On Tue, Oct 24, 2023 at 6:56 AM Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> wrote: > > Now ptep_clear_flush_young() is only called by folio_referenced() to > check if the folio was referenced, and now it will call a tlb flush on > ARM64 architecture. However the tlb flush can be expensive on ARM64 > servers, especially for the systems with a large CPU numbers. > > Similar to the x86 architecture, below comments also apply equally to > ARM64 architecture. So we can drop the tlb flush operation in > ptep_clear_flush_young() on ARM64 architecture to improve the performance. > " > /* Clearing the accessed bit without a TLB flush > * doesn't cause data corruption. [ It could cause incorrect > * page aging and the (mistaken) reclaim of hot pages, but the > * chance of that should be relatively low. ] > * > * So as a performance optimization don't flush the TLB when > * clearing the accessed bit, it will eventually be flushed by > * a context switch or a VM operation anyway. [ In the rare > * event of it not getting flushed for a long time the delay > * shouldn't really matter because there's no real memory > * pressure for swapout to react to. ] > */ > " > Running the thpscale to show some obvious improvements for compaction > latency with this patch: > base patched > Amean fault-both-1 1093.19 ( 0.00%) 1084.57 * 0.79%* > Amean fault-both-3 2566.22 ( 0.00%) 2228.45 * 13.16%* > Amean fault-both-5 3591.22 ( 0.00%) 3146.73 * 12.38%* > Amean fault-both-7 4157.26 ( 0.00%) 4113.67 * 1.05%* > Amean fault-both-12 6184.79 ( 0.00%) 5218.70 * 15.62%* > Amean fault-both-18 9103.70 ( 0.00%) 7739.71 * 14.98%* > Amean fault-both-24 12341.73 ( 0.00%) 10684.23 * 13.43%* > Amean fault-both-30 15519.00 ( 0.00%) 13695.14 * 11.75%* > Amean fault-both-32 16189.15 ( 0.00%) 14365.73 * 11.26%* > base patched > Duration User 167.78 161.03 > Duration System 1836.66 1673.01 > Duration Elapsed 2074.58 2059.75 > > Barry Song submitted a similar patch [1] before, that replaces the > ptep_clear_flush_young_notify() with ptep_clear_young_notify() in > folio_referenced_one(). However, I'm not sure if removing the tlb flush > operation is applicable to every architecture in kernel, so dropping > the tlb flush for ARM64 seems a sensible change. > > Note: I am okay for both approach, if someone can help to ensure that > all architectures do not need the tlb flush when clearing the accessed > bit, then I also think Barry's patch is better (hope Barry can resend > his patch). > > [1] https://lore.kernel.org/lkml/20220617070555.344368-1-21cnbao@xxxxxxxxx/ > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> +Minchan Kim Minchan and I discussed this (again) yesterday -- I'm in favor and he can voice his different opinion on this.
