[RFC PATCH] getcpu_cache system call: caching current CPU number (x86)

[Andy Lutomirski]

On Thu, Jul 16, 2015 at 11:08 AM, Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
> ----- On Jul 14, 2015, at 5:34 AM, Ben Maurer bmaurer@fb.com wrote:
>
>> Mathieu Desnoyers wrote:
>>> If we invoke this per-thread registration directly in the glibc NPTL
>>> implementation,
>>> in start_thread, do you think it would fit your requirements ?
>>
>> I guess this would basically be transparent to the user -- we'd just need to
>> make sure that the registration happens very early, before any chance of
>> calling malloc.
>
> Yes, this is my thinking too.
>
>>
>> That said, having the ability for the kernel to understand that TLS
>> implementation are laid out using the same offset on each thread seems like
>> something that could be valuable long term. Doing so makes it possible to build
>> other TLS-based features without forcing each thread to be registered.
>
> AFAIU, using a fixed hardcoded ABI between kernel and user-space might make
> transition from the pre-existing ABI (where this memory area is not
> reserved) a bit tricky without registering the area, or getting a "feature"
> flag, through a system call.
>
> The related question then becomes: should we issue this system call once
> per process, or once per thread at thread creation ? Issuing it once per
> thread is marginally more costly for thread creation, but seems to be
> easier to deal with internally within the kernel.
>
> We could however ensure that only a single system call is needed per new-coming
> thread, rather than one system call per feature. One way to do this would be
> to register an area that may contain more than just the CPU id. It could
> consist of an expandable structure with fixed offsets. When registered, we
> could pass the size of that structure as an argument to the system call, so
> the kernel knows which features are expected by user-space.

If we actually bit the bullet and implemented per-cpu mappings, we
could have this be completely flexible because there would be no
format at all.  Similarly, if we implemented per-cpu segments,
userspace would need to agree with *itself* how to arbitrate it, but
the kernel wouldn't need to be involved.

With this kind of memory poking, it's definitely messier, which is unfortunate.

--Andy