-----Original Message----- From: Dietmar Eggemann [mailto:dietmar.eggemann@arm.com] Sent: Wednesday, January 13, 2021 12:00 AM To: Morten Rasmussen morten.rasmussen@arm.com; Tim Chen tim.c.chen@linux.intel.com Cc: Song Bao Hua (Barry Song) song.bao.hua@hisilicon.com; valentin.schneider@arm.com; catalin.marinas@arm.com; will@kernel.org; rjw@rjwysocki.net; vincent.guittot@linaro.org; lenb@kernel.org; gregkh@linuxfoundation.org; Jonathan Cameron jonathan.cameron@huawei.com; mingo@redhat.com; peterz@infradead.org; juri.lelli@redhat.com; rostedt@goodmis.org; bsegall@google.com; mgorman@suse.de; mark.rutland@arm.com; sudeep.holla@arm.com; aubrey.li@linux.intel.com; linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org; linux-acpi@vger.kernel.org; linuxarm@openeuler.org; xuwei (O) xuwei5@huawei.com; Zengtao (B) prime.zeng@hisilicon.com; tiantao (H) tiantao6@hisilicon.com Subject: Re: [RFC PATCH v3 0/2] scheduler: expose the topology of clusters and add cluster scheduler
On 11/01/2021 10:28, Morten Rasmussen wrote:
On Fri, Jan 08, 2021 at 12:22:41PM -0800, Tim Chen wrote:
On 1/8/21 7:12 AM, Morten Rasmussen wrote:
On Thu, Jan 07, 2021 at 03:16:47PM -0800, Tim Chen wrote:
On 1/6/21 12:30 AM, Barry Song wrote:
[...]
I think it is going to depend on the workload. If there are dependent tasks that communicate with one another, putting them together in the same cluster will be the right thing to do to reduce communication costs. On the other hand, if the tasks are independent, putting them together
on the same cluster
will increase resource contention and spreading them out will be better.
Agree. That is exactly where I'm coming from. This is all about the task placement policy. We generally tend to spread tasks to avoid resource contention, SMT and caches, which seems to be what you are proposing to extend. I think that makes sense given it can produce significant benefits.
Any thoughts on what is the right clustering "tag" to use to clump related tasks together? Cgroup? Pid? Tasks with same mm?
I think this is the real question. I think the closest thing we have at the moment is the wakee/waker flip heuristic. This seems to be related. Perhaps the wake_affine tricks can serve as starting point?
wake_wide() switches between packing (select_idle_sibling(), llc_size CPUs) and spreading (find_idlest_cpu(), all CPUs).
AFAICS, since none of the sched domains set SD_BALANCE_WAKE, currently all wakeups are (llc-)packed.
Sorry for late response. I was struggling with some other topology issues recently.
For "all wakeups are (llc-)packed", it seems you mean current want_affine is only affecting the new_cpu, and for wake-up path, we will always go to select_idle_sibling() rather than find_idlest_cpu() since nobody sets SD_WAKE_BALANCE in any sched_domain ?
select_task_rq_fair()
for_each_domain(cpu, tmp)
if (tmp->flags & sd_flag) sd = tmp;In case we would like to further distinguish between llc-packing and even narrower (cluster or MC-L2)-packing, we would introduce a 2. level packing vs. spreading heuristic further down in sis().
I didn't get your point on "2 level packing". Would you like to describe more? It seems you mean we need to have separate calculation for avg_scan_cost and sched_feat(SIS_) for cluster (or MC-L2) since cluster and llc are not in the same level physically?
IMHO, Barry's current implementation doesn't do this right now. Instead he's trying to pack on cluster first and if not successful look further among the remaining llc CPUs for an idle CPU.
Yes. That is exactly what the current patch is doing.
Thanks Barry