Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH v3 06/10] sched/fair: Use the prefer_sibling flag of the current sched domain | Date | Fri, 10 Feb 2023 17:12:30 +0000 |
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On 10/02/23 17:53, Peter Zijlstra wrote: > On Fri, Feb 10, 2023 at 02:54:56PM +0000, Valentin Schneider wrote: > >> So something like have SD_PREFER_SIBLING affect the SD it's on (and not >> its parent), but remove it from the lowest non-degenerated topology level? > > So I was rather confused about the whole moving it between levels things > this morning -- conceptually, prefer siblings says you want to try > sibling domains before filling up your current domain. Now, balancing > between siblings happens one level up, hence looking at child->flags > makes perfect sense. > > But looking at the current domain and still calling it prefer sibling > makes absolutely no sense what so ever. >
True :-)
> In that confusion I think I also got the polarity wrong, I thought you > wanted to kill prefer_sibling for the assymetric SMT cases, instead you > want to force enable it as long as there is one SMT child around. > > Whichever way around it we do it, I'm thinking perhaps some renaming > might be in order to clarify things. > > How about adding a flag SD_SPREAD_TASKS, which is the effective toggle > of the behaviour, but have it be set by children with SD_PREFER_SIBLING > or something. >
Or entirely bin SD_PREFER_SIBLING and stick with SD_SPREAD_TASKS, but yeah something along those lines.
> OTOH, there's also > > if (busiest->group_weight == 1 || sds->prefer_sibling) { > > which explicitly also takes the group-of-one (the !child case) into > account, but that's not consistently done. > > sds->prefer_sibling = !child || child->flags & SD_PREFER_SIBLING; > > seems an interesting option,
> except perhaps ASYM_CPUCAPACITY -- I > forget, can CPUs of different capacity be in the same leaf domain? With > big.LITTLE I think not, they had their own cache domains and so you get > at least MC domains per capacity, but DynamiQ might have totally wrecked > that party.
Yeah, newer systems can have different capacities in one MC domain, cf:
b7a331615d25 ("sched/fair: Add asymmetric CPU capacity wakeup scan")
> >> (+ add it to the first NUMA level to keep things as they are, even if TBF I >> find relying on it for NUMA balancing a bit odd). > > Arguably it ought to perhaps be one of those node_reclaim_distance > things. The thing is that NUMA-1 is often fairly quick, esp. these days > where it's basically on die numa.
Right, makes sense, thanks.
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