[PATCH v2] fs/address_space: add alignment padding for i_map and i_mmap_rwsem to mitigate a false sharing.

From: Zhu, Lipeng
Date: Sun Jul 16 2023 - 10:50:37 EST

When running UnixBench/Shell Scripts, we observed high false sharing
for accessing i_mmap against i_mmap_rwsem.

UnixBench/Shell Scripts are typical load/execute command test scenarios,
which concurrently launch->execute->exit a lot of shell commands.
A lot of processes invoke vma_interval_tree_remove which touch
"i_mmap", the call stack:
----vma_interval_tree_remove
|----unlink_file_vma
| free_pgtables
| |----exit_mmap
| | mmput
| | |----begin_new_exec
| | | load_elf_binary
| | | bprm_execve

Meanwhile, there are a lot of processes touch 'i_mmap_rwsem' to acquire
the semaphore in order to access 'i_mmap'.
In existing 'address_space' layout, 'i_mmap' and 'i_mmap_rwsem' are in
the same cacheline.

The patch places the i_mmap and i_mmap_rwsem in separate cache lines
to avoid this false sharing problem.

With this patch, based on kernel v6.4.0, on Intel Sapphire Rapids
112C/224T platform, the score improves by ~5.3%. And perf c2c tool
shows the false sharing is resolved as expected, the symbol
vma_interval_tree_remove disappeared in cache line 0 after this change.

Baseline:
=================================================
Shared Cache Line Distribution Pareto
=================================================
-------------------------------------------------------------
0 3729 5791 0 0 0xff19b3818445c740
-------------------------------------------------------------
3.27% 3.02% 0.00% 0.00% 0x18 0 1 0xffffffffa194403b 604 483 389 692 203 [k] vma_interval_tree_insert [kernel.kallsyms] vma_interval_tree_insert+75 0 1
4.13% 3.63% 0.00% 0.00% 0x20 0 1 0xffffffffa19440a2 553 413 415 962 215 [k] vma_interval_tree_remove [kernel.kallsyms] vma_interval_tree_remove+18 0 1
2.04% 1.35% 0.00% 0.00% 0x28 0 1 0xffffffffa219a1d6 1210 855 460 1229 222 [k] rwsem_down_write_slowpath [kernel.kallsyms] rwsem_down_write_slowpath+678 0 1
0.62% 1.85% 0.00% 0.00% 0x28 0 1 0xffffffffa219a1bf 762 329 577 527 198 [k] rwsem_down_write_slowpath [kernel.kallsyms] rwsem_down_write_slowpath+655 0 1
0.48% 0.31% 0.00% 0.00% 0x28 0 1 0xffffffffa219a58c 1677 1476 733 1544 224 [k] down_write [kernel.kallsyms] down_write+28 0 1
0.05% 0.07% 0.00% 0.00% 0x28 0 1 0xffffffffa219a21d 1040 819 689 33 27 [k] rwsem_down_write_slowpath [kernel.kallsyms] rwsem_down_write_slowpath+749 0 1
0.00% 0.05% 0.00% 0.00% 0x28 0 1 0xffffffffa17707db 0 1005 786 1373 223 [k] up_write [kernel.kallsyms] up_write+27 0 1
0.00% 0.02% 0.00% 0.00% 0x28 0 1 0xffffffffa219a064 0 233 778 32 30 [k] rwsem_down_write_slowpath [kernel.kallsyms] rwsem_down_write_slowpath+308 0 1
33.82% 34.10% 0.00% 0.00% 0x30 0 1 0xffffffffa1770945 779 495 534 6011 224 [k] rwsem_spin_on_owner [kernel.kallsyms] rwsem_spin_on_owner+53 0 1
17.06% 15.28% 0.00% 0.00% 0x30 0 1 0xffffffffa1770915 593 438 468 2715 224 [k] rwsem_spin_on_owner [kernel.kallsyms] rwsem_spin_on_owner+5 0 1
3.54% 3.52% 0.00% 0.00% 0x30 0 1 0xffffffffa2199f84 881 601 583 1421 223 [k] rwsem_down_write_slowpath [kernel.kallsyms] rwsem_down_write_slowpath+84 0 1

With this change:
-------------------------------------------------------------
0 556 838 0 0 0xff2780d7965d2780
-------------------------------------------------------------
0.18% 0.60% 0.00% 0.00% 0x8 0 1 0xffffffffafff27b8 503 453 569 14 13 [k] do_dentry_open [kernel.kallsyms] do_dentry_open+456 0 1
0.54% 0.12% 0.00% 0.00% 0x8 0 1 0xffffffffaffc51ac 510 199 428 15 12 [k] hugepage_vma_check [kernel.kallsyms] hugepage_vma_check+252 0 1
1.80% 2.15% 0.00% 0.00% 0x18 0 1 0xffffffffb079a1d6 1778 799 343 215 136 [k] rwsem_down_write_slowpath [kernel.kallsyms] rwsem_down_write_slowpath+678 0 1
0.54% 1.31% 0.00% 0.00% 0x18 0 1 0xffffffffb079a1bf 547 296 528 91 71 [k] rwsem_down_write_slowpath [kernel.kallsyms] rwsem_down_write_slowpath+655 0 1
0.72% 0.72% 0.00% 0.00% 0x18 0 1 0xffffffffb079a58c 1479 1534 676 288 163 [k] down_write [kernel.kallsyms] down_write+28 0 1
0.00% 0.12% 0.00% 0.00% 0x18 0 1 0xffffffffafd707db 0 2381 744 282 158 [k] up_write [kernel.kallsyms] up_write+27 0 1
0.00% 0.12% 0.00% 0.00% 0x18 0 1 0xffffffffb079a064 0 239 518 6 6 [k] rwsem_down_write_slowpath [kernel.kallsyms] rwsem_down_write_slowpath+308 0 1
46.58% 47.02% 0.00% 0.00% 0x20 0 1 0xffffffffafd70945 704 403 499 1137 219 [k] rwsem_spin_on_owner [kernel.kallsyms] rwsem_spin_on_owner+53 0 1
23.92% 25.78% 0.00% 0.00% 0x20 0 1 0xffffffffafd70915 558 413 500 542 185 [k] rwsem_spin_on_owner [kernel.kallsyms] rwsem_spin_on_owner+5 0 1

v1->v2: change padding to exchange fields.

Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Signed-off-by: Lipeng Zhu <lipeng.zhu@xxxxxxxxx>
---
include/linux/fs.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 133f0640fb24..4a525ed17eab 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -451,11 +451,11 @@ struct address_space {
atomic_t nr_thps;
#endif
struct rb_root_cached i_mmap;
- struct rw_semaphore i_mmap_rwsem;
unsigned long nrpages;
pgoff_t writeback_index;
const struct address_space_operations *a_ops;
unsigned long flags;
+ struct rw_semaphore i_mmap_rwsem;
errseq_t wb_err;
spinlock_t private_lock;
struct list_head private_list;
--
2.39.1