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([fe80::bfe:4ce1:556:4a9d]) by SJ2PR11MB7573.namprd11.prod.outlook.com ([fe80::bfe:4ce1:556:4a9d%5]) with mapi id 15.20.9769.006; Fri, 27 Mar 2026 16:14:37 +0000 Message-ID: Date: Fri, 27 Mar 2026 09:14:34 -0700 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 3/7] fs/resctrl: Disallow the software controller when MBM counters are assignable To: Ben Horgan , CC: , , , , , , , , , , , References: <20260326172551.1553871-1-ben.horgan@arm.com> <20260326172551.1553871-4-ben.horgan@arm.com> From: Reinette Chatre Content-Language: en-US In-Reply-To: <20260326172551.1553871-4-ben.horgan@arm.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: MW4PR03CA0196.namprd03.prod.outlook.com (2603:10b6:303:b8::21) To SJ2PR11MB7573.namprd11.prod.outlook.com (2603:10b6:a03:4d2::10) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email 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lzgtYMjqxloRGceHhnS/jrIWqG7GeYJaWxQ2jlUq2UW2nW4kyPQY1f0PQqC8p2/05UKO71Cia7ZFEmGuQDiLznhTCJWCu80M1/SC7o0rIe4= X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA2PR11MB5001 X-OriginatorOrg: intel.com Hi Ben, On 3/26/26 10:25 AM, Ben Horgan wrote: > The software controller requires that for each MBA control there is one MBM Could you please elaborate the "for each MBA control" distinction here? > counter per monitor group that is assigned to the event backing the > software controller, as per mba_MBps_event. When mbm_event mode is in use, > it is not guaranteed that any particular event will have an assigned > counter. > > Currently, only AMD systems support counter assignment, but the MBA delay > is non-linear and so the software controller is never supported anyway. On > MPAM systems, the MBA delay is linear and so the software controller could > be supported. The MPAM driver, unless a need arises, will not support the > 'default' mbm_assign_mode and will always use the 'mbm_event' mode for > memory bandwidth monitoring. > > Rather than develop a way to guarantee the counter assignment requirements > needed by the software controller, take the pragmatic approach. Don't allow > the software controller to be used at the same time as 'mbm_event' mode. As > MPAM is the only relevant architecture and it will use 'mbm_event' mode > whenever there are assignable MBM counters, for simplicity's sake, don't > allow the software controller when the MBM counters are assignable. > > Implement this by failing the mount if the user requests the software > controller, the mba_MBps option, and the MBM counters are assignable. > > Signed-off-by: Ben Horgan > --- > fs/resctrl/rdtgroup.c | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c > index fa5712db3778..7ef316b24a41 100644 > --- a/fs/resctrl/rdtgroup.c > +++ b/fs/resctrl/rdtgroup.c > @@ -2528,7 +2528,8 @@ static bool supports_mba_mbps(void) > > return (resctrl_is_mbm_enabled() && > r->alloc_capable && is_mba_linear() && > - r->ctrl_scope == rmbm->mon_scope); > + r->ctrl_scope == rmbm->mon_scope && > + !rmbm->mon.mbm_cntr_assignable); > } > > /* > @@ -2943,7 +2944,7 @@ static int rdt_parse_param(struct fs_context *fc, struct fs_parameter *param) > ctx->enable_cdpl2 = true; > return 0; > case Opt_mba_mbps: > - msg = "mba_MBps requires MBM and linear scale MBA at L3 scope"; > + msg = "mba_MBps requires dedicated MBM counters and linear scale MBA at L3 scope"; This looks like the original. I was expecting: https://lore.kernel.org/lkml/cac437e2-8139-4833-9cbd-55d626062730@arm.com/ ? > if (!supports_mba_mbps()) > return invalfc(fc, msg); > ctx->enable_mba_mbps = true; Reinette