Re: [RFC PATCH 1/2] thread_local_abi system call: caching current CPU number (x86)

Andi Kleen
2015-12-13T18:15:32Z

> This getcpu cache is an alternative to the sched_getcpu() vdso which has
> a few benefits:

Note the first version of getcpu() I proposed had a cache. But it was
rejected.

> - It is faster to do a memory read that to call a vDSO,
> - This cached value can be read from within an inline assembly, which
> makes it a useful building block for restartable sequences.

On x86 we already have the de-facto ABI of using LSL with the magic
segment directly. While that is a few cycles slower than a memory load
I question the difference is big enough to justify a new system call,
and risk slow page fault in context switches.

BTW the vdso could be also optimized I think. For example glibc today
does some stupid (slow) things with it, like doing double iindirect
jumps.

-Andi