> -----Original Message----- > From: Dietmar Eggemann [mailto:dietmar.eggemann@xxxxxxx] > Sent: Wednesday, January 13, 2021 1:53 AM > To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>; Morten Rasmussen > <morten.rasmussen@xxxxxxx>; Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> > Cc: valentin.schneider@xxxxxxx; catalin.marinas@xxxxxxx; will@xxxxxxxxxx; > rjw@xxxxxxxxxxxxx; vincent.guittot@xxxxxxxxxx; lenb@xxxxxxxxxx; > gregkh@xxxxxxxxxxxxxxxxxxx; Jonathan Cameron <jonathan.cameron@xxxxxxxxxx>; > mingo@xxxxxxxxxx; peterz@xxxxxxxxxxxxx; juri.lelli@xxxxxxxxxx; > rostedt@xxxxxxxxxxx; bsegall@xxxxxxxxxx; mgorman@xxxxxxx; > mark.rutland@xxxxxxx; sudeep.holla@xxxxxxx; aubrey.li@xxxxxxxxxxxxxxx; > linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; > linux-acpi@xxxxxxxxxxxxxxx; linuxarm@xxxxxxxxxxxxx; xuwei (O) > <xuwei5@xxxxxxxxxx>; Zengtao (B) <prime.zeng@xxxxxxxxxxxxx>; tiantao (H) > <tiantao6@xxxxxxxxxxxxx> > Subject: Re: [RFC PATCH v3 0/2] scheduler: expose the topology of clusters and > add cluster scheduler > > On 08/01/2021 22:30, Song Bao Hua (Barry Song) wrote: > > > >> -----Original Message----- > >> From: Morten Rasmussen [mailto:morten.rasmussen@xxxxxxx] > >> Sent: Saturday, January 9, 2021 4:13 AM > >> To: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> > >> Cc: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>; > >> valentin.schneider@xxxxxxx; catalin.marinas@xxxxxxx; will@xxxxxxxxxx; > >> rjw@xxxxxxxxxxxxx; vincent.guittot@xxxxxxxxxx; lenb@xxxxxxxxxx; > >> gregkh@xxxxxxxxxxxxxxxxxxx; Jonathan Cameron > <jonathan.cameron@xxxxxxxxxx>; > >> mingo@xxxxxxxxxx; peterz@xxxxxxxxxxxxx; juri.lelli@xxxxxxxxxx; > >> dietmar.eggemann@xxxxxxx; rostedt@xxxxxxxxxxx; bsegall@xxxxxxxxxx; > >> mgorman@xxxxxxx; mark.rutland@xxxxxxx; sudeep.holla@xxxxxxx; > >> aubrey.li@xxxxxxxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; > >> linux-kernel@xxxxxxxxxxxxxxx; linux-acpi@xxxxxxxxxxxxxxx; > >> linuxarm@xxxxxxxxxxxxx; xuwei (O) <xuwei5@xxxxxxxxxx>; Zengtao (B) > >> <prime.zeng@xxxxxxxxxxxxx>; tiantao (H) <tiantao6@xxxxxxxxxxxxx> > >> Subject: Re: [RFC PATCH v3 0/2] scheduler: expose the topology of clusters > and > >> add cluster scheduler > >> > >> On Thu, Jan 07, 2021 at 03:16:47PM -0800, Tim Chen wrote: > >>> On 1/6/21 12:30 AM, Barry Song wrote: > >>>> ARM64 server chip Kunpeng 920 has 6 clusters in each NUMA node, and each > >>>> cluster has 4 cpus. All clusters share L3 cache data while each cluster > >>>> has local L3 tag. On the other hand, each cluster will share some > >>>> internal system bus. This means cache is much more affine inside one cluster > >>>> than across clusters. > >>> > >>> There is a similar need for clustering in x86. Some x86 cores could share > >> L2 caches that > >>> is similar to the cluster in Kupeng 920 (e.g. on Jacobsville there are 6 > clusters > >>> of 4 Atom cores, each cluster sharing a separate L2, and 24 cores sharing > >> L3). > >>> Having a sched domain at the L2 cluster helps spread load among > >>> L2 domains. This will reduce L2 cache contention and help with > >>> performance for low to moderate load scenarios. > >> > >> IIUC, you are arguing for the exact opposite behaviour, i.e. balancing > >> between L2 caches while Barry is after consolidating tasks within the > >> boundaries of a L3 tag cache. One helps cache utilization, the other > >> communication latency between tasks. Am I missing something? > > > > Morten, this is not true. > > > > we are both actually looking for the same behavior. My patch also > > has done the exact same behavior of spreading with Tim's patch. > > That's the case for the load-balance path because of the extra Sched > Domain (SD) (CLS/MC_L2) below MC. > > But in wakeup you add code which leads to a different packing strategy. Yes, but I put a note for the 1st case: "Case 1. we have two tasks *without* any relationship running in a system with 2 clusters and 8 cpus" so for tasks without wake-up relationship, the current patch will only result in spreading. Anyway, I will also test Tim's benchmark in kunpeng920 with the SCHED_CLUTER to see what will happen. Till now, benchmark has only covered the case to figure out the benefit of changing wake-up path. I would also be interested in figuring out what we have got from the change of load_balance(). > > It