Commit 233e7aca authored Sep 21, 2020 by Barry Song Committed by Peter Zijlstra Sep 25, 2020

sched/fair: Use dst group while checking imbalance for NUMA balancer

Barry Song noted the following

Something is wrong. In find_busiest_group(), we are checking if
src has higher load, however, in task_numa_find_cpu(), we are
checking if dst will have higher load after balancing. It seems
it is not sensible to check src.

It maybe cause wrong imbalance value, for example,

if dst_running = env->dst_stats.nr_running + 1 results in 3 or
above, and src_running = env->src_stats.nr_running - 1 results
in 1;

The current code is thinking imbalance as 0 since src_running is
smaller than 2. This is inconsistent with load balancer.

Basically, in find_busiest_group(), the NUMA imbalance is ignored if moving
a task "from an almost idle domain" to a "domain with spare capacity". This
patch forbids movement "from a misplaced domain" to "an almost idle domain"
as that is closer to what the CPU load balancer expects.

This patch is not a universal win. The old behaviour was intended to allow
a task from an almost idle NUMA node to migrate to its preferred node if
the destination had capacity but there are corner cases. For example,
a NAS compute load could be parallelised to use 1/3rd of available CPUs
but not all those potential tasks are active at all times allowing this
logic to trigger. An obvious example is specjbb 2005 running various
numbers of warehouses on a 2 socket box with 80 cpus.

