linux-kernel - Re: [RFD] CAT user space interface revisited

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Date:	Tue, 24 Nov 2015 16:27:54 +0800
From:	Chao Peng <chao.p.peng@...ux.intel.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Vikas Shivappa <vikas.shivappa@...el.com>,
	Tejun Heo <tj@...nel.org>, Yu Fenghua <fenghua.yu@...el.com>
Subject: Re: [RFD] CAT user space interface revisited

On Wed, Nov 18, 2015 at 10:01:54PM -0200, Marcelo Tosatti wrote:
> > 	tglx
> 
> Again: you don't need to look into the MSR table and relate it 
> to tasks if you store the data as:
> 
> 	task group 1 = {
> 			reservation-1 = {size = 80Kb, type = data, socketmask = 0xffff},
> 			reservation-2 = {size = 100Kb, type = code, socketmask = 0xffff}
> 	}
> 	
> 	task group 2 = {
> 			reservation-1 = {size = 80Kb, type = data, socketmask = 0xffff},
> 			reservation-3 = {size = 200Kb, type = code, socketmask = 0xffff}
> 	}
> 
> Task group 1 and task group 2 share reservation-1.

Because there is only size but not CBM position info, I guess for
different reservations they will not overlap each other, right?

Personally I like this way of exposing minimal information to userspace.
I can think it working well except for one concern of losing flexibility:

For instance, there is a box for which the full CBM is 0xfffff. After
cache reservation creating/freeing for a while we then have reservations:

reservation1: 0xf0000
reservation2: 0x00ff0

Now people want to request a reservation which size is 0xff, so how
will kernel do at this time? It could return just error or do some
moving/merging (e.g. for reservation2: 0x00ff0 => 0x0ff00) and then
satisfy the request. But I don't know if the moving/merging will cause
delay for tasks that is using it.

Thanks,
Chao
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