On Fri, May 28, 2021 at 07:39:29AM -0700, Dave Hansen wrote:

On 5/28/21 1:55 AM, Mel Gorman wrote:

quoted

-	 * onlined.
-	 */
-	nr_local_cpus = max(1U, cpumask_weight(cpumask_of_node(zone_to_nid(zone)))) + cpu_online;
-	high = total_pages / nr_local_cpus;
+	 * onlined. For memory nodes that have no CPUs, split pcp->high across
+	 * all online CPUs to mitigate the risk that reclaim is triggered
+	 * prematurely due to pages stored on pcp lists.
+	 */
+	nr_split_cpus = cpumask_weight(cpumask_of_node(zone_to_nid(zone))) + cpu_online;
+	if (!nr_split_cpus)
+		nr_split_cpus = num_online_cpus();
+	high = total_pages / nr_split_cpus;

Updated version looks fine to me, thanks!

BTW, to do some of this testing, Feng was doing a plain old kernel
build.  On the one system where this got run, he noted a ~2% regression
in build times.  Nothing major, but you might want to be on the lookout
in case 0day or the other test harnesses find something similar once
this series gets to them.

What type of system was it?

I noticed minor differences for some thread counts on kernel compilations
but for CascadeLake at least, it was mostly neutral. Below is an old test
result based on a previous revision.

kernbench
                               5.13.0-rc2             5.13.0-rc2
                                  vanilla       mm-pcpburst-v2r3
Amean     elsp-2        469.22 (   0.00%)      470.03 *  -0.17%*
Amean     elsp-4        251.03 (   0.00%)      250.83 (   0.08%)
Amean     elsp-8        131.39 (   0.00%)      130.89 (   0.38%)
Amean     elsp-16        74.37 (   0.00%)       75.11 (  -0.99%)
Amean     elsp-32        42.10 (   0.00%)       42.20 (  -0.24%)
Amean     elsp-64        32.21 (   0.00%)       32.14 (   0.23%)
Amean     elsp-128       31.59 (   0.00%)       31.68 (  -0.27%)
Amean     elsp-160       31.76 (   0.00%)       31.69 (   0.21%)

A Haswell machine showed the worst results for kernbench

Amean     elsp-2        459.99 (   0.00%)      465.27 *  -1.15%*
Amean     elsp-4        250.76 (   0.00%)      253.17 *  -0.96%*
Amean     elsp-8        141.28 (   0.00%)      141.78 (  -0.36%)
Amean     elsp-16        77.71 (   0.00%)       77.88 (  -0.22%)
Amean     elsp-32        44.09 (   0.00%)       44.40 (  -0.69%)
Amean     elsp-64        33.79 (   0.00%)       33.46 (   0.96%)
Amean     elsp-128       33.14 (   0.00%)       33.26 (  -0.37%)
Amean     elsp-160       33.26 (   0.00%)       33.36 *  -0.30%*

The series with review feedback and dealing with cpuless nodes is queued
and should complete over the weekend.

Acked-by: Dave Hansen <redacted>

Thanks!

-- 
Mel Gorman
SUSE Labs