Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

[Peter Zijlstra]

On Mon, Sep 23, 2013 at 11:13:03AM -0400, Steven Rostedt wrote:
> Well, the point I was trying to do was to let readers go very fast
> (well, with a mb instead of a mutex), and then when the CPU hotplug
> happens, it goes back to the current method.

Well, for that the thing Oleg proposed works just fine and the
preempt_disable() section vs synchronize_sched() is hardly magic.

But I'd really like to get the writer pending case fast too.

> That is, once we set __cpuhp_write, and then run synchronize_srcu(),
> the system will be in a state that does what it does today (grabbing
> mutexes, and upping refcounts).

Still no point in using srcu for this; preempt_disable +
synchronize_sched() is similar and much faster -- its the rcu_sched
equivalent of what you propose.

> I thought the whole point was to speed up the get_online_cpus() when no
> hotplug is happening. This does that, and is rather simple. It only
> gets slow when hotplug is in effect.

No, well, it also gets slow when a hotplug is pending, which can be
quite a while if we go sprinkle get_online_cpus() all over the place and
the machine is busy.

One we start a hotplug attempt we must wait for all readers to quiesce
-- since the lock is full reader preference this can take an infinite
amount of time -- while we're waiting for this all 4k+ CPUs will be
bouncing the one mutex around on every get_online_cpus(); of which we'll
have many since that's the entire point of making them cheap, to use
more of them.
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