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SubjectRe: [PATCH] 6/6 cacheline align files_lock
>> I'm getting a lot of cacheline bounce from .text.lock.file_table due to 
>> false sharing of the cahceline. The following patch just aligns the lock
>> in it's own cacheline.
>
> What about fixing it's use instead?

Seriously, I'd love to do that ... files_lock is the highest item of
the whole kernel compile profile overall, and accounts for about 10%
of all systime.

By I inlining the spinlocks, I can see the heavy users seem to be:

6278 166.0% get_empty_filp
5482 239.3% __fput
4770 254.7% file_move
-14464 -100.0% .text.lock.file_table

It seems to lock several different things though - free lists, tty lists,
etc, etc. I could break it up to one per list, but that means taking two
locks to shift between lists, which I'd prefer not to do if possible.

Andrew was talking about something more dramatic for it, but I'm not
sure exactly what ... any suggestions on how to drive a stake through
it's heart gladly accepted.

Ironically, the latest scheduler changes *seem* to get rid of about
57% of the cost of it .... I have *no* idea why. Diffprofile between
64 and 64-bk is just wierd:

7222 45.1% page_remove_rmap
521 18.4% __copy_from_user_ll
422 8.9% __copy_to_user_ll
334 105.4% __wake_up
248 48.5% pte_alloc_one
243 205.9% pgd_ctor
211 4.6% vm_enough_memory
198 6.3% zap_pte_range
179 29.3% kmem_cache_free
118 12.2% clear_page_tables
99 13.2% __pte_chain_free
84 6.1% do_no_page
76 6.6% release_pages
74 9.0% strnlen_user
67 5.6% free_hot_cold_page
65 14.4% copy_page_range
...
-54 -26.1% generic_file_open
-54 -7.8% dentry_open
-55 -16.3% do_lookup
-61 -1.7% find_get_page
-74 -58.7% __wake_up_sync
-74 -8.2% current_kernel_time
-82 -16.1% file_ra_state_init
-127 -8.0% schedule
-132 -19.1% fd_install
-148 -2.2% page_add_rmap
-246 -3.0% d_lookup
-263 -9.3% atomic_dec_and_lock
-407 -25.7% file_move
-455 -20.5% __fput
-853 -21.1% get_empty_filp
-870 -1.9% default_idle
-1724 -1.0% total
-8473 -59.8% .text.lock.file_table

page_remove_rmap I'm not too worried about, as I know objrmap will kill
that already (tried to combine them last night, but got an oops, which
I'm just about to try to recreate with just virgin -bk). But *why*
things are shifting about so much from (presumably) just the scheduler
changes is a mystery to me at least.

Any ideas?

M.
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