Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of running thread

From: Peter Zijlstra
Date: Thu Feb 25 2016 - 12:04:42 EST

On Thu, Feb 25, 2016 at 04:55:26PM +0000, Mathieu Desnoyers wrote:
> ----- On Feb 25, 2016, at 4:56 AM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote:
> The restartable sequences are intrinsically designed to work
> on per-cpu data, so they need to fetch the current CPU number
> within the rseq critical section. This is where the getcpu_cache
> system call becomes very useful when combined with rseq:
> getcpu_cache allows reading the current CPU number in a
> fraction of cycle.

Yes yes, I know how restartable sequences work.

But what I worry about is that they want a cpu number and a sequence
number, and for performance it would be very good if those live in the
same cacheline.

That means either getcpu needs to grow a seq number, or restartable
sequences need to _also_ provide the cpu number.