Re: [PATCH] vmalloc: Remove alloc_map from vmap_block.

[Chanho Min]

>I started looking for workloads to profile but then lost interest.
>The current code can theoretically end up walking through a lot of
>partially used blocks if a string of allocations never fit any of
>them.  The number of these blocks depends on previous allocations that
>leave them unusable for future allocations and whether any other
>vmalloc/vmap user recently flushed them all.  So it's painful to think
>about it and hard to impossible to pin down should this ever actually
>result in a performance problem.

vm_map_ram() is allowed to be called by external kernel module.
I profiled some kernel module as bellow perf log. Its mapping behavior
was most of the workload. yes, we can improve its inefficient mapping.
But, This shows the allocation bitmap has the potential to cause significant
overhead.

# Overhead          Command        Shared Object
                  Symbol
# ........  ...............  ...................
.............................................
#
    42.74%  XXXXXXTextureSc  [kernel.kallsyms]    [k] __reg_op
            |
            --- __reg_op
               |
               |--5.39%-- 0xaf57de00
               |          |
               |           --100.00%-- malloc
               |
               |--2.35%-- 0xaf57da00
               |          |
               |           --100.00%-- malloc
               |
               |--2.10%-- 0xaf57ce00
               |          |
               |           --100.00%-- malloc
               |
               |--1.46%-- 0xaf57c800
               |          |
               |           --100.00%-- malloc
               |
               |--1.43%-- 0xaf57dc00
               |          |
               |           --100.00%-- malloc
               |
               |--1.36%-- 0xaf57c200
               |          |
               |           --100.00%-- malloc
               |
               |--1.34%-- 0xaf57c000
               |          |
               |           --100.00%-- malloc
               |
               |--1.26%-- 0xae915400
               |          |
               |           --100.00%-- malloc
               |
               |--0.80%-- 0xae914200
               |          |
               |           --100.00%-- malloc
               |
               |--0.79%-- 0xaf57ca00
               |          |
               |           --100.00%-- malloc
               |
               |--0.67%-- 0xaf57d000
               |          |
               |           --100.00%-- malloc
               |
               |--0.52%-- 0xaf57cc00
               |          |
               |           --100.00%-- malloc
                --80.54%-- [...]
    17.39%  XXXXXXTextureSc  [kernel.kallsyms]    [k]
bitmap_find_free_region
            |
            --- bitmap_find_free_region
               |
               |--99.93%-- vm_map_ram
               |          0x7f00097c
               |          0x7f00985c
               |          |
               |          |--99.79%-- 0x7f009948
               |          |          |
               |          |          |--50.24%-- 0x7f00ab90
               |          |          |          0x7f006c50
               |          |          |          0x7f00e948
               |          |          |          0x7f019630
               |          |          |          0x7f00f0ac
               |          |          |          0x7f002384
               |          |          |          vfs_ioctl
               |          |          |          do_vfs_ioctl
               |          |          |          sys_ioctl
               |          |          |          ret_fast_syscall
               |          |          |
               |          |          |--49.60%-- 0x7f00acfc
               |          |          |          0x7f006bfc
               |          |          |          0x7f018fac
               |          |          |          0x7f00f0ac
               |          |          |          0x7f002384
               |          |          |          vfs_ioctl
               |          |          |          do_vfs_ioctl
               |          |          |          sys_ioctl
               |          |          |          ret_fast_syscall
               |          |          |          malloc
               |          |           --0.16%-- [...]
...

Thanks
Chanho Min
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