From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-174.mta0.migadu.com (out-174.mta0.migadu.com [91.218.175.174]) (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 CBC653AE1A8 for ; Tue, 24 Mar 2026 06:14:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.174 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774332873; cv=none; b=nfOcNHeWIc+5+TDUwZBxjS9iZ6apa9piENdTW6o2hF7A9kUQyOpS9Xo4OstwyBncpDhSQRvsYFKWPzrKzPuSS0rX9foeBpUdxNh/QBsFQ3QtBUrDMDgfPEN1ovJb/ojcBZ/V3tX8OoS+vjsEvq2InqF5gTavE4OyxrEM8P1wtxs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774332873; c=relaxed/simple; bh=skR628SMCx9KTemD4Cg3GA18jPV5b9gBdn2QAV+ZaTY=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version:Content-Type; b=QmRofWNcwhC2OIWfp8s3SSAXEPSEnQkzCzU+mDsALdsZU+2bcnaAJRaHTW6BDiCMn2krGoNOOcXHAemTy/4WtxduAJ79FMgS2DlVGrv2k6KMdqBoXBO6WMnCPG6D5ziFBFvVp4HWk23A4ovEADnrZfBrRiB8eNoeBR9Q6CJr8ko= 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=hnT6R2oX; arc=none smtp.client-ip=91.218.175.174 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="hnT6R2oX" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1774332868; 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=ICRwvZNxMVB+eDUAUeHAtU147HuuApF1zRxwwY2qVB4=; b=hnT6R2oXEZmInaXSy3cUMJ/hU2EBTkHqrDBMOTe86YmiKmkBh4ZMxGwIf3PT6Bbodw5rsH zp6vfO4HhkUvXVnkryl6PMVzOGOSTIkhVAQ45c8jFi4YMTXWmFXvOOAep/e0TNDHsvvvoc 0XvhJ3OfAPbCQDqd1ZxdNUPU56+eQf8= From: Lance Yang To: akpm@linux-foundation.org Cc: lance.yang@linux.dev, peterz@infradead.org, david@kernel.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 Subject: Re: [PATCH v7 0/2] skip redundant sync IPIs when TLB flush sent them Date: Tue, 24 Mar 2026 14:14:19 +0800 Message-Id: <20260324061419.48613-1-lance.yang@linux.dev> In-Reply-To: <20260323135317.0b702a575eeef93332ba2519@linux-foundation.org> References: <20260323135317.0b702a575eeef93332ba2519@linux-foundation.org> Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On Mon, Mar 23, 2026 at 01:53:17PM -0700, Andrew Morton wrote: >On Mon, 9 Mar 2026 10:07:09 +0800 Lance Yang wrote: > >> Hi all, >> >> 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]. >> >> This series introduces 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. >> >> Two-step plan as David suggested[4]: >> >> Step 1 (this series): Skip redundant sync when we're 100% certain the TLB >> flush sent IPIs. INVLPGB is excluded because when supported, we cannot >> guarantee IPIs were sent, keeping it clean and simple. >> >> Step 2 (future work): Send targeted IPIs only to CPUs actually doing >> software/lockless page table walks, benefiting all architectures. >> >> Regarding Step 2, it obviously only applies to setups where Step 1 does not >> apply: like x86 with INVLPGB or arm64. Step 2 work is ongoing; early >> attempts showed ~3% GUP-fast overhead. Reducing the overhead requires more >> work and tuning; it will be submitted separately once ready. >> >> ... >> >> arch/x86/include/asm/tlb.h | 17 ++++++++++++++++- >> arch/x86/include/asm/tlbflush.h | 2 ++ >> arch/x86/kernel/smpboot.c | 1 + >> arch/x86/mm/tlb.c | 15 +++++++++++++++ >> include/asm-generic/tlb.h | 17 +++++++++++++++++ >> mm/mmu_gather.c | 15 +++++++++++++++ >> 6 files changed, 66 insertions(+), 1 deletion(-) > >Kinda straddles both MM and x86. > >I expect a v8 based on David's comments. Yes, a v8 is on the way. >One merge path is for the x86 people to take this, noting David's acks. > >The other merge path is via mm.git, if the x86 people can please >perform review. > >And... mm.git is basically full (overflowing) for this cycle and >review/test has some catching up to do. So I'd prefer to only take the >important things. This patchset is a performance improvement but >contains no measurements to demonstrate the benefit, so I'm not able to >determine its importance! That's a fair point. I should have included numbers from the start. On a 64-core Intel x86 server, the CAL interrupt count in /proc/interrupts dropped from 646,316 to 785 when collapsing a 20 GiB range with this series applied. The larger the system, the more costly redundant broadcast IPIs become. Thanks, Lance