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Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vmqdL-0000000GJ5I-0aps; Mon, 02 Feb 2026 09:54:15 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 98BFF303031; Mon, 02 Feb 2026 10:54:14 +0100 (CET) Date: Mon, 2 Feb 2026 10:54:14 +0100 From: Peter Zijlstra To: Lance Yang Cc: akpm@linux-foundation.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 v4 0/3] targeted TLB sync IPIs for lockless page table walkers Message-ID: <20260202095414.GE2995752@noisy.programming.kicks-ass.net> References: <20260202074557.16544-1-lance.yang@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260202074557.16544-1-lance.yang@linux.dev> X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 678DAA000B X-Stat-Signature: pty3cdysczqubx3i9mhsqzrh3ya4c9iq X-Rspam-User: X-HE-Tag: 1770026067-689593 X-HE-Meta: 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 tkzvk9b5 9p2wB2damcZ2RBernPMyS1UhuJC5/yUvkqLPw+JI6WSd9KZOLT4fzjoS2EMmDtNgdWvHad4daVxMgiPMjp+mtqZNeGZNUkRWVMNLSa9d8R8mjEdPQJqqGoVW6C/1n/3/t5FCQ8M3rVWGGxiI3nwOjtXzYvlEVVs0uvuswHvqyiiYNaINClbx1nFRbiaB74V1TUawh/TYa0K+NXeHc/3e+VL2o7g== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Feb 02, 2026 at 03:45:54PM +0800, Lance Yang wrote: > When freeing or unsharing page tables we send an IPI to synchronize with > concurrent lockless page table walkers (e.g. GUP-fast). Today we broadcast > that IPI to all CPUs, which is costly on large machines and hurts RT > workloads[1]. > > This series makes those IPIs targeted. We track which CPUs are currently > doing a lockless page table walk for a given mm (per-CPU > active_lockless_pt_walk_mm). When we need to sync, we only IPI those CPUs. > GUP-fast and perf_get_page_size() set/clear the tracker around their walk; > tlb_remove_table_sync_mm() uses it and replaces the previous broadcast in > the free/unshare paths. I'm confused. This only happens when !PT_RECLAIM, because if PT_RECLAIM __tlb_remove_table_one() actually uses RCU. So why are you making things more expensive for no reason?