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X-CSE-ConnectionGUID: 09IcPqIiT+aI/Qu18/FsWQ== X-CSE-MsgGUID: 2Q9tEp9wRV2z667CGghnug== X-IronPort-AV: E=McAfee;i="6700,10204,11274"; a="33080597" X-IronPort-AV: E=Sophos;i="6.12,202,1728975600"; d="scan'208";a="33080597" Received: from orviesa005.jf.intel.com ([10.64.159.145]) by orvoesa112.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 02 Dec 2024 08:50:04 -0800 X-CSE-ConnectionGUID: cLFqW7hmRoyF0YweZdrt4Q== X-CSE-MsgGUID: NCBAz5d8SdukCe5l/awmtg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.12,202,1728975600"; d="scan'208";a="98125931" Received: from sramkris-mobl1.amr.corp.intel.com (HELO [10.124.222.246]) ([10.124.222.246]) by orviesa005-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 02 Dec 2024 08:50:03 -0800 Message-ID: <8c6ace22-38d7-4178-ae55-9a9cdcd981d9@intel.com> Date: Mon, 2 Dec 2024 08:50:02 -0800 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: [tip:x86/mm] [x86/mm/tlb] 209954cbc7: will-it-scale.per_thread_ops 13.2% regression To: Mathieu Desnoyers , Peter Zijlstra , Rik van Riel Cc: kernel test robot , oe-lkp@lists.linux.dev, lkp@intel.com, linux-kernel@vger.kernel.org, x86@kernel.org, Ingo Molnar , Linus Torvalds , Mel Gorman References: <202411282207.6bd28eae-lkp@intel.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/28/24 11:46, Mathieu Desnoyers wrote: > AFAIU, this commit changes the way TLB flushes are inhibited when > context switching away from a mm. This means that one additional TLB > flush is performed to a given CPU even after it has context switched > away from the mm, and only then is the mm_cpumask cleared for that CPU. > > This could result in additional TLB flush IPI overhead in specific > scenarios where the IPIs are typically triggered after a thread has > context-switched out. I can see how that might generally be a problem, but for this particular workload: > https://github.com/antonblanchard/will-it-scale/blob/master/tests/tlb_flush2.c I'm not sure how it would apply. will-it-scale should create one big process that runs for quite a long while and is bound to one CPU. The threads can get scheduled out as other things run, but they should pop right back on to the CPU. There shouldn't be a lot of context switching in this workload. It would be great if someone could reproduce this and double-check that the theory about what's making it regress is really holding in practice.