Re: [RESEND PATCH v8 0/8] Reduce TLB flushes by 94% by improving folio migration

Byungchul Park
2024-02-29T09:28:17Z

On Mon, Feb 26, 2024 at 12:06:05PM +0900, Byungchul Park wrote:
> Hi everyone,
>
> While I'm working with a tiered memory system e.g. CXL memory, I have
> been facing migration overhead esp. TLB shootdown on promotion or
> demotion between different tiers. Yeah.. most TLB shootdowns on
> migration through hinting fault can be avoided thanks to Huang Ying's
> work, commit 4d4b6d66db ("mm,unmap: avoid flushing TLB in batch if PTE
> is inaccessible"). See the following link:
>
> https://lore.kernel.org/lkml/20231115025755.GA29979@system.software.com/
>
> However, it's only for ones using hinting fault. I thought it'd be much
> better if we have a general mechanism to reduce the number of TLB
> flushes and TLB misses, that we can ultimately apply to any type of
> migration, I tried it only for tiering for now tho.
>
> I'm suggesting a mechanism called MIGRC that stands for 'Migration Read
> Copy', to reduce TLB flushes by keeping source and destination of folios
> participated in the migrations until all TLB flushes required are done,
> only if those folios are not mapped with write permission PTE entries.
>
> 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 migrc mechanism so as to perform TLB flush
> required right away.
>
> I observed a big improvement of TLB flushes # and TLB misses # at the
> following evaluation using XSBench like:
>
> 1. itlb flush was reduced by 93.9%.
> 2. dtlb thread was reduced by 43.5%.
> 3. stlb flush was reduced by 24.9%.

Hi guys,

The TLB flush reduction is 25% ~ 94%, IMO, it's unbelievable.

While modern computer architectures are typically capable of handling a
large number of TLB flush hardware requests without significant
performance degradation, it's still important to minimize the number of
unnecessary hardware events whenever possible.

The impact of excessive TLB flushes on system performance can vary
depending on factors such as the amount of TLB miss overhead your
particular system experiences. Nevertheless, reducing the freqency of
TLB flushes can contribute to greater overall system stablity and
performance.

I'm convinced this mechanism could help your systems operate better with
much less TLB flushes and misses.

