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. 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.
