Re: [PATCH V1 1/1] sched/numa: Fix mm numa_scan_seq based unconditional scan

[Mel Gorman]

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