Messages in this thread | | | Date | Tue, 8 Jun 2021 15:09:37 -0400 | From | Joel Fernandes <> | Subject | Re: iowait boost is broken |
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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
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