Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences system call (v12)

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Apr 1, 2018, at 12:13 PM, One Thousand Gnomes gnomes@lxorguk.ukuu.org.uk wrote:

> On Tue, 27 Mar 2018 12:05:23 -0400
> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> 
>> Expose a new system call allowing each thread to register one userspace
>> memory area to be used as an ABI between kernel and user-space for two
>> purposes: user-space restartable sequences and quick access to read the
>> current CPU number value from user-space.
> 
> What is the *worst* case timing achievable by using the atomics ? What
> does it do to real time performance requirements ?

Given that there are two system calls introduced in this series (rseq and
cpu_opv), can you clarify which system call you refer to in the two questions
above ?

For rseq, given that its userspace works pretty much like a read seqlock
(it retries on failure), it has no impact whatsoever on scheduler behavior.
So characterizing its worst case timing does not appear to be relevant.

> For cpu_opv you now
> give an answer but your answer is assuming there isn't another thread
> actively thrashing the cache or store buffers, and that the user didn't
> sneakily pass in a page of uncacheable memory (eg framebuffer, or GPU
> space).

Are those considered as device pages ?

> 
> I don't see anything that restricts it to cached pages. With that check
> in place for x86 at least it would probably be ok and I think the sneaky
> attacks to make it uncacheable would fail becuase you've got the pages
> locked so trying to give them to an accelerator will block until you are
> done.
> 
> I still like the idea it's just the latencies concern me.

Indeed, cpu_opv touches pages that are shared with user-space with
preemption off, so this one affects the scheduler latency. The worse-case
timings I measured for cpu_opv were with cache-cold memory. So I expect that
another thread actively trashing the cache would be in the same ballpark
figure. It does not account for a concurrent thread thrashing the store
buffers though.

The checks enforcing which pages can be touched by cpu_opv operations are
done within cpu_op_check_page(). is_zone_device_page() is used to ensure no
device page is touched with preempt disabled. I understand that you would
prefer to disallow pages of uncacheable memory as well, which I'm fine with.
Is there an API similar to is_zone_device_page() to check whether a page is
uncacheable ?

> 
>>        Restartable sequences are atomic  with  respect  to  preemption
>>        (making  it atomic with respect to other threads running on the
>>        same CPU), as well as  signal  delivery  (user-space  execution
>>        contexts nested over the same thread).
> 
> CPU generally means 'big lump with legs on it'. You are not atomic to the
> same CPU, because that CPU may have 30+ cores with 8 threads per core.
> 
> It could do with some better terminology (hardware thread, CPU context ?)

Would you be OK with Christoph's terminology of "Hardware Execution Context" ?

> 
>>        In  a  typical  usage scenario, the thread registering the rseq
>>        structure will be performing  loads  and  stores  from/to  that
>>        structure.  It  is  however also allowed to read that structure
>>        from other threads.  The rseq field updates  performed  by  the
>>        kernel  provide  relaxed  atomicity  semantics, which guarantee
>>        that other threads performing relaxed atomic reads of  the  cpu
>>        number cache will always observe a consistent value.
> 
> So what happens to your API if the kernel atomics get improved ? You are
> effectively exporting rseq behaviour from private to public.

Relaxed atomics is pretty much the loosest kind of consistency we can
provide before we start allowing the compiler to do load/store tearing
(it's basically a volatile store of a word-aligned word). It does not
involve any kind of memory barrier whatsoever. I expect that the atomics
that may evolve in the future will be those with release/acquire and
implicit barriers semantics. The relaxed atomicity does not cover any of
these.

Thanks,

Mathieu

> 
> Alan

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