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Date: Mon, 22 May 2023 14:29:31 +0800
From: Chen Yu <yu.c.chen@...el.com>
To: Yicong Yang <yangyicong@...wei.com>
CC: <peterz@...radead.org>, <mingo@...hat.com>,
<juri.lelli@...hat.com>, <vincent.guittot@...aro.org>,
<tim.c.chen@...ux.intel.com>, <gautham.shenoy@....com>,
<linux-kernel@...r.kernel.org>,
<linux-arm-kernel@...ts.infradead.org>, <dietmar.eggemann@....com>,
<rostedt@...dmis.org>, <bsegall@...gle.com>, <bristot@...hat.com>,
<prime.zeng@...wei.com>, <yangyicong@...ilicon.com>,
<jonathan.cameron@...wei.com>, <ego@...ux.vnet.ibm.com>,
<srikar@...ux.vnet.ibm.com>, <linuxarm@...wei.com>,
<21cnbao@...il.com>, <guodong.xu@...aro.org>,
<hesham.almatary@...wei.com>, <john.garry@...wei.com>,
<shenyang39@...wei.com>, <kprateek.nayak@....com>,
<wuyun.abel@...edance.com>, <tim.c.chen@...el.com>
Subject: Re: [RESEND PATCH v7 2/2] sched/fair: Scan cluster before scanning
LLC in wake-up path
Hi Yicong,
On 2022-09-15 at 15:34:23 +0800, Yicong Yang wrote:
> From: Barry Song <song.bao.hua@...ilicon.com>
>
> For platforms having clusters like Kunpeng920, CPUs within the same cluster
> have lower latency when synchronizing and accessing shared resources like
> cache. Thus, this patch tries to find an idle cpu within the cluster of the
> target CPU before scanning the whole LLC to gain lower latency.
>
> Testing has been done on Kunpeng920 by pinning tasks to one numa and two
> numa. On Kunpeng920, Each numa has 8 clusters and each cluster has 4 CPUs.
>
> With this patch, We noticed enhancement on tbench within one numa or cross
> two numa.
>
> On numa 0:
> 6.0-rc1 patched
> Hmean 1 351.20 ( 0.00%) 396.45 * 12.88%*
> Hmean 2 700.43 ( 0.00%) 793.76 * 13.32%*
> Hmean 4 1404.42 ( 0.00%) 1583.62 * 12.76%*
> Hmean 8 2833.31 ( 0.00%) 3147.85 * 11.10%*
> Hmean 16 5501.90 ( 0.00%) 6089.89 * 10.69%*
> Hmean 32 10428.59 ( 0.00%) 10619.63 * 1.83%*
> Hmean 64 8223.39 ( 0.00%) 8306.93 * 1.02%*
> Hmean 128 7042.88 ( 0.00%) 7068.03 * 0.36%*
>
> On numa 0-1:
> 6.0-rc1 patched
> Hmean 1 363.06 ( 0.00%) 397.13 * 9.38%*
> Hmean 2 721.68 ( 0.00%) 789.84 * 9.44%*
> Hmean 4 1435.15 ( 0.00%) 1566.01 * 9.12%*
> Hmean 8 2776.17 ( 0.00%) 3007.05 * 8.32%*
> Hmean 16 5471.71 ( 0.00%) 6103.91 * 11.55%*
> Hmean 32 10164.98 ( 0.00%) 11531.81 * 13.45%*
> Hmean 64 17143.28 ( 0.00%) 20078.68 * 17.12%*
> Hmean 128 14552.70 ( 0.00%) 15156.41 * 4.15%*
> Hmean 256 12827.37 ( 0.00%) 13326.86 * 3.89%*
>
> Note neither Kunpeng920 nor x86 Jacobsville supports SMT, so the SMT branch
> in the code has not been tested but it supposed to work.
>
May I know if this is the latest version to support cluster based wakeup?
I did a double check on Jacobsville(24 CPUs, 1 socket) with this patch applied.
