On Fri, Oct 20, 2023 at 09:27:46PM +0530, Raghavendra K T wrote:
> Since commit fc137c0ddab2 ("sched/numa: enhance vma scanning logic")
>
> NUMA Balancing allows updating PTEs to trap NUMA hinting faults if the
> task had previously accessed VMA. However unconditional scan of VMAs are
> allowed during initial phase of VMA creation until process's
> mm numa_scan_seq reaches 2 even though current task had not accessed VMA.
>
> Rationale:
> - Without initial scan subsequent PTE update may never happen.
> - Give fair opportunity to all the VMAs to be scanned and subsequently
> understand the access pattern of all the VMAs.
>
> But it has a corner case where, if a VMA is created after some time,
> process's mm numa_scan_seq could be already greater than 2.
>
> For e.g., values of mm numa_scan_seq when VMAs are created by running
> mmtest autonuma benchmark briefly looks like:
> start_seq=0 : 459
> start_seq=2 : 138
> start_seq=3 : 144
> start_seq=4 : 8
> start_seq=8 : 1
> start_seq=9 : 1
> This results in no unconditional PTE updates for those VMAs created after
> some time.
>
> Fix:
> - Note down the initial value of mm numa_scan_seq in per VMA start_seq.
> - Allow unconditional scan till start_seq + 2.
>
> Result:
> SUT: AMD EPYC Milan with 2 NUMA nodes 256 cpus.
> base kernel: upstream 6.6-rc6 with Mels patches [1] applied.
>
> kernbench
> ========== base patched %gain
> Amean elsp-128 165.09 ( 0.00%) 164.78 * 0.19%*
>
> Duration User 41404.28 41375.08
> Duration System 9862.22 9768.48
> Duration Elapsed 519.87 518.72
>
> Ops NUMA PTE updates 1041416.00 831536.00
> Ops NUMA hint faults 263296.00 220966.00
> Ops NUMA pages migrated 258021.00 212769.00
> Ops AutoNUMA cost 1328.67 1114.69
>
> autonumabench
>
> NUMA01_THREADLOCAL
> ==================
> Amean elsp-NUMA01_THREADLOCAL 81.79 (0.00%) 67.74 * 17.18%*
>
> Duration User 54832.73 47379.67
> Duration System 75.00 185.75
> Duration Elapsed 576.72 476.09
>
> Ops NUMA PTE updates 394429.00 11121044.00
> Ops NUMA hint faults 1001.00 8906404.00
> Ops NUMA pages migrated 288.00 2998694.00
> Ops AutoNUMA cost 7.77 44666.84
>
> Link: https://lore.kernel.org/linux-mm/ZSXF3AFZgIld1meX@xxxxxxxxx/T/
>
> Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxx>
--
Mel Gorman
SUSE Labs
