Message-ID: <44F2A62C.9090609@sw.ru>
Date: Mon, 28 Aug 2006 12:15:40 +0400
From: Kirill Korotaev <dev@sw.ru>
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To: vatsa@in.ibm.com
CC: Ingo Molnar <mingo@elte.hu>, Nick Piggin <nickpiggin@yahoo.com.au>,
       Sam Vilain <sam@vilain.net>, linux-kernel@vger.kernel.org,
       Kirill Korotaev <dev@openvz.org>, Mike Galbraith <efault@gmx.de>,
       Balbir Singh <balbir@in.ibm.com>, sekharan@us.ibm.com,
       Andrew Morton <akpm@osdl.org>, nagar@watson.ibm.com,
       matthltc@us.ibm.com, dipankar@in.ibm.com
Subject: Re: [PATCH 1/7] CPU controller V1 - split runqueue
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Srivatsa Vaddagiri wrote:
> On Fri, Aug 25, 2006 at 04:38:00PM +0400, Kirill Korotaev wrote:
> 
>>Srivatsa,
>>
>>I suggest to split existing runqueue structure
>>into 2 pieces: physical cpu (sd, ...) and
>>virtual cpu (essentially a runqueue - array, nr_running, loac etc.)
>>
>>Then replace all references to cpu as int with vcpu_t pointer.
> 
> 
> That's going to be a massive change! If I understand you correctly,
> things like get_cpu() return virtual CPU number rather than the
> corresponding "physical" CPU (the later is anyway a misnomer on
> virtualized platforms)? Also we have get_cpu() now reading some structure and be
> able to tell which CPU a task is running. Now with virtual CPUs, another
> level of translation is needed? Wonder what the performance impact of
> that would be ..
first, in most places get_cpu() should not be replaced with get_vcpu() 
(e.g. accessing per-CPU arrays requires get_cpu(), not get_vcpu()).
next, these changes are _trivial_ which is good as they don't change logic.

>>What advantages does it give?
>>1. it isolates Linux std scheduler code for scheduling
>>  tasks inside runqueues, while adds possibility
>>  to add cleanly more high-level scheduler, which can select
>>  runqueues to run (lets call it "process groups scheduler" - PGS).
>>2. runqueues can run on arbitrary physical CPUs if needed
>>  which helps to solve balancing problem on SMP.
> 
> How do you see the relation between load-balance done thr sched-domain
> heirarchy today and what will be done thr' virtal runqueues? 
sorry, can't get your question.

>>3. it allows naturally to use different PGS algorithms
>>  on top of Linux one. e.g. yours algorithm (probobalistic) or
>>  fair scheduling algorithms like SFQ, EEVDF, BVT with more 
>>  predictable parameters of QoS.
>>4. it will help us to get to the consensus and commit this work
>>  into mainstream, because different PGS with different properties
>>  will be possible.
>>
>>Part of this idea is implemented in OpenVZ scheduler and in some
>>regards looks very much like your work, so I think if you like the idea
>>we can eloborate.
>>
>>What do you think?
> 
> 
> I believe hypervisors like Xen have a similar approach (virtualing CPU
> resource and running a virtual CPU on any available physical CPU). The 
> worry I have applying this to Linux kernel scheduler is in terms of its 
> invasiveness and thus general acceptability.
When I talked with Nick Piggin on summit he was quite optimistic
with such an approach. And again, this invasiveness is very simple
so I do not forsee much objections.

> I will however let the maintainers 
> decide on that. Sending some patches also probably will help measure this 
> "invasiveness/acceptability".
I propose to work on this together helping each other.
This makes part of your patches simlper and ours as well.
And what is good allows different approaches with different properties to be used.

> I had another question related to real-time tasks. How do you control
> CPU usage of real-time tasks in different containers (especially if they
> are SCHED_FIFO types)? Do they get capped at the bandwidth provided to
> the container?
yes. std Linux scheduler runs these processes as far as fair scheduler allows container
to run.

> Also do you take any special steps to retain interactivity?
no, it turned out to work quite fine w/o changes.

Thanks,
Kirill

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