Re: [RFC PATCH v3 for 4.15 08/24] Provide cpu_opv system call

[Thomas Gleixner]

On Fri, 17 Nov 2017, Mathieu Desnoyers wrote:
> ----- On Nov 17, 2017, at 5:09 AM, Thomas Gleixner tglx@xxxxxxxxxxxxx wrote:
> 7) Allow libraries with multi-part algorithms to work on same per-cpu
>    data without affecting the allowed cpu mask
> 
> I stumbled on an interesting use-case within the lttng-ust tracer
> per-cpu buffers: the algorithm needs to update a "reserve" counter,
> serialize data into the buffer, and then update a "commit" counter
> _on the same per-cpu buffer_. My goal is to use rseq for both reserve
> and commit.
> 
> Clearly, if rseq reserve fails, the algorithm can retry on a different
> per-cpu buffer. However, it's not that easy for the commit. It needs to
> be performed on the same per-cpu buffer as the reserve.
> 
> The cpu_opv system call solves that problem by receiving the cpu number
> on which the operation needs to be performed as argument. It can push
> the task to the right CPU if needed, and perform the operations there
> with preemption disabled.

If your transaction cannot be done in one go, then abusing that byte code
interpreter for concluding it is just hillarious. That whole exercise is a
gazillion times slower than the atomic operations which are neccesary to do
it without all that.

I'm even more convinced now that this is overengineered beyond repair.

Thanks,

	tglx