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b=yM9qm7ugRKpa1d+28Z1NjLNSW05C1mgX6m5igCaRKQc7iIRC5yOMCgi/to4dpT5xN bFALWlmn8qT9kIH6Sm5MIHDDZLqnatqEcA4qWIahjKxwhX8TDamvMVHysNvFFVTzB5 CG/3tt9EDVRK0WRV9LGfwZR3fkNqcAhp0izUQZOs= Date: Fri, 24 Apr 2026 06:30:24 -0700 From: Andrew Morton To: Lance Yang Cc: 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, ljs@kernel.org, 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 7.2 v10 0/2] skip redundant sync IPIs when TLB flush sent them Message-Id: <20260424063024.ce42ee6a5546e4d9337dd007@linux-foundation.org> In-Reply-To: <20260424062528.71951-1-lance.yang@linux.dev> References: <20260424062528.71951-1-lance.yang@linux.dev> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Stat-Signature: raraaax9tf64wa3n3idjjahfizpx6b3n X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: B644F4000F X-Rspam-User: X-HE-Tag: 1777037426-995112 X-HE-Meta: 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 hmaCKsjz StGBYtqzZ1OScKv65zi7z0D8WKrbj/JgtQongIA5qu/AFXTCL47hcVF/6ALIUePAdx1yf5gOHV+BXRo7tz3QIzAQe7ByrhIvZGgQt8AFe/dPQ8epyEbFIgxF/lSxeASua8e6ifAgstL+Lk7bMFZQm9nkH88bLS9YbdMP3hIy+58m+pnW3faAS7Gk+sH1GO3oeKPWnxQGPeqh6h/s4EFfUF9W+lI8/Ry4iqmTL86egvpKL97yaOquV4c1V939PPIulQoMAnqX0wH+Zx2GsLBVKDztMydkkZ7bubIb6DjELaVgSCifDJHyJPIWqzg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, 24 Apr 2026 14:25:26 +0800 Lance Yang wrote: > 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. Sashiko questions: https://sashiko.dev/#/patchset/20260424062528.71951-1-lance.yang@linux.dev (I never know if my Sashiko emails are welcome/useful. Maybe Sashiko said the same stuff about v9 and it's all wrong. But better safe than sorry!)