Byungchul

> 4. dtlb store misses was reduced by 34.2%.
> 5. itlb load misses was reduced by 45.5%.
> 6. The runtime was reduced by 3.5%.
>
> I believe that it would help more with any real cases.
>
> ---
>
> The measurement result:
>
> Architecture - x86_64
> QEMU - kvm enabled, host cpu
> Numa - 2 nodes (16 CPUs 1GB, no CPUs 8GB)
> Linux Kernel - v6.7, numa balancing tiering on, demotion enabled
> Benchmark - XSBench -p 100000000 (-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 v6.7
> ----------------------
> $ 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 100000000
>
> Performance counter stats for 'system wide':
>
> 85647229 itlb.itlb_flush
> 480981504 tlb_flush.dtlb_thread
> 323937200 tlb_flush.stlb_any
> 238381632579 dTLB-load-misses
> 601514255 dTLB-store-misses
> 2974157461 iTLB-load-misses
>
> 2252.883892112 seconds time elapsed
>
> $ cat /proc/vmstat
>
> ...
> numa_pages_migrated 12790664
> pgmigrate_success 26835314
> nr_tlb_remote_flush 3031412
> nr_tlb_remote_flush_received 45234862
> nr_tlb_local_flush_all 216584
> nr_tlb_local_flush_one 740940
> ...
>
> AFTER - mainline v6.7 + 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 100000000
>
> Performance counter stats for 'system wide':
>
> 5240261 itlb.itlb_flush
> 271581774 tlb_flush.dtlb_thread
> 243149389 tlb_flush.stlb_any
> 234502983364 dTLB-load-misses
> 395673680 dTLB-store-misses
> 1620215163 iTLB-load-misses
>
> 2172.283436287 seconds time elapsed
>
> $ cat /proc/vmstat
>
> ...
> numa_pages_migrated 14897064
> pgmigrate_success 30825530
> nr_tlb_remote_flush 198290
> nr_tlb_remote_flush_received 2820156
> nr_tlb_local_flush_all 92048
> nr_tlb_local_flush_one 741401
> ...
>
> ---
>
> Changes from v7:
> 1. Rewrite cover letter to explain what 'migrc' mechasism is.
> (feedbacked by Andrew Morton)
> 2. Supplement the commit message of a patch 'mm: Add APIs to
> free a folio directly to the buddy bypassing pcp'.
> (feedbacked by Andrew Morton)
>
> Changes from v6:
> 1. Fix build errors in case of
> CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH disabled by moving
> migrc_flush_{start,end}() calls from arch code to
> try_to_unmap_flush() in mm/rmap.c.
>
> Changes from v5:
> 1. Fix build errors in case of CONFIG_MIGRATION disabled or
> CONFIG_HWPOISON_INJECT moduled. (feedbacked by kernel test
> bot and Raymond Jay Golo)
> 2. Organize migrc code with two kconfigs, CONFIG_MIGRATION and
> CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH.
>
> Changes from v4:
>
> 1. Rebase on v6.7.
> 2. Fix build errors in arm64 that is doing nothing for TLB flush
> but has CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH. (reported
> by kernel test robot)
> 3. Don't use any page flag. So the system would give up migrc
> mechanism more often but it's okay. The final improvement is
> good enough.
> 4. Instead, optimize full TLB flush(arch_tlbbatch_flush()) by
> avoiding redundant CPUs from TLB flush.
>
> Changes from v3:
>
> 1. Don't use the kconfig, CONFIG_MIGRC, and remove sysctl knob,
> migrc_enable. (feedbacked by Nadav)
> 2. Remove the optimization skipping CPUs that have already
> performed TLB flushes needed by any reason when performing
> TLB flushes by migrc because I can't tell the performance
> difference between w/ the optimization and w/o that.
> (feedbacked by Nadav)
> 3. Minimize arch-specific code. While at it, move all the migrc
> declarations and inline functions from include/linux/mm.h to
> mm/internal.h (feedbacked by Dave Hansen, Nadav)
> 4. Separate a part making migrc paused when the system is in
> high memory pressure to another patch. (feedbacked by Nadav)
> 5. Rename:
> a. arch_tlbbatch_clean() to arch_tlbbatch_clear(),
> b. tlb_ubc_nowr to tlb_ubc_ro,
> c. migrc_try_flush_free_folios() to migrc_flush_free_folios(),
> d. migrc_stop to migrc_pause.
> (feedbacked by Nadav)
> 6. Use ->lru list_head instead of introducing a new llist_head.
> (feedbacked by Nadav)
> 7. Use non-atomic operations of page-flag when it's safe.
> (feedbacked by Nadav)
> 8. Use stack instead of keeping a pointer of 'struct migrc_req'
> in struct task, which is for manipulating it locally.
> (feedbacked by Nadav)
> 9. Replace a lot of simple functions to inline functions placed
> in a header, mm/internal.h. (feedbacked by Nadav)
> 10. Add additional sufficient comments. (feedbacked by Nadav)
> 11. Remove a lot of wrapper functions. (feedbacked by Nadav)
>
> Changes from RFC v2:
>
> 1. Remove additional occupation in struct page. To do that,
> unioned with lru field for migrc's list and added a page
> flag. I know page flag is a thing that we don't like to add
> but no choice because migrc should distinguish folios under
> migrc's control from others. Instead, I force migrc to be
> used only on 64 bit system to mitigate you guys from getting
> angry.
> 2. Remove meaningless internal object allocator that I
> introduced to minimize impact onto the system. However, a ton
> of tests showed there was no difference.
> 3. Stop migrc from working when the system is in high memory
> pressure like about to perform direct reclaim. At the
> condition where the swap mechanism is heavily used, I found
> the system suffered from regression without this control.
> 4. Exclude folios that pte_dirty() == true from migrc's interest
> so that migrc can work simpler.
> 5. Combine several patches that work tightly coupled to one.
> 6. Add sufficient comments for better review.
> 7. Manage migrc's request in per-node manner (from globally).
> 8. Add TLB miss improvement in commit message.
> 9. Test with more CPUs(4 -> 16) to see bigger improvement.
>
> Changes from RFC:
>
> 1. Fix a bug triggered when a destination folio at the previous
> migration becomes a source folio at the next migration,
> before the folio gets handled properly so that the folio can
> play with another migration. There was inconsistency in the
> folio's state. Fixed it.
> 2. Split the patch set into more pieces so that the folks can
> review better. (Feedbacked by Nadav Amit)
> 3. Fix a wrong usage of barrier e.g. smp_mb__after_atomic().
> (Feedbacked by Nadav Amit)
> 4. Tried to add sufficient comments to explain the patch set
> better. (Feedbacked by Nadav Amit)
>
> Byungchul Park (8):
> x86/tlb: Add APIs manipulating tlb batch's arch data
> arm64: tlbflush: Add APIs manipulating tlb batch's arch data
> mm/rmap: Recognize read-only TLB entries during batched TLB flush
> x86/tlb, mm/rmap: Separate arch_tlbbatch_clear() out of
> arch_tlbbatch_flush()
> mm: Separate move/undo doing on folio list from migrate_pages_batch()
> mm: Add APIs to free a folio directly to the buddy bypassing pcp
> mm: Defer TLB flush by keeping both src and dst folios at migration
> mm: Pause migrc mechanism at high memory pressure
>
> arch/arm64/include/asm/tlbflush.h | 19 ++
> arch/x86/include/asm/tlbflush.h | 18 ++
> arch/x86/mm/tlb.c | 2 -
> include/linux/mm.h | 23 ++
> include/linux/mmzone.h | 7 +
> include/linux/sched.h | 9 +
> mm/internal.h | 78 ++++++
> mm/memory.c | 8 +
> mm/migrate.c | 411 ++++++++++++++++++++++++++----
> mm/page_alloc.c | 34 ++-
> mm/rmap.c | 40 ++-
> mm/swap.c | 7 +
> 12 files changed, 597 insertions(+), 59 deletions(-)
>
>
> base-commit: 0dd3ee31125508cd67f7e7172247f05b7fd1753a
> --
> 2.17.1