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

[Thomas Gleixner]

On Fri, 26 Feb 2016, Mathieu Desnoyers wrote:
> ----- On Feb 26, 2016, at 11:29 AM, Thomas Gleixner tglx@xxxxxxxxxxxxx wrote:
> > Right. There is no point in having two calls and two update mechanisms for a
> > very similar purpose.
> > 
> > So let userspace have one struct where cpu/seq and whatever is required for
> > rseq is located and flag at register time which parts of the struct need to be
> > updated.
> 
> If we put both cpu/seq/other in that structure, why not plan ahead and make
> it extensible then ?
> 
> That looks very much like the "Thread-local ABI" series I posted last year.
> See https://lkml.org/lkml/2015/12/22/464
> 
> Here is why I ended up introducing the specialized "getcpu_cache" system call
> rather than the "generic" system call (quote from the getcpu_cache changelog):
> 
>     Rationale for the getcpu_cache system call rather than the thread-local
>     ABI system call proposed earlier:
>     
>     Rather than doing a "generic" thread-local ABI, specialize this system
>     call for a cpu number cache only. Anyway, the thread-local ABI approach
>     would have required that we introduce "feature" flags, which would have
>     ended up reimplementing multiplexing of features on top of a system
>     call. It seems better to introduce one system call per feature instead.
> 
> If everyone end up preferring that we introduce a system call that implements
> many features at once, that's indeed something we can do, but I remember
> being told in the past that this is generally a bad idea.

It's a bad idea if you mix stuff which does not belong together, but if you
have stuff which shares a substantial amount of things then it makes a lot of
sense. Especially if it adds similar stuff into hotpathes.
 
> For one thing, it would make the interface more cumbersome to deal with
> from user-space in terms of feature detection: if we want to make this
> interface extensible, in addition to check -1, errno=ENOSYS, userspace
> would have to deal with a field containing the length of the structure
> as expected by user-space and kernel, and feature flags to see the common
> set of features supported by kernel and user-space.
>
> Having one system call per feature seems simpler to handle in terms of
> feature availability detection from a userspace point of view.

That might well be, but that does not justify two fastpath updates, two
seperate pointers to handle, etc ....

Thanks,

	tglx