Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light" atomicreaders to prevent CPU offline

From: Srivatsa S. Bhat
Date: Wed Dec 05 2012 - 13:54:57 EST

Replaying what Tejun wrote:

Hello, Oleg.

> Replaying what Oleg wrote:
> Hi,
> Sorry I don't understand the context and I can't find this thread
> anywhere, so I am not sure I understand...

Weird, lkml cc is missing. Srivatsa?

[Now fixed. This thread has lkml CC]

>> Replaying what Tejun wrote:
>> So, we basically need percpu_rwlock. We already have percpu_rwsem.
> Yes, and with -mm patches it becomes reader-friendly. In particular
> see
>> Oleg, it seems
>> CPU hotplug needs big-reader rwlock, ideas on how to proceed?
> I am going to convert get_online_cpus() to use percpu_down_read(),
> this looks simple.
> We already discussed this with Paul, see
> and the whole thread.
> In short, all we need is percpu_down_write_recursive_readers() and
> afaics the only complication is lockdep, we need down_read_no_lockdep()
> which (like __up_read) doesn't do rwsem_acquire_read().

So, it's a different thing. There are two mechanism protecting
against cpu hotplug - get_online_cpus() and preempt_disable(). The
former can be used by ones which can sleep and need to protect against
the whole up/down process (DOWN_PREPARE and so on). The latter
protects the last step and can be used when the caller can't sleep.
Replacing get_online_cpus() w/ percpu_rwsem is great but this thread
is about replacing preempt_disable with something finer grained and
less heavy on the writer side - IOW, percpu_rwlock as opposed to
percpu_rwsem, so, I think the end result would be that CPU hotplug
will be protected by percpu_rwsem for the whole part and by
percpu_rwlock for the last commit stage.

The problem seems that we don't have percpu_rwlock yet. It shouldn't
be too difficult to implement, right?


-- tejun

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