Date: Thu, 14 Dec 2017 19:57:08 +0000 (UTC)
From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Chris Lameter <cl@linux.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Andy Lutomirski <luto@amacapital.net>,
        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>, Ben Maurer <bmaurer@fb.com>,
        rostedt <rostedt@goodmis.org>, Josh Triplett <josh@joshtriplett.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>
Message-ID: <1772818221.34575.1513281428902.JavaMail.zimbra@efficios.com>
In-Reply-To: <20171214194853.GE3326@worktop>
References: <20171214161403.30643-1-mathieu.desnoyers@efficios.com> <20171214161403.30643-3-mathieu.desnoyers@efficios.com> <alpine.DEB.2.20.1712141033100.6249@nuc-kabylake> <12046460.34426.1513275177081.JavaMail.zimbra@efficios.com> <alpine.DEB.2.20.1712141238180.8340@nuc-kabylake> <20171214194853.GE3326@worktop>
Subject: Re: [RFC PATCH for 4.16 02/21] rseq: Introduce restartable
 sequences system call (v12)
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Thread-Topic: rseq: Introduce restartable sequences system call (v12)
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----- On Dec 14, 2017, at 2:48 PM, Peter Zijlstra peterz@infradead.org wrote:

> On Thu, Dec 14, 2017 at 12:50:13PM -0600, Christopher Lameter wrote:
>> Ultimately I wish fast increments like done by this_cpu_inc() could be
>> implemented in an efficient way on non x86 platforms that do not have
>> cheap instructions like that.
> 
> So the problem isn't migration; for that we could wrap the operation in
> preempt_disable() which is not more expensive than rseq would be. And a
> lot more deterministic.
> 
> The problem instead is interrupts, which can result in nested load-store
> operations, and that comes apart. This then means having to disable
> interrupts over these things and _that_ is expensive.

Then could we consider checking a per task-struct rseq_cs pointer when
returning from interrupt handler ? This rseq_cs pointer would track
kernel restartable sequences. This would also work for NMI handlers.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com