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

[Peter Zijlstra]

On Tue, Mar 05, 2019 at 05:32:10PM -0500, Mathieu Desnoyers wrote:
> ----- On Mar 5, 2019, at 4:58 PM, Peter Zijlstra peterz@xxxxxxxxxxxxx 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@xxxxxxxxxxxxxx/
> 
> (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.