successive iowait boost
> > doubling does not happen. So heavy I/O waiting code never gets a
> > boost.
> This might indeed be an issue. What's more: the fact that boosting is applied
> per core, any migration will disregard the boosting applied so far, so
> it might start increasing the freq on 'new' CPU from scratch.
> Might, as sugov (and the I/O boosting) has no notion of the source of boosting
> request so it might end up on a core that has already lifted I/O boost.
> This also means, that having different small tasks, waking up from
> I/O within the mentioned TICK_NSEC time window might drive the frequency to max
> even though those would be sporadic wakeups. Things get slightly
> cumbersome as increasing the boost does not necessarily result in the freq
> change, so the above mentioned case would need proper timing but it is possible.
> Also the boost value will not get doubled unless previous one has been applied.
> This might result in misalignment between task wake-ups/placement and sugov's
> freq changes.

Agreed, there are many issues, thanks for highlighting.

> > 3. update_load_avg() is triggered right after the the iowait boost
> > request which makes another cpufreq update request, this request is a
> > non-iowait boost one so it ends up resetting the iowait boost request
> > (in the same path!).
> Not necessarily - this is guarded by the TICK_NSEC you have mentioned:
> in
>     sugov_iowait_reset {
>          ...
>          if (delta_ns <= TICK_NSEC)
>              return;
>          ...
>     }
> So for the particular call sequence the boost should not get reset.
> Another problem is when the non-I/O bound tasks triggers the update
> after the TICK_NSEC has elapsed and before the frequency change is done.
> (see sugov_should_update_freq )

Oh yeah. You are quite right on this particular issue. I agree it shouldn't
reset. So I wonder what I was seeing for #3 then... Hopefully delta_ns can be
trusted..

> > 4. Same as #3 but due the update_blocked_averages from new idle balance path.
> > 
> > Here is a patch that tries to address these problems and I see better
> > cpufreq boosting happening, however it is just a test broken patch to
> > highlight the issues:
> > https://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git/commit/?h=sched/5.4/iowait-boost-debug-1&id=3627d896d499d168fef9a388e5d6b3359acc3423
> > 
> > I think we ought to rewrite the whole mess instead of fixing it since
> > a lot has changed in scheduler code over time it feels. Beata is
> > working on rewriting the whole iowait boost infra, I am glad she has
> > started the work on this and looking forward to helping with the
> > patches.
> > 
> So back to the expectations.
> The main problem, as I see it, is what do we actually want to achieve with
> the I/O boosting? Is it supposed to compensate the time lost while waiting
> for the I/O request to be completed or is is supposed to optimize the rate
> at which I/O requests are being made. Do we want to boost I/O bound tasks by
> default, no limits applied  or should we care about balancing performance
> vs power ? And unless those expectations are clearly stated, we might not
> get too far with any changes, really.

Yeah part of the problem its not clear how much boost is 'ideal'. However, not having any boost at all or a
boost too low for heavy I/O seems a clear issues. I guess some boost will be
better than no boost. But I agree its hard to design an algorithm that fits
everything.

> Smth that I do agree with is what has been suggested few times by Quentin,
> to make the I/O boosting being a per-task feature, not per-core, and that is
> smth I have been experimenting with.

Yeah I agree with that too.

Thanks,
Joel