On Mon, Feb 05, 2024 at 02:22:29PM +0800, JonasZhou wrote: > When running UnixBench/execl, each execl process repeatedly performs > i_mmap_lock_write -> vma_interval_tree_remove/insert -> > i_mmap_unlock_write. As indicated below, when i_mmap and i_mmap_rwsem > are in the same CACHE Line, there will be more HITM. (I wasn't familiar with the term HITM. For anyone else who's unfamiliar, this appears to mean a HIT in another core's cache, which has the cachline in the Modified state) > Func0: i_mmap_lock_write > Func1: vma_interval_tree_remove/insert > Func2: i_mmap_unlock_write > In the same CACHE Line > Process A | Process B | Process C | Process D | CACHE Line state > ----------+-----------+-----------+-----------+----------------- > Func0 | | | | I->M > | Func0 | | | HITM M->S > Func1 | | | | may change to M > | | Func0 | | HITM M->S > Func2 | | | | S->M > | | | Func0 | HITM M->S > > In different CACHE Lines > Process A | Process B | Process C | Process D | CACHE Line state > ----------+-----------+-----------+-----------+----------------- > Func0 | | | | I->M > | Func0 | | | HITM M->S > Func1 | | | | > | | Func0 | | S->S > Func2 | | | | S->M > | | | Func0 | HITM M->S > > The same issue will occur in Unixbench/shell because the shell > launches a lot of shell commands, loads executable files and dynamic > libraries into memory, execute, and exit. OK, I see. > Yes, his commit has been merged into the Linux kernel, but there > is an issue. After moving i_mmap_rwsem below flags, there is a > 32-byte gap between i_mmap_rwsem and i_mmap. However, the struct > address_space is aligned to sizeof(long), which is 8 on the x86-64 > architecture. As a result, i_mmap_rwsem and i_mmap may be placed on > the same CACHE Line, causing a false sharing problem. This issue has > been observed using the perf c2c tool. Got it. OK, let's put this patch in. It's a stopgap measure, clearly. I'll reply to Dave's email with a longer term solution.
