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SubjectRe: [v3 2/3] mm: Defer TLB flush by keeping both src and dst folios at migration
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On 30.10.23 08:25, Byungchul Park wrote:
> Implementation of CONFIG_MIGRC that stands for 'Migration Read Copy'.
> We always face the migration overhead at either promotion or demotion,
> while working with tiered memory e.g. CXL memory and found out TLB
> shootdown is a quite big one that is needed to get rid of if possible.
>
> Fortunately, TLB flush can be defered or even skipped if both source and
> destination of folios during migration are kept until all TLB flushes
> required will have been done, of course, only if the target PTE entries
> have read only permission, more precisely speaking, don't have write
> permission. Otherwise, no doubt the folio might get messed up.
>
> To achieve that:
>
> 1. For the folios that map only to non-writable TLB entries, prevent
> TLB flush at migration by keeping both source and destination
> folios, which will be handled later at a better time.
>
> 2. When any non-writable TLB entry changes to writable e.g. through
> fault handler, give up CONFIG_MIGRC mechanism so as to perform
> TLB flush required right away.
>
> 3. Temporarily stop migrc from working when the system is in very
> high memory pressure e.g. direct reclaim needed.
>
> The measurement result:
>
> Architecture - x86_64
> QEMU - kvm enabled, host cpu
> Numa - 2 nodes (16 CPUs 1GB, no CPUs 8GB)
> Linux Kernel - v6.6-rc5, numa balancing tiering on, demotion enabled
> Benchmark - XSBench -p 50000000 (-p option makes the runtime longer)
>
> run 'perf stat' using events:
> 1) itlb.itlb_flush
> 2) tlb_flush.dtlb_thread
> 3) tlb_flush.stlb_any
> 4) dTLB-load-misses
> 5) dTLB-store-misses
> 6) iTLB-load-misses
>
> run 'cat /proc/vmstat' and pick:
> 1) numa_pages_migrated
> 2) pgmigrate_success
> 3) nr_tlb_remote_flush
> 4) nr_tlb_remote_flush_received
> 5) nr_tlb_local_flush_all
> 6) nr_tlb_local_flush_one
>
> BEFORE - mainline v6.6-rc5
> ------------------------------------------
> $ perf stat -a \
> -e itlb.itlb_flush \
> -e tlb_flush.dtlb_thread \
> -e tlb_flush.stlb_any \
> -e dTLB-load-misses \
> -e dTLB-store-misses \
> -e iTLB-load-misses \
> ./XSBench -p 50000000
>
> Performance counter stats for 'system wide':
>
> 20953405 itlb.itlb_flush
> 114886593 tlb_flush.dtlb_thread
> 88267015 tlb_flush.stlb_any
> 115304095543 dTLB-load-misses
> 163904743 dTLB-store-misses
> 608486259 iTLB-load-misses
>
> 556.787113849 seconds time elapsed
>
> $ cat /proc/vmstat
>
> ...
> numa_pages_migrated 3378748
> pgmigrate_success 7720310
> nr_tlb_remote_flush 751464
> nr_tlb_remote_flush_received 10742115
> nr_tlb_local_flush_all 21899
> nr_tlb_local_flush_one 740157
> ...
>
> AFTER - mainline v6.6-rc5 + CONFIG_MIGRC
> ------------------------------------------
> $ perf stat -a \
> -e itlb.itlb_flush \
> -e tlb_flush.dtlb_thread \
> -e tlb_flush.stlb_any \
> -e dTLB-load-misses \
> -e dTLB-store-misses \
> -e iTLB-load-misses \
> ./XSBench -p 50000000
>
> Performance counter stats for 'system wide':
>
> 4353555 itlb.itlb_flush
> 72482780 tlb_flush.dtlb_thread
> 68226458 tlb_flush.stlb_any
> 114331610808 dTLB-load-misses
> 116084771 dTLB-store-misses
> 377180518 iTLB-load-misses
>
> 552.667718220 seconds time elapsed
>
> $ cat /proc/vmstat
>

So, an improvement of 0.74% ? How stable are the results? Serious
question: worth the churn?

Or did I get the numbers wrong?

> #define node_present_pages(nid) (NODE_DATA(nid)->node_present_pages)
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 5c02720c53a5..1ca2ac91aa14 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -135,6 +135,9 @@ enum pageflags {
> #ifdef CONFIG_ARCH_USES_PG_ARCH_X
> PG_arch_2,
> PG_arch_3,
> +#endif
> +#ifdef CONFIG_MIGRC
> + PG_migrc, /* Page has its copy under migrc's control */
> #endif
> __NR_PAGEFLAGS,
>
> @@ -589,6 +592,10 @@ TESTCLEARFLAG(Young, young, PF_ANY)
> PAGEFLAG(Idle, idle, PF_ANY)
> #endif
>
> +#ifdef CONFIG_MIGRC
> +PAGEFLAG(Migrc, migrc, PF_ANY)
> +#endif

I assume you know this: new pageflags are frowned upon.

--
Cheers,

David / dhildenb

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