linux-kernel - Re: [RFC PATCH for 4.18 12/23] cpu_opv: Provide cpu

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Date:   Thu, 12 Apr 2018 15:59:27 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC PATCH for 4.18 12/23] cpu_opv: Provide cpu_opv system call
 (v7)

----- On Apr 12, 2018, at 3:43 PM, Linus Torvalds torvalds@...ux-foundation.org wrote:

> On Thu, Apr 12, 2018 at 12:27 PM, Mathieu Desnoyers
> <mathieu.desnoyers@...icios.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