looks like that Tim's workload (SPECrate mcf) shows a performance > boost solely because of the changes the additional MC_L2 SD introduces > in load balance. The wakeup path is unchanged, i.e. llc-packing. IMHO we > have to carefully distinguish between packing vs. spreading in wakeup > and load-balance here. > > > Considering the below two cases: > > Case 1. we have two tasks without any relationship running in a system with > 2 clusters and 8 cpus. > > > > Without the sched_domain of cluster, these two tasks might be put as below: > > +-------------------+ +-----------------+ > > | +----+ +----+ | | | > > | |task| |task| | | | > > | |1 | |2 | | | | > > | +----+ +----+ | | | > > | | | | > > | cluster1 | | cluster2 | > > +-------------------+ +-----------------+ > > With the sched_domain of cluster, load balance will spread them as below: > > +-------------------+ +-----------------+ > > | +----+ | | +----+ | > > | |task| | | |task| | > > | |1 | | | |2 | | > > | +----+ | | +----+ | > > | | | | > > | cluster1 | | cluster2 | > > +-------------------+ +-----------------+ > > > > Then task1 and tasks2 get more cache and decrease cache contention. > > They will get better performance. > > > > That is what my original patch also can make. And tim's patch > > is also doing. Once we add a sched_domain, load balance will > > get involved. > > > > > > Case 2. we have 8 tasks, running in a system with 2 clusters and 8 cpus. > > But they are working in 4 groups: > > Task1 wakes up task4 > > Task2 wakes up task5 > > Task3 wakes up task6 > > Task4 wakes up task7 > > > > With my changing in select_idle_sibling, the WAKE_AFFINE mechanism will > > try to put task1 and 4, task2 and 5, task3 and 6, task4 and 7 in same clusters > rather > > than putting all of them in the random one of the 8 cpus. However, the 8 tasks > > are still spreading among the 8 cpus with my change in select_idle_sibling > > as load balance is still working. > > > > +---------------------------+ +----------------------+ > > | +----+ +-----+ | | +----+ +-----+ | > > | |task| |task | | | |task| |task | | > > | |1 | | 4 | | | |2 | |5 | | > > | +----+ +-----+ | | +----+ +-----+ | > > | | | | > > | cluster1 | | cluster2 | > > | | | | > > | | | | > > | +-----+ +------+ | | +-----+ +------+ | > > | |task | | task | | | |task | |task | | > > | |3 | | 6 | | | |4 | |8 | | > > | +-----+ +------+ | | +-----+ +------+ | > > +---------------------------+ +----------------------+ > > Your use-case (#tasks, runtime/period) seems to be perfectly crafted to > show the benefit of your patch on your specific system (cluster-size = > 4). IMHO, this extra infrastructure especially in the wakeup path should > show benefits over a range of different benchmarks. > > > Let's consider the 3rd case, that one would be more tricky: > > > > task1 and task2 have close relationship and they are waker-wakee pair. > > With my current patch, select_idle_sidling() wants to put them in one > > cluster, load balance wants to put them in two clusters. Load balance will > win. > > Then maybe we need some same mechanism like adjusting numa imbalance: > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/ > kernel/sched/fair.c?id=b396f52326de20 > > if we permit a light imbalance between clusters, select_idle_sidling() > > will win. And task1 and task2 get better cache affinity. > > This would look weird to allow this kind of imbalance on CLS (MC_L2) and > NUMA domains but not on the MC domain for example. Yes. I guess I actually meant permitting imbalance between sched_group made by the child sched_cluster domain of the parent sched_mc domain. sched_mc domain +----------------------------------+ | +--------+ +----------+ | | |sched_ | |sched_ | | | |group | |group | | | +--+-----+ +----+-----+ | | | allow small | | | | imbalance | | +----------------------------------+ | | | | | | | | | | + + child domain: child domain: sched_cluster sched_cluster For sched_group within one sched_cluster domain, we don't allow this kind of imbalance. Anyway, I would be happier to see this kind of imbalance is only allowed when we exactly know two tasks in the cluster have wake-up relationship. Right now, SD_NUMA seems to be simply allowing this imbalance without the knowledge of the relationships of tasks causing imbalance. Thanks Barry
