Re: perf: Translating mmap2 ids into socket info?

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Oct 22, 2014 at 12:20:26PM -0400, Don Zickus wrote:
> Hi,
> 
> A question/request came up during our cache to cache analysis.  We were
> wondering if give an unique mmap2 id (major, minor, inode, inode
> generation), if it was possible to determine a cpu socket id that memory
> was attached to at the time of the captured perf event?

No, see below. Also socket is the wrong information, both AMD and Intel
have chips with two nodes in one socket :-)

> We ran into a scenario awhile back where a dcache struct was allocated on
> say node2 and then subsequently free'd.  The memory was thought to be
> re-allocated on node0 for another dcache entry.  It turned out the memory
> was still attached to node2 and was causing slowdowns.

Yes, kernel memory is directly addresses, you basically have a static
address->node mapping, it never changes.

For instance, on my ivb-ep I can find the following in my dmesg:

[    0.000000] NUMA: Initialized distance table, cnt=2
[    0.000000] NUMA: Node 0 [mem 0x00000000-0xbfffffff] + [mem 0x100000000-0x43fffffff] -> [mem 0x00000000-0x43fffffff]
[    0.000000] NODE_DATA(0) allocated [mem 0x43fffc000-0x43fffffff]
[    0.000000] NODE_DATA(1) allocated [mem 0x83fff9000-0x83fffcfff]
[    0.000000]  [ffffea0000000000-ffffea000edfffff] PMD -> [ffff88042fe00000-ffff88043ddfffff] on node 0
[    0.000000]  [ffffea000ee00000-ffffea001cdfffff] PMD -> [ffff88082f600000-ffff88083d5fffff] on node 1
[    0.000000] Zone ranges:
[    0.000000]   DMA      [mem 0x00001000-0x00ffffff]
[    0.000000]   DMA32    [mem 0x01000000-0xffffffff]
[    0.000000]   Normal   [mem 0x100000000-0x83fffffff]
[    0.000000] Movable zone start for each node
[    0.000000] Early memory node ranges
[    0.000000]   node   0: [mem 0x00001000-0x0008dfff]
[    0.000000]   node   0: [mem 0x00100000-0xbad28fff]
[    0.000000]   node   0: [mem 0xbaf90000-0xbafc4fff]
[    0.000000]   node   0: [mem 0xbafda000-0xbb3d3fff]
[    0.000000]   node   0: [mem 0xbdfac000-0xbdffffff]
[    0.000000]   node   0: [mem 0x100000000-0x43fffffff]
[    0.000000]   node   1: [mem 0x440000000-0x83fffffff]
[    0.000000] Initmem setup node 0 [mem 0x00001000-0x43fffffff]
[    0.000000] On node 0 totalpages: 4174137
[    0.000000]   DMA zone: 56 pages used for memmap
[    0.000000]   DMA zone: 21 pages reserved
[    0.000000]   DMA zone: 3981 pages, LIFO batch:0
[    0.000000]   DMA32 zone: 10422 pages used for memmap
[    0.000000]   DMA32 zone: 762284 pages, LIFO batch:31
[    0.000000]   Normal zone: 46592 pages used for memmap
[    0.000000]   Normal zone: 3407872 pages, LIFO batch:31
[    0.000000] Initmem setup node 1 [mem 0x440000000-0x83fffffff]
[    0.000000] On node 1 totalpages: 4194304
[    0.000000]   Normal zone: 57344 pages used for memmap
[    0.000000]   Normal zone: 4194304 pages, LIFO batch:31

> Our cache-to-cache tool noticed the slowdown but we couldn't understand
> why because we had falsely assumed the memory was allocated on the local
> node but instead it was on the remote node.

But in general, you can never say for user memory, since that has the
process page table mapping in between, the user virtual address is
unrelated to backing (and can change frequently and without
notification).

Therefore the mmap(2) information is useless for this, it only concerns
user memory.