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

[Peter Zijlstra]

On Wed, Nov 01, 2023 at 09:21:01AM +0000, Mel Gorman wrote:
> 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>

Thanks, will queue for the next merge window (6.8 I think that is) once
6.7-rc1 comes around.