specjbb
5.9.0-rc4 5.9.0-rc4
vanilla dstbalance-v1r1
Hmean tput-1 46425.00 ( 0.00%) 43394.00 * -6.53%*
Hmean tput-2 98416.00 ( 0.00%) 96031.00 * -2.42%*
Hmean tput-3 150184.00 ( 0.00%) 148783.00 * -0.93%*
Hmean tput-4 200683.00 ( 0.00%) 197906.00 * -1.38%*
Hmean tput-5 236305.00 ( 0.00%) 245549.00 * 3.91%*
Hmean tput-6 281559.00 ( 0.00%) 285692.00 * 1.47%*
Hmean tput-7 338558.00 ( 0.00%) 334467.00 * -1.21%*
Hmean tput-8 340745.00 ( 0.00%) 372501.00 * 9.32%*
Hmean tput-9 424343.00 ( 0.00%) 413006.00 * -2.67%*
Hmean tput-10 421854.00 ( 0.00%) 434261.00 * 2.94%*
Hmean tput-11 493256.00 ( 0.00%) 485330.00 * -1.61%*
Hmean tput-12 549573.00 ( 0.00%) 529959.00 * -3.57%*
Hmean tput-13 593183.00 ( 0.00%) 555010.00 * -6.44%*
Hmean tput-14 588252.00 ( 0.00%) 599166.00 * 1.86%*
Hmean tput-15 623065.00 ( 0.00%) 642713.00 * 3.15%*
Hmean tput-16 703924.00 ( 0.00%) 660758.00 * -6.13%*
Hmean tput-17 666023.00 ( 0.00%) 697675.00 * 4.75%*
Hmean tput-18 761502.00 ( 0.00%) 758360.00 * -0.41%*
Hmean tput-19 796088.00 ( 0.00%) 798368.00 * 0.29%*
Hmean tput-20 733564.00 ( 0.00%) 823086.00 * 12.20%*
Hmean tput-21 840980.00 ( 0.00%) 856711.00 * 1.87%*
Hmean tput-22 804285.00 ( 0.00%) 872238.00 * 8.45%*
Hmean tput-23 795208.00 ( 0.00%) 889374.00 * 11.84%*
Hmean tput-24 848619.00 ( 0.00%) 966783.00 * 13.92%*
Hmean tput-25 750848.00 ( 0.00%) 903790.00 * 20.37%*
Hmean tput-26 780523.00 ( 0.00%) 962254.00 * 23.28%*
Hmean tput-27 1042245.00 ( 0.00%) 991544.00 * -4.86%*
Hmean tput-28 1090580.00 ( 0.00%) 1035926.00 * -5.01%*
Hmean tput-29 999483.00 ( 0.00%) 1082948.00 * 8.35%*
Hmean tput-30 1098663.00 ( 0.00%) 1113427.00 * 1.34%*
Hmean tput-31 1125671.00 ( 0.00%) 1134175.00 * 0.76%*
Hmean tput-32 968167.00 ( 0.00%) 1250286.00 * 29.14%*
Hmean tput-33 1077676.00 ( 0.00%) 1060893.00 * -1.56%*
Hmean tput-34 1090538.00 ( 0.00%) 1090933.00 * 0.04%*
Hmean tput-35 967058.00 ( 0.00%) 1107421.00 * 14.51%*
Hmean tput-36 1051745.00 ( 0.00%) 1210663.00 * 15.11%*
Hmean tput-37 1019465.00 ( 0.00%) 1351446.00 * 32.56%*
Hmean tput-38 1083102.00 ( 0.00%) 1064541.00 * -1.71%*
Hmean tput-39 1232990.00 ( 0.00%) 1303623.00 * 5.73%*
Hmean tput-40 1175542.00 ( 0.00%) 1340943.00 * 14.07%*
Hmean tput-41 1127826.00 ( 0.00%) 1339492.00 * 18.77%*
Hmean tput-42 1198313.00 ( 0.00%) 1411023.00 * 17.75%*
Hmean tput-43 1163733.00 ( 0.00%) 1228253.00 * 5.54%*
Hmean tput-44 1305562.00 ( 0.00%) 1357886.00 * 4.01%*
Hmean tput-45 1326752.00 ( 0.00%) 1406061.00 * 5.98%*
Hmean tput-46 1339424.00 ( 0.00%) 1418451.00 * 5.90%*
Hmean tput-47 1415057.00 ( 0.00%) 1381570.00 * -2.37%*
Hmean tput-48 1392003.00 ( 0.00%) 1421167.00 * 2.10%*
Hmean tput-49 1408374.00 ( 0.00%) 1418659.00 * 0.73%*
Hmean tput-50 1359822.00 ( 0.00%) 1391070.00 * 2.30%*
Hmean tput-51 1414246.00 ( 0.00%) 1392679.00 * -1.52%*
Hmean tput-52 1432352.00 ( 0.00%) 1354020.00 * -5.47%*
Hmean tput-53 1387563.00 ( 0.00%) 1409563.00 * 1.59%*
Hmean tput-54 1406420.00 ( 0.00%) 1388711.00 * -1.26%*
Hmean tput-55 1438804.00 ( 0.00%) 1387472.00 * -3.57%*
Hmean tput-56 1399465.00 ( 0.00%) 1400296.00 * 0.06%*
Hmean tput-57 1428132.00 ( 0.00%) 1396399.00 * -2.22%*
Hmean tput-58 1432385.00 ( 0.00%) 1386253.00 * -3.22%*
Hmean tput-59 1421612.00 ( 0.00%) 1371416.00 * -3.53%*
Hmean tput-60 1429423.00 ( 0.00%) 1389412.00 * -2.80%*
Hmean tput-61 1396230.00 ( 0.00%) 1351122.00 * -3.23%*
Hmean tput-62 1418396.00 ( 0.00%) 1383098.00 * -2.49%*
Hmean tput-63 1409918.00 ( 0.00%) 1374662.00 * -2.50%*
Hmean tput-64 1410236.00 ( 0.00%) 1376216.00 * -2.41%*
Hmean tput-65 1396405.00 ( 0.00%) 1364418.00 * -2.29%*
Hmean tput-66 1395975.00 ( 0.00%) 1357326.00 * -2.77%*
Hmean tput-67 1392986.00 ( 0.00%) 1349642.00 * -3.11%*
Hmean tput-68 1386541.00 ( 0.00%) 1343261.00 * -3.12%*
Hmean tput-69 1374407.00 ( 0.00%) 1342588.00 * -2.32%*
Hmean tput-70 1377513.00 ( 0.00%) 1334654.00 * -3.11%*
Hmean tput-71 1369319.00 ( 0.00%) 1334952.00 * -2.51%*
Hmean tput-72 1354635.00 ( 0.00%) 1329005.00 * -1.89%*
Hmean tput-73 1350933.00 ( 0.00%) 1318942.00 * -2.37%*
Hmean tput-74 1351714.00 ( 0.00%) 1316347.00 * -2.62%*
Hmean tput-75 1352198.00 ( 0.00%) 1309974.00 * -3.12%*
Hmean tput-76 1349490.00 ( 0.00%) 1286064.00 * -4.70%*
Hmean tput-77 1336131.00 ( 0.00%) 1303684.00 * -2.43%*
Hmean tput-78 1308896.00 ( 0.00%) 1271024.00 * -2.89%*
Hmean tput-79 1326703.00 ( 0.00%) 1290862.00 * -2.70%*
Hmean tput-80 1336199.00 ( 0.00%) 1291629.00 * -3.34%*

