linux-kernel - Re: [PATCH for 5.1 0/3] Restartable Sequences updates for 5.1

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Date:   Wed, 6 Mar 2019 09:21:30 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "H.J. Lu" <hjl.tools@...il.com>,
        libc-alpha <libc-alpha@...rceware.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>, 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>, Andi Kleen <andi@...stfloor.org>,
        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>,
        Joel Fernandes <joelaf@...gle.com>,
        Carlos O'Donell <carlos@...hat.com>,
        Florian Weimer <fweimer@...hat.com>
Subject: Re: [PATCH for 5.1 0/3] Restartable Sequences updates for 5.1

On Tue, Mar 05, 2019 at 05:32:10PM -0500, Mathieu Desnoyers wrote:
> ----- On Mar 5, 2019, at 4:58 PM, Peter Zijlstra peterz@...radead.org wrote:
> 
> > On Tue, Mar 05, 2019 at 03:18:35PM -0500, Mathieu Desnoyers wrote:
> >> * NUMA node ID in TLS
> >> 
> >> Having the NUMA node ID available in a TLS variable would allow glibc to
> >> perform interesting NUMA performance improvements within its locking
> >> implementation, so I have a patch adding NUMA node ID support to rseq
> >> as a new rseq system call flag.
> > 
> > Details? There's just not much room in the futex word, and futexes
> > themselves are not numa aware.
> 
> It was discussed in this libc-alpha mailing list thread:
> 
> https://public-inbox.org/libc-alpha/CAMe9rOo7i_-keOooa0D+P_wzatVCdKkTRiFiJ-cxpnvi+eApuQ@mail.gmail.com/
> 
> (adding the relevant people in CC)
> 
> I'd like to hear in more details on how they intend to design
> NUMA-aware spinlocks within glibc. All I know is that quick
> access to the node ID would help for this.

Userspace spinlocks are a trainwreck anyway. The only case where they
can possibly work is when there's only a single thread on every cpu.
Pretty much any other scenario is fail; see why we have paravirt
spinlocks.