Messages in this thread | | | Date | Thu, 12 Apr 2018 15:59:27 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH for 4.18 12/23] cpu_opv: Provide cpu_opv system call (v7) |
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----- On Apr 12, 2018, at 3:43 PM, Linus Torvalds torvalds@linux-foundation.org wrote:
> On Thu, Apr 12, 2018 at 12:27 PM, Mathieu Desnoyers > <mathieu.desnoyers@efficios.com> wrote: >> The cpu_opv system call executes a vector of operations on behalf of >> user-space on a specific CPU with preemption disabled. It is inspired >> by readv() and writev() system calls which take a "struct iovec" >> array as argument. > > Do we really want the page pinning? > > This whole cpu_opv thing is the most questionable part of the series, > and the page pinning is the most questionable part of cpu_opv for me.
What are your concerns about page pinning ?
Do you have an alternative approach in mind ?
> Can we plan on merging just the plain rseq parts *without* this all > first, and then see the cpu_opv thing as a "maybe future expansion" > part.
The main problem with the incremental approach is that it won't deal with remote CPU data accesses, and won't deal with cpu hotplug in non-racy ways. For *some* of the use-cases, the other issues solved by cpu_opv can be worked-around in user-space, at the cost of making the userspace code a mess, and in many cases slower than if we can rely on cpu_opv for the fallback.
All the rseq test-cases depend on cpu_opv as they stand now. Without cpu_opv to handle the corner-cases, things become much more messy on the user-space side.
Thanks,
Mathieu
> > I think that would make Andy happier too. > > Linus
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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