Re: [PATCH 5/5] xen: RCU: avoid busy waiting until the end of grace period.

From: Julien Grall <julien.grall@arm.com>
To: Dario Faggioli <dario.faggioli@citrix.com>,
	xen-devel@lists.xenproject.org
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
	nd@arm.com, Stefano Stabellini <sstabellini@kernel.org>,
	Jan Beulich <jbeulich@suse.com>
Subject: Re: [PATCH 5/5] xen: RCU: avoid busy waiting until the end of grace period.
Date: Tue, 1 Aug 2017 09:54:40 +0100	[thread overview]
Message-ID: <ac3e66e4-3962-4fae-3fc2-56410f845ad4@arm.com> (raw)
In-Reply-To: <150114249858.22910.4601418126082976816.stgit@Solace>

Hi Dario,

On 27/07/2017 09:01, Dario Faggioli wrote:
> Instead of having the CPU where a callback is queued, busy
> looping on rcu_pending(), use a timer.
>
> In fact, we let the CPU go idla,e but we program a timer
> that will periodically wake it up, for checking whether the
> grace period has actually ended.
>
> It is kind of similar to introducing a periodic tick, but
> with a much more limited scope, and a lot less overhead. In
> fact, this timer is:
> - only active for the CPU(s) that have callbacks queued,
>   waiting for the end of a grace period;
> - only active when those CPU(s) are idle (and stopped as
>   soon as they resume execution).

If I read this correctly, it means on ARM the idling will now get 
interrupted periodically. This is a bit unfortunate, given that if you 
have a CPU doing nothing, you would still interrupt it intermittently.

I was expected that we could remove the CPU from the RCU whilst it is 
idle. Is there any reason for not doing that?

Cheers,

-- 
Julien Grall

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