From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2FD97390988; Mon, 23 Mar 2026 11:04:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774263862; cv=none; b=eWlutesyjPIco8ZzPZvRvp5QsSmShRXRmGy2ioILuCrPTMoGPw+KyB0elVr1b277CkQQfC4NvzYOi5DEp/z3XE0hgeKnJJUvshrmcwpf307oI4wyDIB+512BOFnbqOoQU/ZKxdGHJCu5QfL1W1f0qzMycNiydEETZ3DpGlWAT7A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774263862; c=relaxed/simple; bh=cZpSLIsH4XFvNpI5LDZxz95hRu5Out/Dx+WnDsOPv7Q=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=WddwPJs7RxqRPs2gFMVsWZCMsmSO6lN4gFQADHUAUiuXIKqK3ewTbKO4AinVQfd8N5iRk1sIznCQws348KNCRvGsYl8g4d+wvYRvj9RqTS4Qim08j9W9YZl2FjCstblVuebfd3BDFFYTMx1VEcy9pVaqHc5Hh3W+eYkbajkCq8g= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=O8uzlx4d; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="O8uzlx4d" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B458EC4CEF7; Mon, 23 Mar 2026 11:04:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774263861; bh=cZpSLIsH4XFvNpI5LDZxz95hRu5Out/Dx+WnDsOPv7Q=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; 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 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 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 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