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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 16 Apr 2018 12:26:35 -0700
Message-ID: <CA+55aFxtZXi05UNLmtbKcZ=2N_ROE5pLnu9+v6FaXiuK=VbErA@mail.gmail.com>
Subject: Re: [RFC PATCH for 4.18 12/23] cpu_opv: Provide cpu_opv system call (v7)
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Dave Watson <davejwatson@fb.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>,
        Paul Turner <pjt@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Andrew Hunter <ahh@google.com>,
        Andi Kleen <andi@firstfloor.org>, Chris Lameter <cl@linux.com>,
        Ben Maurer <bmaurer@fb.com>, rostedt <rostedt@goodmis.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>
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On Mon, Apr 16, 2018 at 12:21 PM, Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
>
> And I try very hard to avoid being told I'm the one breaking
> user-space. ;-)

You *can't* be breaking user space. User space doesn't use this yet.

That's actually why I'd like to start with the minimal set - to make
sure we don't introduce features that will come back to bite us later.

The one compelling use case I saw was a memory allocator that used
this for getting per-CPU (vs per-thread) memory scaling.

That code didn't need the cpu_opv system call at all.

And if somebody does a ldload of a malloc library, and then wants to
analyze the behavior of a program, maybe they should ldload their own
malloc routines first? That's pretty much par for the course for those
kinds of projects.

So I'd much rather we first merge the non-contentious parts that
actually have some numbers for "this improves performance and makes a
nice fancy malloc possible".

As it is, the cpu_opv seems to be all about theory, not about actual need.

              Linus