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

[Mathieu Desnoyers]

----- Original Message -----
> On Thu, May 21, 2015 at 10:44:47AM -0400, Mathieu Desnoyers wrote:
> > Expose a new system call allowing userspace threads to register
> > a TLS area used as an ABI between the kernel and userspace to
> > share information required to create efficient per-cpu critical
> > sections in user-space.
> > 
> > This ABI consists of a thread-local structure containing:
> > 
> > - a nesting count surrounding the critical section,
> > - a signal number to be sent to the thread when preempting a thread
> >   with non-zero nesting count,
> > - a flag indicating whether the signal has been sent within the
> >   critical section,
> > - an integer where to store the current CPU number, updated whenever
> >   the thread is preempted. This CPU number cache is not strictly
> >   needed, but performs better than getcpu vdso.
> > 
> > This approach is inspired by Paul Turner and Andrew Hunter's work
> > on percpu atomics, which lets the kernel handle restart of critical
> > sections, ref.
> > http://www.linuxplumbersconf.org/2013/ocw/system/presentations/1695/original/LPC%20-%20PerCpu%20Atomics.pdf
> > 
> > What is done differently here compared to percpu atomics: we track
> > a single nesting counter per thread rather than many ranges of
> > instruction pointer values. We deliver a signal to user-space and
> > let the logic of restart be handled in user-space, thus moving
> > the complexity out of the kernel. The nesting counter approach
> > allows us to skip the complexity of interacting with signals that
> > would be otherwise needed with the percpu atomics approach, which
> > needs to know which instruction pointers are preempted, including
> > when preemption occurs on a signal handler nested over an instruction
> > pointer of interest.
> > 
> > Advantages of this approach over percpu atomics:
> > - kernel code is relatively simple: complexity of restart sections
> >   is in user-space,
> > - easy to port to other architectures: just need to reserve a new
> >   system call,
> > - for threads which have registered a TLS structure, the fast-path
> >   at preemption is only a nesting counter check, along with the
> >   optional store of the current CPU number, rather than comparing
> >   instruction pointer with possibly many registered ranges,
> > 
> > Caveats of this approach compared to the percpu atomics:
> > - We need a signal number for this, so it cannot be done without
> >   designing the application accordingly,
> > - Handling restart in user-space is currently performed with page
> >   protection, for which we install a SIGSEGV signal handler. Again,
> >   this requires designing the application accordingly, especially
> >   if the application installs its own segmentation fault handler,
> > - It cannot be used for tracing of processes by injection of code
> >   into their address space, due to interactions with application
> >   signal handlers.
> > 
> > The user-space proof of concept code implementing the restart section
> > can be found here: https://github.com/compudj/percpu-dev
> > 
> > Benchmarking sched_getcpu() vs tls cache approach. Getting the
> > current CPU number:
> > 
> > - With Linux vdso:            12.7 ns
> > - With TLS-cached cpu number:  0.3 ns
> > 
> > We will use the TLS-cached cpu number for the following
> > benchmarks.
> > 
> > On an Intel(R) Xeon(R) CPU E5-2630 v3 @ 2.40GHz, comparison
> > with a baseline running very few load/stores (no locking,
> > no getcpu, assuming one thread per CPU with affinity),
> > against locking scheme based on "lock; cmpxchg", "cmpxchg"
> > (using restart signal), load-store (using restart signal).
> > This is performed with 32 threads on a 16-core, hyperthread
> > system:
> > 
> >                  ns/loop      overhead (ns)
> > Baseline:          3.7           0.0
> > lock; cmpxchg:    22.0          18.3
> > cmpxchg:          11.1           7.4
> > load-store:        9.4           5.7
> > 
> > Therefore, the load-store scheme has a speedup of 3.2x over the
> > "lock; cmpxchg" scheme if both are using the tls-cache for the
> > CPU number. If we use Linux sched_getcpu() for "lock; cmpxchg"
> > we reach of speedup of 5.4x for load-store+tls-cache vs
> > "lock; cmpxchg"+vdso-getcpu.
> > 
> > I'm sending this out to trigger discussion, and hopefully to see
> > Paul and Andrew's patches being posted publicly at some point, so
> > we can compare our approaches.
> 
> The idea seems sensible.  One quick comment: as with any new syscall,
> please include a flags argument.

Sure, I'll add it right away. Thanks for the feedback!

Mathieu

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