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 37E7B376494 for ; Fri, 30 Jan 2026 12:21:48 +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=1769775711; cv=none; b=A2qRvrv6rHKO6wkmMfaEI+vhWQDLqMvkcgnzZm+E1bkxf6rgbAGwkxFYBHFLvcjiN+txVIV+CG2rzgHS0ByslY0mIijzy43irdvw8To8ohOpcPYmqLfX4T1NVtpshGlfhtU9XUWY6IbNQu//g37nWcw82XMGbbK8gBsA17u8WYU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769775711; c=relaxed/simple; bh=4rW3klVonaRjqtTArWPp0tognmUQmSzmkxdoBF4gL3w=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=e4+UYLlnPG0AfA0e8xcSUYer4vW9Ig3lBf7cJs3pS/B0oRKPYFFN6pXGp6qXgas7GJtHcq6/G+8THJbxzgmr1BYsnVYlsZYpRPeH54BLMNqpKg1t8jIBi+o0uwjcCYYSBaG9qVWWqTfkE3Zblb9nkJHvE5PGCgy52vdfURgLn7Q= 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 F4148153B; Fri, 30 Jan 2026 04:21:41 -0800 (PST) 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 9921D3F632; Fri, 30 Jan 2026 04:21:43 -0800 (PST) Message-ID: Date: Fri, 30 Jan 2026 12:21:42 +0000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 45/47] arm_mpam: Add workaround for T241-MPAM-4 To: Fenghua Yu Cc: amitsinght@marvell.com, baisheng.gao@unisoc.com, baolin.wang@linux.alibaba.com, carl@os.amperecomputing.com, dave.martin@arm.com, david@kernel.org, dfustini@baylibre.com, gshan@redhat.com, james.morse@arm.com, jonathan.cameron@huawei.com, kobak@nvidia.com, lcherian@marvell.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, peternewman@google.com, punit.agrawal@oss.qualcomm.com, quic_jiles@quicinc.com, reinette.chatre@intel.com, rohit.mathew@arm.com, scott@os.amperecomputing.com, sdonthineni@nvidia.com, tan.shaopeng@fujitsu.com, xhao@linux.alibaba.com, catalin.marinas@arm.com, will@kernel.org, corbet@lwn.net, maz@kernel.org, oupton@kernel.org, joey.gouly@arm.com, suzuki.poulose@arm.com, kvmarm@lists.linux.dev References: <20260112165914.4086692-1-ben.horgan@arm.com> <20260112165914.4086692-46-ben.horgan@arm.com> <23f08379-301d-4ffc-bf6f-d6a7bb952724@nvidia.com> From: Ben Horgan Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Hi Fenghua, On 1/29/26 22:14, Fenghua Yu wrote: > Hi, Ben, > > On 1/19/26 12:56, Ben Horgan wrote: >> Hi Fenghua, >> >> On 1/15/26 23:20, Fenghua Yu wrote: >>> Hi, Shanker and Ben, >>> >>> On 1/12/26 08:59, Ben Horgan wrote: >>>> From: Shanker Donthineni >>>> >>>> In the T241 implementation of memory-bandwidth partitioning, in the >>>> absence >>>> of contention for bandwidth, the minimum bandwidth setting can >>>> affect the >>>> amount of achieved bandwidth. Specifically, the achieved bandwidth >>>> in the >>>> absence of contention can settle to any value between the values of >>>> MPAMCFG_MBW_MIN and MPAMCFG_MBW_MAX.  Also, if MPAMCFG_MBW_MIN is set >>>> zero (below 0.78125%), once a core enters a throttled state, it will >>>> never >>>> leave that state. >>>> >>>> The first issue is not a concern if the MPAM software allows to program >>>> MPAMCFG_MBW_MIN through the sysfs interface. This patch ensures program >>>> MBW_MIN=1 (0.78125%) whenever MPAMCFG_MBW_MIN=0 is programmed. >>> >>> When MBW_MIN=1, min mem bw can be very low when contention. This may >>> drop mem access performance. Is it possible to set MBW_MIN bigger so >>> that ensure the floor of mem access is high? >> >> Isn't that a policy decision rather than something we should be putting >> in a quirk framework? > > MBW_MIN is 1% or 5% less than MBW_MAX. > > The lower MBW_MIN hints hardware to lower mem bandwidth when mem access > contention. That causes memory performance degradation. > > Is it possible to do the following changes to fix the performance issue? > 1. By default min mbw is equal to max mbw. So hardware won't lower > performance unless it's needed. This can fix the current performance issue. > 2. Add a new schemata line (e.g. MBI:=x;=y;...) to specify min > mbw just like max mbw specified by schemata line "MB:...". User can use > this line to change min mbw per partition per node. This could be added > in the future. Thanks for bringing this up. It raises some more general queries about the handling of mbw_min and so I'll move the discussion to [PATCH v3 41/47] arm_mpam: Generate a configuration for min controls as it isn't specific to the nvidia quirks. > > Thanks. > > -Fenghua Thanks, Ben