Messages in this thread | | | Subject | Re: [PATCH] sched/fair: Clear target from cpus to scan in select_idle_cpu | From | Yicong Yang <> | Date | Fri, 26 Nov 2021 17:38:19 +0800 |
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On 2021/11/25 20:46, Yicong Yang wrote: > On 2021/11/25 19:17, Mel Gorman wrote: >> On Wed, Nov 24, 2021 at 04:54:01PM +0800, Yicong Yang wrote: >>> Commit 56498cfb045d noticed that "When select_idle_cpu starts scanning for >>> an idle CPU, it starts with a target CPU that has already been checked >>> by select_idle_sibling. This patch starts with the next CPU instead." >>> It only changed the scanning start cpu to target + 1 but still leave >>> the target in the scanning cpumask. The target still have a chance to be >>> checked in the last turn. Fix this by clear the target from the cpus >>> to scan. >>> >>> Fixes: 56498cfb045d ("sched/fair: Avoid a second scan of target in select_idle_cpu") >>> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com> >> >> Did you check the performance of this? When I tried something like this >> in a different context, I found that the cost of clearing the bit was >> more expensive than simply using target + 1. For the target to be >> rescanned, the whole mask would have to be scanned as no other CPUs are >> idle which is the unlikely case. By clearing the bit, a cost is always >> incurred even if the first CPU scanned is idle. >> > > Not yet, it's from code. I've launched some tests and we'll see the results tomorrow. > > We traced the scanning here and seems the case that scan the whole LLC without > finding an idle cpu has some proportion. On 4-NUMA 128-Core Kunpeng 920 server > tested with mysql, there is ~1% probability for not finding and idle cpu when > sysbench threads is 128. The probability will increase when the load increases. > . >
Hi Mel,
I tested hackbench and tbench on our machine with numactl -N 0 run-mmtests.sh -c $config
config-workload-hackbench-process-pipes 5.16-rc1 5.16-rc1+patch Amean 1 0.5178 ( 0.00%) 0.5207 ( -0.56%) Amean 4 1.0108 ( 0.00%) 0.9274 ( 8.25%) Amean 7 1.9349 ( 0.00%) 1.8508 ( 4.35%) Amean 12 3.4179 ( 0.00%) 3.3170 ( 2.95%) Amean 21 5.9209 ( 0.00%) 5.8878 ( 0.56%) Amean 30 6.8677 ( 0.00%) 6.6241 * 3.55%* Amean 48 10.3759 ( 0.00%) 9.5785 * 7.69%* Amean 64 13.4606 ( 0.00%) 12.3713 * 8.09%*
config-network-tbench 5.16-rc1 5.16-rc1+patch Hmean 1 324.56 ( 0.00%) 324.01 * -0.17%* Hmean 2 650.91 ( 0.00%) 646.89 * -0.62%* Hmean 4 1291.16 ( 0.00%) 1298.56 * 0.57%* Hmean 8 2625.06 ( 0.00%) 2615.81 * -0.35%* Hmean 16 5293.86 ( 0.00%) 5267.24 * -0.50%* Hmean 32 8464.34 ( 0.00%) 9578.40 * 13.16%* Hmean 64 7417.02 ( 0.00%) 7218.91 * -2.67%* Hmean 128 6313.71 ( 0.00%) 6180.67 * -2.11%*
Thanks.
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