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b=O8uzlx4dv+oYAEjjcSqdVpXYZ8B1iP9u6eefUS7KnBdO1jV1nDLkt8M4cv+flrJ4x JF7n9LdVHizBHwgYFeftNAhGfNvKisywSC2AaXUa41lyhkyqMUb8NppSToaU+zt0oA UAtjne/8nIy5mv0bKnjNILw1rFYh6TWOnXJddrcLMCseV0/5HUCRM4Jypn9BKiYQho iGxT3MH3lNx89NIH5zl8sSPrXHrzz9E9C/kMhCuZPS2bPCSZy0MuA6DTcYRzcfqsAT TdXdgeuNufG5FxoEffzcSDEbgMga4n7kdNwccYNXqRFy5B9qmW8CcaHnCjmD2sCqFS WZIMdntBTtMgw== Message-ID: <955fb2e9-b3f9-479a-806d-79877635816c@kernel.org> Date: Mon, 23 Mar 2026 12:04:12 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 1/2] mm/mmu_gather: prepare to skip redundant sync IPIs To: Lance Yang , akpm@linux-foundation.org Cc: peterz@infradead.org, dave.hansen@intel.com, dave.hansen@linux.intel.com, ypodemsk@redhat.com, hughd@google.com, will@kernel.org, aneesh.kumar@kernel.org, npiggin@gmail.com, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com, arnd@arndb.de, lorenzo.stoakes@oracle.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, shy828301@gmail.com, riel@surriel.com, jannh@google.com, jgross@suse.com, seanjc@google.com, pbonzini@redhat.com, boris.ostrovsky@oracle.com, virtualization@lists.linux.dev, kvm@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, ioworker0@gmail.com References: <20260309020711.20831-1-lance.yang@linux.dev> <20260309020711.20831-2-lance.yang@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: D2E7112000F X-Stat-Signature: 51388o8djsmbxechpfnddws9khq1oh9t X-HE-Tag: 1774263862-386169 X-HE-Meta: U2FsdGVkX1+LF4oYsvAvlTziT2+czCIPPWtUXAr4Xw32YM/ngE7TMqSkeF5d847e/l0PmMD/1XDyo2iuPWMFqH17TH5Y/3myH9A804z9cu7Scobl+anxhyusZAzKb2oYGjCSwXXzmrLzAPBBpQoAEk0XPtRnwccWL7vxhCHoK+E5mtcNRVn/glgiz+hV7fqB53g/yq15md05+0HpAEKa72mfNM/gtFEJzZx20Xi5nd6AuiH7b2uj6tTTJULQG3CuR0+FvcGkCkIJAu4L+Ufhm25V/UdCzQmouNzBZcm2RqNMsxz9D31aVltQQejqxhnB5BszW9KY+V4Fr/hpsmTkTjAOcb3pCCPty8asfTttBWyTfLbrdI1esRLTDXQ2mucjCk8hntt8HGKNmYAbQlW09TOJBS9fT89lfPcbkEhqat7cTgjCr1773V8CMdiKfrBbbxOLZXRjqf1EvEtfi51QB7ZSY1zzvlkompjt3htyBPulsE1U5cn1FIYMqoyF7VymKzObVZTsnth8Xzsj0y9vBnXCuRNJuaws6RYSS0PXPDsQpsVxJ3SNofPVlUfqo94HIIJejSi672o/qZvZtrrZVeSxPOUCbI0dB9on7sLU82dcmcze6YqDvtpt0+27J33lX9F8h+X+qOygh1cOfQ07RNCjXeMb36/GRrj1xF/pVoyGZCaOc8kiucMCnpQvKlDy7gfGbHmEiOjTslqT4/h/HSI42U+92AeOR1th/+L8lCFEJMqm6vDwGYKHOqYyyWk6aIzJCwkjrH9h9IOlqWFf8YOp4yBcyhcKQd7W29utd04HfTIt0h2x4+x/c4sZiyf/zKCQd9cdk3oAbN98cvKofgDDwBzPABSA5vB2yhVbw0YO6hhPyudZqD3p28sGg/W46t4Kn8b1tZD5h5NIfJNipr//qFewbE3aQ3rrvursmBKypNtnCR6H17w/tQxD+EX8/Hb+HMsfazu2FPXLMmx fjq4Le4E wKN/mxjp/D9/ddizHcndE+1pQqZFFjhP7qMOeL6gtjNHhI0RHz+w1gUlYAZVeX8R0XzeqPpH9rpzHh4GXh4HQVjIvFkEmd4NlO/jQN1UvWsySXxLq1cp8bUSUDA3A+8Eu8mou9+mDuG2gg+NujkYNu9D8smzUSwXR4SvcXAWwFu9ReDLl2gdT5Yzicw9nA5qceqhxZ4jUNZ74mJGsg8M87jpu6rLjybxb46LHl/fl4BIOynu7VAzv3qFYvydKvM1bPo+zFucm8Zq3h4av8IosgHLaDpeMKq3osH8SZr9oyenluTfZ8goOBTPQQzgBRyY7R6ktBP3rUXpvykFleVfJQ4h1tEnNSul9gx4WyWf9iQzu90I= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3/9/26 03:07, Lance Yang wrote: > From: Lance Yang > > When page table operations require synchronization with software/lockless > walkers, they call tlb_remove_table_sync_{one,rcu}() after flushing the > TLB (tlb->freed_tables or tlb->unshared_tables). > > On architectures where the TLB flush already sends IPIs to all target CPUs, > the subsequent sync IPI broadcast is redundant. This is not only costly on > large systems where it disrupts all CPUs even for single-process page table > operations, but has also been reported to hurt RT workloads[1]. > > Introduce tlb_table_flush_implies_ipi_broadcast() to check if the prior TLB > flush already provided the necessary synchronization. When true, the sync > calls can early-return. > > A few cases rely on this synchronization: > > 1) hugetlb PMD unshare[2]: The problem is not the freeing but the reuse > of the PMD table for other purposes in the last remaining user after > unsharing. > > 2) khugepaged collapse[3]: Ensure no concurrent GUP-fast before collapsing > and (possibly) freeing the page table / re-depositing it. > > Currently always returns false (no behavior change). The follow-up patch > will enable the optimization for x86. > > [1] https://lore.kernel.org/linux-mm/1b27a3fa-359a-43d0-bdeb-c31341749367@kernel.org/ > [2] https://lore.kernel.org/linux-mm/6a364356-5fea-4a6c-b959-ba3b22ce9c88@kernel.org/ > [3] https://lore.kernel.org/linux-mm/2cb4503d-3a3f-4f6c-8038-7b3d1c74b3c2@kernel.org/ > > Suggested-by: David Hildenbrand (Arm) > Signed-off-by: Lance Yang > --- Acked-by: David Hildenbrand (Arm) -- Cheers, David