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

[Mathieu Desnoyers]

----- On Feb 26, 2016, at 6:04 PM, H. Peter Anvin hpa@xxxxxxxxx wrote:

> On February 26, 2016 12:24:15 PM PST, Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>>----- On Feb 26, 2016, at 1:01 PM, Thomas Gleixner tglx@xxxxxxxxxxxxx
>>wrote:
>>
>>> 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 ....
>>
>>Keeping two separate pointers in the task_struct rather than a single
>>one
>>might indeed be unwelcome, but I'm not sure I fully grasp the fast path
>>argument in this case: getcpu_cache only sets a notifier thread flag
>>on thread migration, whereas AFAIU rseq adds code to context switch and
>>signal
>>delivery, which are prone to have a higher impact.
>>
>>Indeed both will have their own code in the resume notifier, but is it
>>really
>>a fast path ?
>>
>>From my point of view, making it easy for userspace to just enable
>>getcpu_cache
>>without having the scheduler and signal delivery fast-path overhead of
>>rseq seems
>>like a good thing. I'm not all that sure that saving an extra pointer
>>in
>>task_struct justifies the added system call interface complexity.
>>
>>Thanks,
>>
>>Mathieu
> 
> I think it would be a good idea to make this a general pointer for the kernel to
> be able to write per thread state to user space, which obviously can't be done
> with the vDSO.
> 
> This means the libc per thread startup should query the kernel for the size of
> this structure and allocate thread local data accordingly.  We can then grow
> this structure if needed without making the ABI even more complex.
> 
> This is more than a system call: this is an entirely new way for userspace to
> interact with the kernel.  Therefore we should make it a general facility.

I'm really glad to see I'm not the only one seeing potential for
genericity here. :-) This is exactly what I had in mind
last year when proposing the thread_local_abi() system call:
a generic way to register an extensible per-thread data structure
so the kernel can communicate with user-space and vice-versa.

Rather than having the libc query the kernel for size of the structure,
I would recommend that libc tells the kernel the size of the thread-local
ABI structure it supports. The idea here is that both the kernel and libc
need to know about the fields in that structure to allow a two-way
interaction. Fields known only by either the kernel or userspace
are useless for a given thread anyway. This way, libc could statically
define the structure.

I would be tempted to also add "features" flags, so both user-space
and the kernel could tell each other what they support: user-space
would announce the set of features it supports, and it could also
query the kernel for the set of supported features. One simple approach
would be to use a uint64_t as type for those feature flags, and
reserve the last bit for extending to future flags if we ever have
more than 64.

Thoughts ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com