On Tue, Dec 04, 2018 at 09:43:53PM -0800, Alison Schofield wrote:
> On Tue, Dec 04, 2018 at 10:21:45AM +0100, Peter Zijlstra wrote:
> > On Mon, Dec 03, 2018 at 11:39:58PM -0800, Alison Schofield wrote:
> >
> > > +static int mktme_build_leadcpus_mask(void)
> > > +{
> > > + int online_cpu, mktme_cpu;
> > > + int online_pkgid, mktme_pkgid = -1;
> > > +
> > > + if (!zalloc_cpumask_var(&mktme_leadcpus, GFP_KERNEL))
> > > + return -ENOMEM;
> > > +
> > > + for_each_online_cpu(online_cpu) {
> > > + online_pkgid = topology_physical_package_id(online_cpu);
> > > +
> > > + for_each_cpu(mktme_cpu, mktme_leadcpus) {
> > > + mktme_pkgid = topology_physical_package_id(mktme_cpu);
> > > + if (mktme_pkgid == online_pkgid)
> > > + break;
> > > + }
> > > + if (mktme_pkgid != online_pkgid)
> > > + cpumask_set_cpu(online_cpu, mktme_leadcpus);
> >
> > Do you really need LOCK prefixed bit set here?
> No. Changed to __cpumask_set_cpu(). Will check for other instances
> where I can skip LOCK prefix.
>
> > How is that serialized and kept relevant in the face of hotplug?
> mktme_leadcpus is updated on hotplug startup and teardowns.
Not in this patch it is not. That is added in a subsequent patch, which
means that during bisection hotplug is utterly wrecked if you happen to
land between these patches, that is bad.
> > Also, do you really need O(n^2) to find the first occurence of a value
> > in an array?
> How about this O(n)?
>
> unsigned long *pkg_map;
> int cpu, pkgid;
>
> if (!zalloc_cpumask_var(&mktme_leadcpus, GFP_KERNEL))
> return -ENOMEM;
>
> pkg_map = bitmap_zalloc(topology_max_packages(), GFP_KERNEL);
> if (!pkg_map) {
> free_cpumask_var(mktme_leadcpus);
> return -ENOMEM;
> }
> for_each_online_cpu(cpu) {
> pkgid = topology_physical_package_id(cpu);
> if (!test_and_set_bit(pkgid, pkg_map))
You again don't need that LOCK prefix here.
__test_and_set_bit() :-)
> __cpumask_set_cpu(cpu, mktme_leadcpus);
> }
Right.
