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

From: H. Peter Anvin
Date: Wed Feb 24 2016 - 15:21:01 EST

On February 23, 2016 8:09:23 PM PST, Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>----- On Feb 23, 2016, at 8:36 PM, H. Peter Anvin hpa@xxxxxxxxx wrote:
>> On 02/23/2016 03:28 PM, Mathieu Desnoyers wrote:
>>> 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
>>> ARM show a 44x speedup. They show a 14x speedup on x86-64 compared
>>> 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 ?

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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