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Mon, 9 Nov 2026 19:21:56 +0000 Message-ID: <131cd15b-9498-4487-a5ce-ed6a47ef8ddd@intel.com> Date: Mon, 9 Nov 2026 11:21:53 -0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] fs/resctrl: Disallow monitor group rename when the RMID changes To: Ben Horgan , Jinjie Ruan , , , , , , , , , , , , , References: <20261105123524.2282090-1-ruanjinjie@huawei.com> <7a8d6a15-5a10-4035-b825-4436cba537ee@arm.com> <599b7b91-f6c8-4a8e-87a0-c023f3fd58ed@arm.com> <6c5d88a9-f86b-4255-82cc-bee70c2d7736@intel.com> <113c13ba-e1de-44b1-9a6f-4bd94ecbd71e@arm.com> Content-Language: en-US From: Reinette Chatre In-Reply-To: <113c13ba-e1de-44b1-9a6f-4bd94ecbd71e@arm.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW4PR04CA0209.namprd04.prod.outlook.com (2603:10b6:303:86::34) To IA0PR11MB8379.namprd11.prod.outlook.com (2603:10b6:208:488::20) X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA0PR11MB8379:EE_|LV2PR11MB185511:EE_ X-MS-Office365-Filtering-Correlation-Id: b2b67784-8435-4281-aafd-08df3e96b823 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|376014|7416014|23010399003|921020|56012099006|11063799006|261022105027099003|261021121427099003|261021123027099003|261026121527099003|261028144727099003|261009223027099003|261026140227099003|4143699003|261102123227099003|10067099003|18002099003|22082099003; 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Where the RMID is only meaningful >>>>> together with the CLOSID, as on arm64 MPAM where the RMID is a PMG that >>>>> extends the PARTID, this move does not work. The rename succeeds, but: >>>>> >>>>> - The group's monitoring data is reset. Its counters are identified by >>>>> the CLOSID and RMID together, so after the move the group reads the >>>>> counters of the new CLOSID instead of the ones its tasks used. >>>>> >>>>> - The tasks are not moved. mongrp_reparent() sets the new parent and >>>>> CLOSID of the group before rdt_move_group_tasks() looks for the tasks >>>>> to update, and the search then matches on the new CLOSID, which none >>>>> of the tasks carries yet. The tasks keep the old CLOSID and the old >>>>> control group's allocation, and the group does not count them. >>>>> >>>>> The move has never worked on these systems, so rejecting it does not >>>>> take away behavior that an existing user can rely on. >>>>> >>>>> Reject the move with -EOPNOTSUPP when it changes the RMID index of the >>>>> group, and document the restriction in resctrl.rst. Where the RMID does >>>>> not depend on the CLOSID, such as on x86, the move keeps the same index >>>>> and is still allowed. >>>>> >>>>> Fixes: 3e9b35823aab ("arm_mpam: resctrl: Add rmid index helpers") >>>>> Signed-off-by: Peter Newman >>>>> Signed-off-by: Jinjie Ruan >>>>> Cc: # 7.1.x >>>>> Link: https://lore.kernel.org/lkml/20241205153845.394714-1-peternewman@google.com/ >>>>> Link: https://lore.kernel.org/lkml/20261031101052.1692230-1-ruanjinjie@huawei.com/ >>>>> --- >>>>> v2: >>>>> - Drop the Suggested-by tag. >>>>> - Describe the systems by whether the RMID depends on the CLOSID rather >>>>> than by MPAM, in both the changelog and resctrl.rst. >>>>> - Explain that moving a monitor group is already broken on these systems >>>>> because the tasks are not moved, so no existing user relies on it. >>>>> - v1: https://lore.kernel.org/all/20261104101913.586595-1-ruanjinjie@huawei.com/ >>>>> --- >>>>> Documentation/filesystems/resctrl.rst | 7 ++++--- >>>>> fs/resctrl/rdtgroup.c | 13 +++++++++++++ >>>>> 2 files changed, 17 insertions(+), 3 deletions(-) >>>>> >>>>> diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst >>>>> index b52795e03303..9ae34aace0ff 100644 >>>>> --- a/Documentation/filesystems/resctrl.rst >>>>> +++ b/Documentation/filesystems/resctrl.rst >>>>> @@ -552,9 +552,10 @@ will automatically remove all MON groups below it. >>>>> Moving MON group directories to a new parent CTRL_MON group is supported >>>>> for the purpose of changing the resource allocations of a MON group >>>>> without impacting its monitoring data or assigned tasks. This operation >>>>> -is not allowed for MON groups which monitor CPUs. No other move >>>>> -operation is currently allowed other than simply renaming a CTRL_MON or >>>>> -MON group. >>>>> +is not allowed for MON groups which monitor CPUs, and fails with >>>>> +-EOPNOTSUPP on systems where the RMID depends on the CLOSID. No other >>>>> +move operation is currently allowed other than simply renaming a >>>>> +CTRL_MON or MON group. >>>> >>>> A user may not readily know whether RMID depends on CLOSID. To document >>>> this properly we need something specific we can point the user too. >>>> Perhaps this can be a file in the info directory specifying whether the >>>> move is possible or not, info/moveable_mon_group, as we discussed in v1. >>>> I suggest a precursor patch adding this file, followed by this patch >>>> which can then refer to the file. >>> >>> On the other hand, that does add an extra dependency for any backporting >>> of this fix. >> Does this fix really depend on a new user interface? >> >> I understand this is helpful to user space and this can be added, but I do >> not see how this fix requires it. > > Not strictly, no. Up to a point the documentation does but we could go > back to the documentation in v1 which mentions MPAM for the fix and > then, as a new feature, add the new interface on top of that. The > documentation can then be updated to mention the new interface only at > the time the new interface is added. Existing documentation makes a promise that monitoring data is not impacted by a move. The fix could start by just updating that snippet to match how other failures are handled. To keep things together I'll combine a suggestion in this regard with my other feedback in response to the patch self. Reinette