Messages in this thread | | | Date | Thu, 10 Oct 2013 21:34:22 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2 |
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On Thu, Oct 10, 2013 at 12:16:16PM -0700, Linus Torvalds wrote: > On Thu, Oct 10, 2013 at 12:04 PM, Steven Rostedt <rostedt@goodmis.org> wrote: > > > > I'm wondering if we can have a for_each_cpu() that only disables > > preemption in the loop. > > I think we'd generally want to have it be something the loop asks for. > > If the loop is just some kind of "gather statistics" thing, I don't > think it's required. The cost per loop is so low (usually adding up a > couple of words) that the downside drowns the upside. > > And we could easily look at MAXSMP (or NR_CPUS) at compile-time, and > not do it for common small values (although it looks like Fedora > defaults to 128 CPU's for their distro kernels, which seems a bit > excessive - too many by far for normal people, too few for the crazy > big ones).
Alternatively we could write it something like:
rcu_read_lock(); for_each_online_node(node) { for_each_cpu(cpu, cpumask_of_node(node)) { ... } cond_resched_rcu(); } rcu_read_unlock();
But yes, I think this pattern (and variations) should work for most cases.
The one I'm struggling with atm is kernel/cpuset.c: rebuild_sched_domains_locked(). Although I'm thinking we should do that seqlock style, just build it and verify if its still valid, if not, try again -- although I'm sure it'll be 'fun' to get correct.
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