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Date: Thu, 25 Feb 2016 18:04:16 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
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Subject: Re: [PATCH v4 1/5] getcpu_cache system call: cache CPU number of
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On Thu, Feb 25, 2016 at 04:55:26PM +0000, Mathieu Desnoyers wrote:
> ----- On Feb 25, 2016, at 4:56 AM, Peter Zijlstra peterz@infradead.org wrote:
> The restartable sequences are intrinsically designed to work
> on per-cpu data, so they need to fetch the current CPU number
> within the rseq critical section. This is where the getcpu_cache
> system call becomes very useful when combined with rseq:
> getcpu_cache allows reading the current CPU number in a
> fraction of cycle.

Yes yes, I know how restartable sequences work.

But what I worry about is that they want a cpu number and a sequence
number, and for performance it would be very good if those live in the
same cacheline.

That means either getcpu needs to grow a seq number, or restartable
sequences need to _also_ provide the cpu number.