Messages in this thread |  | | Date | Fri, 2 Feb 2018 20:59:43 +0100 | From | Peter Zijlstra <> | Subject | Re: [RESEND RFC PATCH V3] sched: Improve scalability of select_idle_sibling using SMT balance |
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On Fri, Feb 02, 2018 at 11:53:40AM -0500, Steven Sistare wrote: > On 2/1/2018 7:33 AM, Peter Zijlstra wrote: > > On Mon, Jan 29, 2018 at 03:31:02PM -0800, subhra mazumdar wrote: > >> + rcu_read_lock(); > >> + sd = rcu_dereference(per_cpu(sd_llc, this_cpu)); > >> + if (util) { > >> + for_each_lower_domain(sd) { > >> + if (sd->level == 0) > >> + break; > > > > afaict you really only need this for the core, and here you're assuming > > everything below the LLC is cores. Would it not be much clearer if you > > introduce sd_core. > > > > As is, for_each_lower_domain includes the starting domain, sd->group > > then is the first core group for this cpu. But then you continue to the > > smt domain (on Intel, on other architectures there could be a cluster > > domain in between) and then you bail using that sd->level == 0 hack > > because otherwise things would go *bang*. > > Hi Peter, > > The code here and in smt_balance intentionally visits each level between > the llc and smt, including core-cluster on architectures that define it. > smt_balance thus has the chance to randomly pick a better cluster, > and then within that cluster randomly pick a better core. It makes sense, > as resources are shared within a cluster, and choosing a less loaded cluster > should give better performance. As you suggest in a few other places, > it would be nice to see performance results for this case. We have > SPARC processors with core clusters. >
But then you get that atomic crud to contend on the cluster level, which is even worse than it contending on the core level.
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