Re: [RFC PATCH v3] membarrier: provide core serialization

[Will Deacon <will.deacon@arm.com>]

On Fri, Sep 01, 2017 at 12:10:07PM -0400, Mathieu Desnoyers wrote:
> Add a new MEMBARRIER_FLAG_SYNC_CORE flag to the membarrier
> system call. It allows membarrier to issue core serializing barriers in
> addition to memory barriers on target threads whenever a membarrier
> command is performed.
> 
> It is relevant for reclaim of JIT code, which requires to issue core
> serializing barriers on all threads running on behalf of a process
> after ensuring the old code is not visible anymore, before re-using
> memory for new code.
> 
> The new MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED used with
> MEMBARRIER_FLAG_SYNC_CORE flag registers the current process as
> requiring core serialization. It may block. It can be used to ensure
> MEMBARRIER_CMD_PRIVATE_EXPEDITED never blocks, even the first time it is
> invoked by a process with the MEMBARRIER_FLAG_SYNC_CORE flag.
> 
> * Scheduler Overhead Benchmarks
> 
> Intel(R) Xeon(R) CPU E5-2630 v3 @ 2.40GHz
> Linux v4.13-rc6
> 
> Inter-thread scheduling
> taskset 01 ./perf bench sched pipe -T
> 
>                        Avg. usecs/op         Std.Dev. usecs/op
> Before this change:         2.55                   0.10
> With this change:           2.49                   0.08
> SYNC_CORE processes:        2.70                   0.10
> 
> Inter-process scheduling
> taskset 01 ./perf bench sched pipe
> 
> Before this change:         2.93                   0.13
> With this change:           2.93                   0.13
> SYNC_CORE processes:        3.20                   0.06
> 
> Changes since v2:
> - Rename MEMBARRIER_CMD_REGISTER_SYNC_CORE to
>   MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED,

I'm still not convinced that this registration step is needed (at least
for arm, power and x86), but my previous comments were ignored.

> - Introduce the "MEMBARRIER_FLAG_SYNC_CORE" flag.
> - Introduce CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE, only implemented by
>   x86 32/64 initially.
> - Introduce arch_membarrier_user_icache_flush, a no-op on x86 32/64,
>   which can be implemented on architectures with incoherent data and
>   instruction caches. It is associated with
>   CONFIG_ARCH_HAS_MEMBARRIER_USER_ICACHE_FLUSH.

Given that MEMBARRIER_FLAG_SYNC_CORE is about flushing the internal CPU
pipeline (iiuc), could we rename this so that it doesn't mention the
I-cache, please? I-cache flushing is a very different operation on most
architectures I'm aware of, and on arm64 it's even available to userspace
(and broadcast in hardware to other cores).

Will