linux-kernel - Re: [PATCH RFC 1/3] Add a trigger API for efficient non-blocking waiting

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Message-Id: <200808281427.24566.borntraeger@de.ibm.com>
Date:	Thu, 28 Aug 2008 14:27:24 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Jens Axboe <jens.axboe@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH RFC 1/3] Add a trigger API for efficient non-blocking waiting

Sorry for answering late, but here are some comments.

Am Samstag, 16. August 2008 schrieb Jeremy Fitzhardinge:
[...]
> 
>  - In a virtual environment, spinning virtual CPUs just waste CPU
>    resources, and may steal CPU time from vCPUs which need it to make
>    progress.  The trigger API allows the vCPUs to give up their CPU
>    entirely.  The s390 people observed a problem with stop_machine
>    taking a very long time (seconds) when there are more vcpus than
>    available cpus.

Yes, we have seen some real contention if the number of vcpus is much higher 
than the number of real cpus. 

> 
> The trigger API is simple:
> 
> To initialize a trigger, you can either do it statically with:
> 
>    DEFINE_TRIGGER(trigger);
> 
> or dynamically with
> 
>    trigger_init(&trigger);
> 
> Then to use it, the wait side does:
> 
>    trigger_reset(&trigger);
> 
>    while(!condition)
> 	trigger_wait(&trigger);
> 
>    trigger_finish(&trigger);
> 
> and when the condition is set true:
> 
>    condition = true;
>    trigger_kick(&trigger);

Hmm, I currently try to build something  with sigp_stop and sigp_start on 
s390, not sure yet if this interface allows me to do that. At the moment 
something is wrong in my prototype. 
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