Re: [PATCH] arm64: mm: drop tlb flush operation when clearing the access bit

[Yu Zhao <yuzhao@google.com>]

On Tue, Oct 24, 2023 at 6:56 AM Baolin Wang
<baolin.wang@linux.alibaba.com> 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@gmail.com/
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>

+Minchan Kim

Minchan and I discussed this (again) yesterday -- I'm in favor and he
can voice his different opinion on this.