Overall there are obvious improvements for netperf/tbench in throughput:
netperf
=======
case load baseline(std%) compare%( std%)
TCP_RR 6-threads 1.00 ( 0.59) +6.63 ( 0.71)
TCP_RR 12-threads 1.00 ( 0.25) +5.90 ( 0.16)
TCP_RR 18-threads 1.00 ( 0.39) +9.49 ( 0.49)
TCP_RR 24-threads 1.00 ( 0.95) +2.61 ( 0.94)
TCP_RR 30-threads 1.00 ( 5.01) +2.37 ( 3.82)
TCP_RR 36-threads 1.00 ( 3.73) +2.02 ( 2.97)
TCP_RR 42-threads 1.00 ( 3.88) +1.99 ( 3.96)
TCP_RR 48-threads 1.00 ( 1.39) +1.74 ( 1.50)
UDP_RR 6-threads 1.00 ( 1.31) +5.04 ( 1.70)
UDP_RR 12-threads 1.00 ( 0.30) +8.18 ( 0.20)
UDP_RR 18-threads 1.00 ( 0.37) +10.94 ( 0.59)
UDP_RR 24-threads 1.00 ( 0.84) +1.12 ( 0.99)
UDP_RR 30-threads 1.00 ( 4.70) +1.61 ( 6.54)
UDP_RR 36-threads 1.00 ( 10.53) +1.71 ( 2.67)
UDP_RR 42-threads 1.00 ( 2.52) +0.63 ( 3.60)
UDP_RR 48-threads 1.00 ( 1.61) +0.12 ( 1.27)
tbench
======
case load baseline(std%) compare%( std%)
loopback 6-threads 1.00 ( 0.60) +2.94 ( 0.23)
loopback 12-threads 1.00 ( 0.11) +4.27 ( 0.23)
loopback 18-threads 1.00 ( 0.12) +13.45 ( 0.14)
loopback 24-threads 1.00 ( 0.13) +0.69 ( 0.24)
loopback 30-threads 1.00 ( 0.34) +0.42 ( 0.15)
loopback 36-threads 1.00 ( 0.29) +0.58 ( 0.07)
loopback 42-threads 1.00 ( 0.06) +0.38 ( 0.45)
loopback 48-threads 1.00 ( 0.04) +0.15 ( 0.68)
schbench
========
case load baseline(std%) compare%( std%)
normal 1-mthreads 1.00 ( 4.56) +3.23 ( 0.00)
normal 2-mthreads 1.00 ( 0.00) +0.00 ( 0.00)
normal 4-mthreads 1.00 ( 11.00) -8.82 ( 16.66)
normal 8-mthreads 1.00 ( 7.10) -4.49 ( 3.26)
hackbench
=========
case load baseline(std%) compare%( std%)
process-pipe 1-groups 1.00 ( 0.62) +4.71 ( 0.96)
process-pipe 2-groups 1.00 ( 0.84) +3.56 ( 2.35)
process-pipe 4-groups 1.00 ( 1.56) +6.74 ( 0.74)
process-pipe 8-groups 1.00 ( 14.27) +0.85 ( 8.34)
process-sockets 1-groups 1.00 ( 0.36) -8.05 ( 1.54)
process-sockets 2-groups 1.00 ( 3.19) +1.77 ( 2.39)
process-sockets 4-groups 1.00 ( 1.86) -29.10 ( 2.63)
process-sockets 8-groups 1.00 ( 1.77) -2.94 ( 1.55)
threads-pipe 1-groups 1.00 ( 0.74) +6.62 ( 0.94)
threads-pipe 2-groups 1.00 ( 1.28) +7.50 ( 0.93)
threads-pipe 4-groups 1.00 ( 0.80) +8.72 ( 4.54)
threads-pipe 8-groups 1.00 ( 8.77) +6.49 ( 7.49)
threads-sockets 1-groups 1.00 ( 0.43) -4.35 ( 0.27)
threads-sockets 2-groups 1.00 ( 0.35) -5.60 ( 1.86)
threads-sockets 4-groups 1.00 ( 0.61) -26.87 ( 2.35)
threads-sockets 8-groups 1.00 ( 0.81) -6.60 ( 0.62)
And there is regression from hackbench in socket mode, especially in
4 groups case.
