Messages in this thread | | | Date | Wed, 15 Apr 2020 11:36:17 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 2/4] cpumask: Make cpumask_any() truly random |
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On Tue, Apr 14, 2020 at 12:19:56PM -0400, Steven Rostedt wrote:
> > +/** > > + * cpumask_any - pick a "random" cpu from *srcp > > + * @srcp: the input cpumask > > + * > > + * Returns >= nr_cpu_ids if no cpus set. > > + */ > > +int cpumask_any(const struct cpumask *srcp) > > +{ > > + int next, prev; > > + > > + /* NOTE: our first selection will skip 0. */ > > + prev = __this_cpu_read(distribute_cpu_mask_prev); > > + > > + next = cpumask_next(prev, srcp); > > + if (next >= nr_cpu_ids) > > + next = cpumask_first(srcp); > > + > > + if (next < nr_cpu_ids) > > + __this_cpu_write(distribute_cpu_mask_prev, next); > > Do we care if this gets preempted and migrated to a new CPU where we read > "prev" from one distribute_cpu_mask_prev on one CPU and write it to another > CPU?
I don't think we do; that just adds to the randomness ;-), but you do raise a good point in that __this_cpu_*() ops assume preemption is already disabled, which is true of the one exiting cpumask_any_and_distribute() caller, but is no longer true after patch 1, and this patch repeats the mistake.
So either we need to disable preemption across the function or transition to this_cpu_*() ops.
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