Messages in this thread | | | Date | Thu, 10 Oct 2013 20:50:32 +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 02:43:27PM -0400, Steven Rostedt wrote: > On Thu, 10 Oct 2013 11:10:35 -0700 > Linus Torvalds <torvalds@linux-foundation.org> wrote: > .. now we can free all the percpu data and kill the CPU .. > > > > without any locking anywhere - not stop-machine, not anything. If > > somebody is doing a "for_each_cpu()" (under just a regular > > rcu_read_lock()) and they see the bit set while it's going down, who > > cares? The CPU is still there, the data is accessible.. > > The problem is that rcu_read_lock() requires preemption disabled unless > you are using the preemptable rcu tree version. There's always > srcu_read_lock() but that's not so free. It's basically the same as > what Peter is doing.
No, srcu is actually more expensive in the fast path. Although possibly we could make SRCU more complex ;-)
> There's places in the kernel that does for_each_cpu() that I'm sure you > don't want to disable preemption for. Especially when you start having > 4096 CPU machines!
:-)
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