In 4 groups case, the fd descriptors of each hackbench group is 3, so there
are 3 x 4 x 2 = 24 tasks in the system, which is the same number
as the online CPUs.
I added schedstats trace and found that it was due to target CPU(becauase the
idle CPU scan in select_idle_sibling() failed) is chosen more offen than
the previous CPU with this patch applied. And with this patch applied, when
there are 4 groups of hackbench, some CPUs are around 80% utilization, while
without the patch applied, every CPU is nearly 100% utilized. This suggested
that, task migration is unnecessary in this case, just to put the wakee on its
previous CPU is optimal and could mitigate race condition. I did an experiment
to keep the cpus_share_cache() as it is when checking prev cpu and recent_used_cpu,
the regression was gone(comment below).
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> [https://lore.kernel.org/lkml/Ytfjs+m1kUs0ScSn@worktop.programming.kicks-ass.net]
> Tested-by: Yicong Yang <yangyicong@...ilicon.com>
> Signed-off-by: Barry Song <song.bao.hua@...ilicon.com>
> Signed-off-by: Yicong Yang <yangyicong@...ilicon.com>
> Reviewed-by: Tim Chen <tim.c.chen@...ux.intel.com>
> Reviewed-by: Chen Yu <yu.c.chen@...el.com>
> ---
> kernel/sched/fair.c | 30 +++++++++++++++++++++++++++---
> kernel/sched/sched.h | 1 +
> kernel/sched/topology.c | 10 ++++++++++
> 3 files changed, 38 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 4e5b171b1171..e6505b0764c0 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6444,6 +6444,30 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> }
> }
>
> + if (static_branch_unlikely(&sched_cluster_active)) {
> + struct sched_domain *sdc = rcu_dereference(per_cpu(sd_cluster, target));
> +
> + if (sdc) {
> + for_each_cpu_wrap(cpu, sched_domain_span(sdc), target + 1) {
> + if (!cpumask_test_cpu(cpu, cpus))
> + continue;
> +
> + if (has_idle_core) {
> + i = select_idle_core(p, cpu, cpus, &idle_cpu);
> + if ((unsigned int)i < nr_cpumask_bits)
> + return i;
> + } else {
> + if (--nr <= 0)
> + return -1;
> + idle_cpu = __select_idle_cpu(cpu, p);
> + if ((unsigned int)idle_cpu < nr_cpumask_bits)
> + return idle_cpu;
> + }
> + }
> + cpumask_andnot(cpus, cpus, sched_domain_span(sdc));
> + }
> + }
> +
> for_each_cpu_wrap(cpu, cpus, target + 1) {
> if (has_idle_core) {
> i = select_idle_core(p, cpu, cpus, &idle_cpu);
> @@ -6451,7 +6475,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> return i;
>
> } else {
> - if (!--nr)
> + if (--nr <= 0)
This change seems to not be needed because if the cluster scan has run out of nr budget,
it will return -1 there, and there's no need to check nr here. But yes, with this
change the code is more readable.
> return -1;
> idle_cpu = __select_idle_cpu(cpu, p);
> if ((unsigned int)idle_cpu < nr_cpumask_bits)
> @@ -6550,7 +6574,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> /*
> * If the previous CPU is cache affine and idle, don't be stupid:
> */
> - if (prev != target && cpus_share_cache(prev, target) &&
> + if (prev != target && cpus_share_lowest_cache(prev, target) &&
This change impacts hackbench in socket mode a bit. It seems that for hackbench even
putting the wakee on its previous CPU in the same LLC is better than putting it on
current cluster. But it seems to be hackbench specific.
thanks,
Chenyu
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