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 for more information:
https://lore.kernel.org/lkml/20231115025755.GA29979@xxxxxxxxxxxxxxxxxxx/
However, it's only for ones using hinting fault. I thought it'd be much
better if we have a general mechanism to reduce all tlb numbers that we
can ultimately apply to any type of migration.
I'm suggesting a mechanism called MIGRC that stands for 'Migration Read
Copy', to reduce tlb numbers by deferring tlb flush until the source
folios at migration actually become used, of course, only if the target
PTE don't have write permission.
To achieve that:
1. For the folios that map only to non-writable tlb entries, prevent
tlb flush during migration but perform it just before the source
folios actually become used out of buddy or pcp.
2. When any non-writable tlb entry changes to writable e.g. through
fault handler, give up migrc mechanism and perform tlb flush
required right away.
No matter what type of workload is used for performance evaluation, the
result would be positive thanks to the unconditional reduction of tlb
flushes, tlb misses and interrupts. For the test, I picked up XSBench
that is widely used for performance analysis on high performance
computing architectures - https://github.com/ANL-CESAR/XSBench.
The result would depend on memory latency and how often reclaim runs,
which implies tlb miss overhead and how many times migration happens.
The slower the memory is and the more reclaim runs, the better migrc
works so as to obtain the better result. In my system, the result
shows:
1. itlb flushes are reduced over 90%.
2. itlb misses are reduced over 30%.
3. All the other tlb numbers also get enhanced.
4. tlb shootdown interrupts are reduced over 90%.
5. The test program runtime is reduced over 5%.
The test envitonment:
Architecture - x86_64
QEMU - kvm enabled, host cpu
Numa - 2 nodes (16 CPUs 1GB, no CPUs 99GB)
Linux Kernel - v6.9-rc4, numa balancing tiering on, demotion enabled
< measurement: raw data - tlb and interrupt numbers >
$ 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 -t 16 -p 50000000
$ grep "TLB shootdowns" /proc/interrupts
BEFORE
------
40417078 itlb.itlb_flush
234852566 tlb_flush.dtlb_thread
153192357 tlb_flush.stlb_any
119001107892 dTLB-load-misses
307921167 dTLB-store-misses
1355272118 iTLB-load-misses
TLB: 1364803 1303670 1333921 1349607
1356934 1354216 1332972 1342842
1350265 1316443 1355928 1360793
1298239 1326358 1343006 1340971
TLB shootdowns
AFTER
-----
3316495 itlb.itlb_flush
138912511 tlb_flush.dtlb_thread
115199341 tlb_flush.stlb_any
117610390021 dTLB-load-misses
198042233 dTLB-store-misses
840066984 iTLB-load-misses
TLB: 117257 119219 117178 115737
117967 118948 117508 116079
116962 117266 117320 117215
105808 103934 115672 117610
TLB shootdowns
< measurement: user experience - runtime >
$ time XSBench -t 16 -p 50000000
BEFORE
------
Threads: 16
Runtime: 968.783 seconds
Lookups: 1,700,000,000
Lookups/s: 1,754,778
15208.91s user 141.44s system 1564% cpu 16:20.98 total
AFTER
-----
Threads: 16
Runtime: 913.210 seconds
Lookups: 1,700,000,000
Lookups/s: 1,861,565
14351.69s user 138.23s system 1565% cpu 15:25.47 total
---
Changes from v8:
1. Rebase on the latest, v6.9-rc4.
2. Supplement comments and commit message.
3. Change the candidate to apply migrc mechnism:
BEFORE - The source folios at demotion and promotion.
AFTER - The souce folios at any type of migration.
4. Change how migrc mechanism works:
BEFORE - Reduce tlb flushes by deferring folio_free() for
source folios during demotion and promotion.
AFTER - Reduce tlb flushes by deferring tlb flush until they
actually become used, out of pcp or buddy. The
current version of migrc does *not* defer calling
folio_free() but let it go as it is as the same as
vanilla kernel, with the folios marked kind of 'need
to tlb flush'. And then handle the flush when the
page exits from pcp or buddy so as to prevent
changing vm stats e.g. free pages.
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 parts from migrate_pages_batch()
mm: buddy: make room for a new variable, mgen, in struct page
mm: add folio_put_mgen() to deliver migrc's generation number to pcp
or buddy
mm: defer tlb flush until the source folios at migration actually get
used
arch/arm64/include/asm/tlbflush.h | 18 ++
arch/x86/include/asm/tlbflush.h | 18 ++
arch/x86/mm/tlb.c | 2 -
include/linux/mm.h | 22 ++
include/linux/mm_types.h | 39 ++-
include/linux/sched.h | 10 +
mm/compaction.c | 10 +
mm/internal.h | 85 +++++-
mm/memory.c | 8 +
mm/migrate.c | 487 ++++++++++++++++++++++++++----
mm/page_alloc.c | 125 ++++++--
mm/page_reporting.c | 10 +
mm/rmap.c | 40 ++-
mm/swap.c | 19 +-
14 files changed, 795 insertions(+), 98 deletions(-)
base-commit: 0bbac3facb5d6cc0171c45c9873a2dc96bea9680
--
2.17.1
