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

Byungchul Park
2024-02-26T03:06:31Z

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%.
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