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Searching for a particular memory block by id is an O(n) operation because each memory block's underlying device is kept in an unsorted linked list on the subsystem bus. We can cut the lookup cost to O(log n) if we cache each memory block in an xarray. This time complexity improvement is significant on systems with many memory blocks. For example: 1. A 128GB POWER9 VM with 256MB memblocks has 512 blocks. With this change memory_dev_init() completes ~12ms faster and walk_memory_blocks() completes ~12ms faster. Before: [ 0.005042] memory_dev_init: adding memory blocks [ 0.021591] memory_dev_init: added memory blocks [ 0.022699] walk_memory_blocks: walking memory blocks [ 0.038730] walk_memory_blocks: walked memory blocks 0-511 After: [ 0.005057] memory_dev_init: adding memory blocks [ 0.009415] memory_dev_init: added memory blocks [ 0.010519] walk_memory_blocks: walking memory blocks [ 0.014135] walk_memory_blocks: walked memory blocks 0-511 2. A 256GB POWER9 LPAR with 256MB memblocks has 1024 blocks. With this change memory_dev_init() completes ~88ms faster and walk_memory_blocks() completes ~87ms faster. Before: [ 0.252246] memory_dev_init: adding memory blocks [ 0.395469] memory_dev_init: added memory blocks [ 0.409413] walk_memory_blocks: walking memory blocks [ 0.433028] walk_memory_blocks: walked memory blocks 0-511 [ 0.433094] walk_memory_blocks: walking memory blocks [ 0.500244] walk_memory_blocks: walked memory blocks 131072-131583 After: [ 0.245063] memory_dev_init: adding memory blocks [ 0.299539] memory_dev_init: added memory blocks [ 0.313609] walk_memory_blocks: walking memory blocks [ 0.315287] walk_memory_blocks: walked memory blocks 0-511 [ 0.315349] walk_memory_blocks: walking memory blocks [ 0.316988] walk_memory_blocks: walked memory blocks 131072-131583 3. A 32TB POWER9 LPAR with 256MB memblocks has 131072 blocks. With this change we complete memory_dev_init() ~37 minutes faster and walk_memory_blocks() at least ~30 minutes faster. The exact timing for walk_memory_blocks() is missing, though I observed that the soft lockups in walk_memory_blocks() disappeared with the change, suggesting that lower bound. Before: [ 13.703907] memory_dev_init: adding blocks [ 2287.406099] memory_dev_init: added all blocks [ 2347.494986] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160 [ 2527.625378] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160 [ 2707.761977] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160 [ 2887.899975] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160 [ 3068.028318] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160 [ 3248.158764] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160 [ 3428.287296] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160 [ 3608.425357] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160 [ 3788.554572] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160 [ 3968.695071] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160 [ 4148.823970] [c000000014c5bb60] [c000000000869af4] walk_memory_blocks+0x94/0x160 After: [ 13.696898] memory_dev_init: adding blocks [ 15.660035] memory_dev_init: added all blocks (the walk_memory_blocks traces disappear) There should be no significant negative impact for machines with few memory blocks. A sparse xarray has a small footprint and an O(log n) lookup is negligibly slower than an O(n) lookup for only the smallest number of memory blocks. 1. A 16GB x86 machine with 128MB memblocks has 132 blocks. With this change memory_dev_init() completes ~300us faster and walk_memory_blocks() completes no faster or slower. The improvement is pretty close to noise. Before: [ 0.224752] memory_dev_init: adding memory blocks [ 0.227116] memory_dev_init: added memory blocks [ 0.227183] walk_memory_blocks: walking memory blocks [ 0.227183] walk_memory_blocks: walked memory blocks 0-131 After: [ 0.224911] memory_dev_init: adding memory blocks [ 0.226935] memory_dev_init: added memory blocks [ 0.227089] walk_memory_blocks: walking memory blocks [ 0.227089] walk_memory_blocks: walked memory blocks 0-131 [david@redhat.com: document the locking] Link: http://lkml.kernel.org/r/bc21eec6-7251-4c91-2f57-9a0671f8d414@redhat.com Signed-off-by: Scott Cheloha <cheloha@linux.ibm.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Acked-by: David Hildenbrand <david@redhat.com> Acked-by: Nathan Lynch <nathanl@linux.ibm.com> Acked-by: Michal Hocko <mhocko@suse.com> Cc: Rafael J. Wysocki <rafael@kernel.org> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Rick Lindsley <ricklind@linux.vnet.ibm.com> Cc: Scott Cheloha <cheloha@linux.ibm.com> Link: http://lkml.kernel.org/r/20200121231028.13699-1-cheloha@linux.ibm.com Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.