linux-kernel - Re: [PATCH mpam mpam/snapshot/v6.14-rc1 3/5] arm_mpam: Provide conversion method for new closid/rmid pairs

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Message-ID: <Z7M7R8uf4g2C68cO@e133380.arm.com>
Date: Mon, 17 Feb 2025 13:36:07 +0000
From: Dave Martin <Dave.Martin@....com>
To: Zeng Heng <zengheng4@...wei.com>
Cc: james.morse@....com, bobo.shaobowang@...wei.com,
	jonathan.cameron@...wei.com, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH mpam mpam/snapshot/v6.14-rc1 3/5] arm_mpam: Provide
 conversion method for new closid/rmid pairs

Hi,

On Mon, Feb 17, 2025 at 02:18:44PM +0800, Zeng Heng wrote:
> Hi Martin,
> 
> On 2025/2/17 11:18, Zeng Heng wrote:
> > The MPAM driver statically assigns all reqPARTIDs to respective intPARTIDs.
> > For the new rmid allocation strategy, it will check if there is an
> > available rmid of any reqPARTID which belongs to the input closid, not just
> > the rmids belonging to the closid.
> > 
> > For a mixture of MSCs system, for MSCs that do not support narrow-partid,
> > we use the PARTIDs exceeding the number of closids as reqPARTIDs for
> > expanding the monitoring groups.
> > 
> > In order to keep the existing resctrl API interface, the rmid contains both
> > req_idx and PMG information instead of PMG only under the MPAM driver. The
> > req_idx represents the req_idx-th sub-monitoring group under the control
> > group. The new rmid would be like:
> > 
> >      rmid = (req_idx << shift | pmg).
> > 
> 
> To consider future compatibility with dynamically allocated reqpartid,
> should I refactor the rmid?
> 
> Instead of defining rmid.req_idx, we could place the entire reqpartid
> directly within rmid. In This way, the allocation of reqpartid will no
> longer be constrained by the static allocation of closid, facilitating
> future compatibility with dynamic allocation mechanisms.
> 
> In this case, it needs to refactor the resctrl_arch_rmid_idx_encode()
> and resctrl_arch_rmid_idx_decode(), and we can simplify
> closid_rmid2reqpartid() to rmid2reqpartid().
> 
> What are your thoughts on this idea? Thank you in advance for your
> reply.
> 
> Best regards,
> Zeng Heng

Does this mean that the RMID must be expanded to cover all possible
(reqPARTID, PMG) combinations?

A single reqPARTID cannot be allocated to two different resctrl control
groups at the same time, even though a PMG value can be reused across
multiple control groups -- so it sounds like your proposal would
require changes in the resctrl core code as well as possibly requiring
a larger rmid_ptrs table.

But I might have misunderstood what you are proposing here...

Can you illustrate with one or two examples?

Cheers
---Dave