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X-CSE-ConnectionGUID: yy10pP/gS22rNUPQP5410A== X-CSE-MsgGUID: NO6xrnwFS3GDQ3mEtunGkQ== X-IronPort-AV: E=McAfee;i="6700,10204,11307"; a="35565015" X-IronPort-AV: E=Sophos;i="6.12,293,1728975600"; d="scan'208";a="35565015" Received: from fmviesa008.fm.intel.com ([10.60.135.148]) by fmvoesa113.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 06 Jan 2025 07:54:51 -0800 X-CSE-ConnectionGUID: c19zz9oMQEy2zacs0dpSCg== X-CSE-MsgGUID: 3dC5QvYpSIaXMj3S3AL+0g== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.12,293,1728975600"; d="scan'208";a="102673031" Received: from lbogdanm-mobl3.ger.corp.intel.com (HELO [10.124.220.224]) ([10.124.220.224]) by fmviesa008-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 06 Jan 2025 07:54:50 -0800 Message-ID: <5bda5b49-60a0-41d5-8bd9-c52794a645a8@intel.com> Date: Mon, 6 Jan 2025 07:54:50 -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: [PATCH 01/12] x86/mm: make MMU_GATHER_RCU_TABLE_FREE unconditional To: Peter Zijlstra , Borislav Petkov Cc: Rik van Riel , x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, dave.hansen@linux.intel.com, luto@kernel.org, tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, akpm@linux-foundation.org, nadav.amit@gmail.com, zhengqi.arch@bytedance.com, linux-mm@kvack.org References: <20241230175550.4046587-1-riel@surriel.com> <20241230175550.4046587-2-riel@surriel.com> <20241230184118.GMZ3LpTn3dHvu_bq-p@fat_crate.local> <20250102195609.GB7274@noisy.programming.kicks-ass.net> <20250103121843.GDZ3fVo0r0JRsGImBS@fat_crate.local> <20250104162713.GA22803@noisy.programming.kicks-ass.net> 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 1/4/25 08:27, Peter Zijlstra wrote: >> Or should we make this unconditional on all native because we don't care about >> the overhead and would like to have simpler code. I mean, disabling IRQs vs >> batching and allocating memory...? > The disabling IRQs on the GUP-fast side stays, it acts as a > RCU-read-side section -- also mmu_gather reverts to sending IPIs if it > runs out of memory (extremely rare). > > I don't think there is measurable overhead from doing the separate table > batching, but I'm sure the robots will tell us. We should _try_ to make it unconditional for simplicity if nothing else. BTW, a few years back, some folks at Intel turned on MMU_GATHER_RCU_TABLE_FREE and ran the usual 0day/LKP tests. I _think_ it was when we were exploring the benefits of Intel's IPI-free TLB flushing mechanism. We didn't find anything remarkable either way (IIRC).