The performance at the mid-point is better but not universally better. The
patch is a mixed bag depending on the workload, machine and overall
levels of utilisation. Sometimes it's better (sometimes much better),
other times it is worse (sometimes much worse). Given that there isn't a
universally good decision in this section and more people seem to prefer
the patch then it may be best to keep the LB decisions consistent and
revisit imbalance handling when the load balancer code changes settle down.

Jirka Hladky added the following observation.

Our results are mostly in line with what you see. We observe
big gains (20-50%) when the system is loaded to 1/3 of the
maximum capacity and mixed results at the full load - some
workloads benefit from the patch at the full load, others not,
but performance changes at the full load are mostly within the
noise of results (+/-5%). Overall, we think this patch is helpful.

[mgorman@techsingularity.net: Rewrote changelog]
Fixes: fb86f5b2 ("sched/numa: Use similar logic to the load balancer for moving between domains with spare capacity")
Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200921221849.GI3179@techsingularity.net

parent 6e749913

kernel/sched/fair.c

+4 −4

Original line number	Diff line number	Diff line
		@@ -1550,7 +1550,7 @@ struct task_numa_env {
		static unsigned long cpu_load(struct rq *rq);
		static unsigned long cpu_runnable(struct rq *rq);
		static unsigned long cpu_util(int cpu);
		static inline long adjust_numa_imbalance(int imbalance, int src_nr_running);
		static inline long adjust_numa_imbalance(int imbalance, int nr_running);

		static inline enum
		numa_type numa_classify(unsigned int imbalance_pct,
		@@ -1930,7 +1930,7 @@ static void task_numa_find_cpu(struct task_numa_env *env,
		src_running = env->src_stats.nr_running - 1;
		dst_running = env->dst_stats.nr_running + 1;
		imbalance = max(0, dst_running - src_running);
		imbalance = adjust_numa_imbalance(imbalance, src_running);
		imbalance = adjust_numa_imbalance(imbalance, dst_running);

		/* Use idle CPU if there is no imbalance */
		if (!imbalance) {
		@@ -8967,7 +8967,7 @@ next_group:
		}
		}

		static inline long adjust_numa_imbalance(int imbalance, int src_nr_running)
		static inline long adjust_numa_imbalance(int imbalance, int nr_running)
		{
		unsigned int imbalance_min;

		@@ -8976,7 +8976,7 @@ static inline long adjust_numa_imbalance(int imbalance, int src_nr_running)
		* tasks that remain local when the source domain is almost idle.
		*/
		imbalance_min = 2;
		if (src_nr_running <= imbalance_min)
		if (nr_running <= imbalance_min)
		return 0;

		return imbalance;

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