On Fri, Dec 11, 2020 at 10:51:17AM +0100, Vincent Guittot wrote: > On Thu, 10 Dec 2020 at 12:04, Mel Gorman <mgor...@techsingularity.net> wrote: > > > > On Thu, Dec 10, 2020 at 10:38:37AM +0100, Vincent Guittot wrote: > > > > while testing your patchset and Aubrey one on top of tip, I'm facing > > > > some perf regression on my arm64 numa system on hackbench and reaim. > > > > The regression seems to comes from your patchset but i don't know > > > > which patch in particular yet > > > > > > > > hackbench -l 256000 -g 1 > > > > > > > > v5.10-rc7 + tip/sched/core 13,255(+/- 3.22%) > > > > with your patchset 15.368(+/- 2.74) -15.9% > > > > > > > > I'm also seeing perf regression on reaim but this one needs more > > > > investigation before confirming > > > > > > > > TBH, I was not expecting regressions. I'm running more test to find > > > > which patch is the culprit > > > > > > The regression comes from patch 3: sched/fair: Do not replace > > > recent_used_cpu with the new target > > > > > > > That's not entirely surprising. The intent of the patch is to increase the > > hit rate of p->recent_used_cpu but it's not a guaranteed win due to two > > corner cases. If multiple tasks have the same p->recent_used_cpu, they can > > race to use that CPU and stack as a result instead of searching the domain. > > If SMT is enabled then p->recent_used_cpu can point to an idle CPU that has > > a busy sibling which the search would have avoided in select_idle_core(). > > > > I think you are using processes and sockets for hackbench but as you'll > > see later, hackbench can be used both to show losses and gains. > > I run more hackbench tests with pipe and socket and both show > regression with patch 3 whereas this is significant improvement with > other patches and Aubrey's one >
Is SMT enabled on your test machine? If not, then patch 4 should make no difference but if SMT is enabled, I wonder how this untested version of patch 3 behaves for you. The main difference is that the recent used cpu is used as a search target so that it would still check if it's an idle core and if not, fall through so it's used as an idle CPU after checking it's allowed by p->cpus_ptr. diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 5c41875aec23..63980bcf6e70 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6275,21 +6275,14 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target) return prev; } - /* Check a recently used CPU as a potential idle candidate: */ + /* Check a recently used CPU as a search target: */ recent_used_cpu = p->recent_used_cpu; + p->recent_used_cpu = prev; if (recent_used_cpu != prev && recent_used_cpu != target && cpus_share_cache(recent_used_cpu, target) && - (available_idle_cpu(recent_used_cpu) || sched_idle_cpu(recent_used_cpu)) && - cpumask_test_cpu(p->recent_used_cpu, p->cpus_ptr) && - asym_fits_capacity(task_util, recent_used_cpu)) { - /* - * Replace recent_used_cpu with prev as it is a potential - * candidate for the next wake: - */ - p->recent_used_cpu = prev; - return recent_used_cpu; - } + (available_idle_cpu(recent_used_cpu) || sched_idle_cpu(recent_used_cpu))) + target = recent_used_cpu; /* * For asymmetric CPU capacity systems, our domain of interest is @@ -6768,9 +6761,6 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags) } else if (wake_flags & WF_TTWU) { /* XXX always ? */ /* Fast path */ new_cpu = select_idle_sibling(p, prev_cpu, new_cpu); - - if (want_affine) - current->recent_used_cpu = cpu; } rcu_read_unlock(); -- Mel Gorman SUSE Labs