[v4 0/3] Reduce TLB flushes under some specific conditions

[Byungchul Park]

Hi everyone,

While I'm working with 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").

However, it's only for ones using hinting fault. I thought it'd be much
better if we have a general mechanism to reduce # of TLB flushes and
TLB misses, that we can apply to any type of migration. I tried it only
for tiering migration for now tho.

I'm suggesting a mechanism 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 at all. I worked Based on v6.6-rc5.

Can you believe it? I saw the number of TLB full flush reduced about
80% and iTLB miss reduced about 50%, and the time wise performance
always shows at least 1% stable improvement with the workload I tested
with, XSBench. However, I believe that it would help more with other
ones or any real ones. It'd be appreciated to let me know if I'm missing
something.

	Byungchul

---

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 (3):
  mm/rmap: Recognize read-only TLB entries during batched TLB flush
  mm: Defer TLB flush by keeping both src and dst folios at migration
  mm: Pause migrc mechanism at high memory pressure

 arch/x86/include/asm/tlbflush.h |   3 +
 arch/x86/mm/tlb.c               |  11 ++
 include/linux/mm_types.h        |  21 +++
 include/linux/mmzone.h          |   9 ++
 include/linux/page-flags.h      |   4 +
 include/linux/sched.h           |   7 +
 include/trace/events/mmflags.h  |   3 +-
 mm/internal.h                   |  78 ++++++++++
 mm/memory.c                     |  11 ++
 mm/migrate.c                    | 266 ++++++++++++++++++++++++++++++++
 mm/page_alloc.c                 |  30 +++-
 mm/rmap.c                       |  35 ++++-
 12 files changed, 475 insertions(+), 3 deletions(-)

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2.17.1