From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-177.mta1.migadu.com (out-177.mta1.migadu.com [95.215.58.177]) (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 B756E257448 for ; Thu, 18 Dec 2025 14:36:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.177 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766068585; cv=none; b=IPhLz+fTQcdmJxyzzUzJIKOTpmuEfvkFjwM9IHy6eQF18l3kuGzh/oFlDRSFv33F5NP1kzV7TOOK74loDRnndNMJh4zaf+x81gXQCVhVTu72ukiaQYdkpHBGc9e/SMDJLkUF4/iFX9OJGtt8926cNHz/jsHxqD4iabta9jO+S98= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766068585; c=relaxed/simple; bh=4KEVoBX1tq64wU+qAS265Rmxx0K+GFXcp+VSKNYxIp4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ItG9r1ZtaUClPDDrXYT5BAmAqLqc8vIV0YO53tMFvXYqK5/pu6nTQIRWWk9lNxhEeUd3fqCl/E5r7dKyNGtnx1eqFeshJgubheNr+CU8opaV8GYoZOodCek3b6YGj9yotEqVhMObcfvBbQM7QX6/J8WXVca7EU7sJs4w1NWcg0Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=PpMbVtrz; arc=none smtp.client-ip=95.215.58.177 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="PpMbVtrz" Message-ID: <6fdf89ee-3f6a-420f-b4d6-b03e3e2c8c9b@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1766068571; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=8/obYsmYE7DxZF0F7CimsIs8ULL/trNGoQu3yuIxJ4Q=; b=PpMbVtrzF9RqtCYbqHrg5kVIozY0nQPPfUZDye+GfxYxEfRSyrmGid/c8J5DlSEP9uuEaZ Rg366J3xLe78dFngON/IJ7OO8n1xI7PKzXlwpQYQvzYImbl3cCw37jX/Pq7r9o4a6Q0eDX kD2GtDWnn4voFSAKU1hfnCKTYllD5wI= Date: Thu, 18 Dec 2025 22:35:59 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH RFC 3/3] mm/khugepaged: skip redundant IPI in collapse_huge_page() To: "David Hildenbrand (Red Hat)" Cc: will@kernel.org, aneesh.kumar@kernel.org, npiggin@gmail.com, peterz@infradead.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, 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, ioworker0@gmail.com, shy828301@gmail.com, riel@surriel.com, jannh@google.com, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, akpm@linux-foundation.org References: <20251213080038.10917-1-lance.yang@linux.dev> <20251213080038.10917-4-lance.yang@linux.dev> <948d425a-2d6e-4439-a280-0ca9e7521b13@kernel.org> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: <948d425a-2d6e-4439-a280-0ca9e7521b13@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2025/12/18 21:13, David Hildenbrand (Red Hat) wrote: > On 12/13/25 09:00, Lance Yang wrote: >> From: Lance Yang >> >> Similar to the hugetlb PMD unsharing optimization, skip the second IPI >> in collapse_huge_page() when the TLB flush already provides necessary >> synchronization. >> >> Before commit a37259732a7d ("x86/mm: Make MMU_GATHER_RCU_TABLE_FREE >> unconditional"), bare metal x86 didn't enable MMU_GATHER_RCU_TABLE_FREE. >> In that configuration, tlb_remove_table_sync_one() was a NOP. GUP-fast >> synchronization relied on IRQ disabling, which blocks TLB flush IPIs. >> >> When Rik made MMU_GATHER_RCU_TABLE_FREE unconditional to support AMD's >> INVLPGB, all x86 systems started sending the second IPI. However, on >> native x86 this is redundant: >> >>    - pmdp_collapse_flush() calls flush_tlb_range(), sending IPIs to all >>      CPUs to invalidate TLB entries >> >>    - GUP-fast runs with IRQs disabled, so when the flush IPI completes, >>      any concurrent GUP-fast must have finished >> >>    - tlb_remove_table_sync_one() provides no additional synchronization >> >> On x86, skip the second IPI when running native (without paravirt) and >> without INVLPGB. For paravirt with non-native flush_tlb_multi and for >> INVLPGB, conservatively keep both IPIs. >> >> Use tlb_table_flush_implies_ipi_broadcast(), consistent with the hugetlb >> optimization. >> >> Suggested-by: David Hildenbrand (Red Hat) >> Signed-off-by: Lance Yang >> --- >>   mm/khugepaged.c | 7 ++++++- >>   1 file changed, 6 insertions(+), 1 deletion(-) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index 97d1b2824386..06ea793a8190 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -1178,7 +1178,12 @@ static int collapse_huge_page(struct mm_struct >> *mm, unsigned long address, >>       _pmd = pmdp_collapse_flush(vma, address, pmd); >>       spin_unlock(pmd_ptl); >>       mmu_notifier_invalidate_range_end(&range); >> -    tlb_remove_table_sync_one(); >> +    /* >> +     * Skip the second IPI if the TLB flush above already synchronized >> +     * with concurrent GUP-fast via broadcast IPIs. >> +     */ >> +    if (!tlb_table_flush_implies_ipi_broadcast()) >> +        tlb_remove_table_sync_one(); > > We end up calling > >     flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE); > >     -> flush_tlb_mm_range(freed_tables = true) > >     -> flush_tlb_multi(mm_cpumask(mm), info); > > So freed_tables=true and we should be doing the right thing. Yep ;) > BTW, I was wondering whether we should embed that > tlb_table_flush_implies_ipi_broadcast() check in > tlb_remove_table_sync_one() instead. > It then relies on the caller to do the right thing (flush with > freed_tables=true or unshared_tables = true). > > Thoughts? Good point! Let me check the other callers to ensure they are all preceded by a flush with freed_tables=true (or unshared_tables). Will get back to you with what I find :) Cheers, Lance