From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 0F16B3FB7D8 for ; Wed, 18 Mar 2026 15:55:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773849333; cv=none; b=TwMilEZzTtGpIBYKUqgDA+ytQtTPTk5iKcwfPWDEZXjnyuSqi+1f86SVAfNfS+9TII92acEX4FeP4wECHVzo1A4DVuVMIJclc46Rq5cyKOHyJpxfnlW47EuWWjOsaLXaK4WsUtlgGFtBsMTHGQQeHZDvbrzKenm0A3MYO/vHfOc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773849333; c=relaxed/simple; bh=OHQqGQSeknYrqAUBuj0Fk+ec/Yb+CoA5oKTk0s3GvHI=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=rouFkwBKg+reaxR7uKHYBf3HZNma0pOBT4nnKrdZcXykWDVMFUqNmJjQzoaeBBOILXSFvRGTEbpja2xoQq5A6UmzMgyYfXTKgEK6biFj4CzVarCPDhdYoaeD7fRDgl1Lds1fJxui2GzvtVA5SZaLZDmB9z9LvlHs6oe4QOxZtWo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 5E5452103; Wed, 18 Mar 2026 08:55:23 -0700 (PDT) Received: from [10.1.196.46] (e134344.arm.com [10.1.196.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 92D493F915; Wed, 18 Mar 2026 08:55:27 -0700 (PDT) Message-ID: <729fd48c-f25b-430a-95e1-49e6156364f8@arm.com> Date: Wed, 18 Mar 2026 15:55:26 +0000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Thunderbird Daily Subject: Re: [PATCH v2 4/6] fs/resctrl: Disallow the software controller when mbm counters are assignable From: Ben Horgan To: Reinette Chatre , linux-kernel@vger.kernel.org Cc: tony.luck@intel.com, Dave.Martin@arm.com, james.morse@arm.com, babu.moger@amd.com, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, fenghuay@nvidia.com, tan.shaopeng@fujitsu.com References: <20260313174524.3482767-1-ben.horgan@arm.com> <20260313174524.3482767-5-ben.horgan@arm.com> <43a47ac0-5d49-47da-9740-9789df9bda96@intel.com> Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Hi Reinette, On 3/18/26 15:44, Ben Horgan wrote: > > Apologies if the commit message aren't clear. I do see that I was confused in this one between > counters being assignable, i.e. mbm_event mode possible, and the user being the being able to > assign and unassign counters, i.e. mbm_event mode enabled. Do you prefer this as a commit message? > > fs/resctrl: Disallow the software controller when using mbm_event mode > > The software controller requires that there is one MBM counter, for each > domain, per monitor group that is assigned to the event backing the > software controller (the mba_MBps_event). When mbm_event mode is in use > this event backing cannot be guaranteed as there may be insufficient > counters, the user may unassign the counter or mbm_assign_on_mkdir may be > set to 0. Another case which could be mentioned here and results in unassigned events is when mbm_assign_on_mkdir is set to 1 but one of the domains was offline when the monitor group was created. > > Currently, only AMD systems support counter assignment but the MBA is non > linear and so the software controller is never supported anyway. For MPAM > systems the MBA is linear and it could support the software controller. > Adding support would require guaranteeing that the events backing the > software controller are always assigned. In order to support mbm_event mode > for MPAM, which is needed to support memory bandwidth monitors, MBM, on > platforms with fewer bandwidth counters (per domain) than monitor groups, > take the pragmatic approach and fail the mount if the user requests the > software controller, the mba_MBps option, when the mbm_event mode is > enabled. Furthermore, disable switching to mbm_event mode when the software > controller is enabled. If, in the future, a way for mbm_event mode and the > software controller to function at the same time is introduced these > restrictions can be adapted. Thanks, Ben