linux-kernel - Re: [RFC PATCH v3 for 4.15 08/24] Provide cpu

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Message-ID: <20171117191547.GI2482@two.firstfloor.org>
Date:   Fri, 17 Nov 2017 11:15:47 -0800
From:   Andi Kleen <andi@...stfloor.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andi Kleen <andi@...stfloor.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        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>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>,
        Chris Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>,
        rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC PATCH v3 for 4.15 08/24] Provide cpu_opv system call

> The most straight forward is to have a mechanism which forces everything
> into the slow path in case of debugging, lack of progress, etc. The slow

That's the abort address, right?

For the generic case the fall back path would require disabling preemption
unfortunately, for which we don't have a mechanism in user space.

I think that is what Mathieu tried to implement here with this call.

There may be some special cases where it's possible without preemption
control, e.g. a malloc could just not use the per cpu cache. But I doubt
that is possible in all cases that Mathieu envisions. 

But again would be a different code path, and I question the need for it when
we can just let the operator of the debugger deal with it.

-Andi