Re: [PATCH v4 0/5] getcpu_cache system call for 4.6

On February 23, 2016 8:09:23 PM PST, Mathieu Desnoyers [off-list ref] wrote:

----- On Feb 23, 2016, at 8:36 PM, H. Peter Anvin hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org wrote:

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On 02/23/2016 03:28 PM, Mathieu Desnoyers wrote:

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Hi,

Here is a patchset implementing a cache for the CPU number of the
currently running thread in user-space.

Benchmarks comparing this approach to a getcpu based on system call

on

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ARM show a 44x speedup. They show a 14x speedup on x86-64 compared

to

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executing lsl from a vDSO through glibc.

I'm added a man page in the changelog of patch 1/3, which shows an
example usage of this new system call.

This series is based on v4.5-rc5, submitted for Linux 4.6.

Feedback is welcome,

What is the resulting context switch overhead?

The getcpu_cache only adds code to the thread migration path,
and to the resume notifier. The context switch path per se is
untouched. I would therefore expect the overhead on context
switch to be within the noise, except if stuff like hackbench
would be so sensitive to the size of struct task_struct that
a single extra pointer added at the end of struct task_struct
would throw off the benchmarks.

Is that what you are concerned about ?

Thanks,

Mathieu

Yes, I'd like to see numbers.  It is way easy to handwave small changes away, but they add up over time.  Without numbers it is a bit hard to quantify the pro vs con.
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