Re: [RFC PATCH] percpu system call: fast userspace percpu critical sections

[Andy Lutomirski]

On Tue, May 26, 2015 at 2:18 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Tue, May 26, 2015 at 2:04 PM, Mathieu Desnoyers
>>
>>>
>>> It's too bad that not all architectures have a single-instruction
>>> unlocked compare-and-exchange.
>>
>> Based on my benchmarks, it's not clear that single-instruction
>> unlocked CAS is actually faster than doing the same with many
>> instructions.
>
> True, but with a single instruction the user can't get preempted in the middle.
>
> Looking at your code, it looks like percpu_user_sched_in has some
> potentially nasty issues with page faults.  Avoiding touching user
> memory from the scheduler would be quite nice from an implementation
> POV, and the x86-specific gs hack wins in that regard.

ARM has "TLB lockdown entries" which could, I think, be used to
implement per-cpu or per-thread mappings.  I'm actually rather
surprised that Linux doesn't already use a TLB lockdown entry for TLS.
(Hmm.  Maybe it's because the interface to write the entries requires
actually touching the page.  Maybe not -- the ARM docs, in general,
seem to be much less clear than the Intel and AMD docs.)

ARM doesn't seem to have any single-instruction compare-exchange or
similar instruction, though, so this might be all that useful.  On the
other hand, ARM can probably do reasonably efficient per-cpu memory
allocation and such with a single ldrex/strex pair.

--Andy
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