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DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1774406632; 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=ei/9l0yY7Rn5LSa4qZLzjXvD8FXeXbiuykIdzrEGGI8=; b=JqJYxCEq+lrhrLpLxEyPYrKajW5e8CXwMDWT8tH66YKkSs1lIqcLLBSimJDzH2ilNjg2fr XqMyL9OU6mgfjoBf8JGd1h7NnAf5wUyg9Sl9kJtncCi0D3BfYneybyTawTqCbvGyi29LLu TyhR/3ijGiBXv1xiv7Di0IYSdgxmA4Y= From: Lance Yang To: akpm@linux-foundation.org, peterz@infradead.org, dave.hansen@intel.com, david@kernel.org Cc: 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, Lance Yang Subject: Re: [PATCH v8 0/2] skip redundant sync IPIs when TLB flush sent them Date: Wed, 25 Mar 2026 10:43:37 +0800 Message-Id: <20260325024337.56681-1-lance.yang@linux.dev> In-Reply-To: <20260324114339.2c0777f7b2c5483281a15667@linux-foundation.org> References: <20260324114339.2c0777f7b2c5483281a15667@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 1911CA0002 X-Stat-Signature: ch5af7amzrt4qpq75t48ppbmt79bksqg X-Rspam-User: X-HE-Tag: 1774406634-751087 X-HE-Meta: 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 vnAT6iSS dy/n8pdN5DpHAddzg56IxSzgiBNrOd45jmjYTbFrxXaVnMD1PD9WtFwA8H3gLKxUMQA0iyeWTq/iMNQnOFyDGmCaO4D5eemo6QN2Ac5Gxfj+7hsKbIB248A0YtCni3Zod5QZmh1x8523QmC8u81bmKFIYVrLCAF7jVlDPpM7gr/7gHQxToYFXopjW28cz2bnp9BrWCnQlJs+ugDdWPyQ1rljF4g== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Mar 24, 2026 at 11:43:39AM -0700, Andrew Morton wrote: >On Tue, 24 Mar 2026 16:52:36 +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. >> >> 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. > >Well that's nice. > >Which other architectures could utilize this? Thanks! RISC-V looks like a candidate, and if I get some time I'll dive into it. > >> David Hildenbrand did the initial implementation. I built on his work and >> relied on off-list discussions to push it further - thanks a lot David! >> >> ... >> >> arch/x86/include/asm/tlb.h | 18 +++++++++++++++++- >> 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, 67 insertions(+), 1 deletion(-) > >Can the x86 maintainers please review these changes? Yes, an x86 review would be much appreciated, please! After commit a37259732a7d ("x86/mm: Make MMU_GATHER_RCU_TABLE_FREE unconditional"), tlb_remove_table_sync_one() is no longer a NOP, even on native x86 without INVLPGB, so it ends up issuing a redundant IPI broadcast. Thanks, Lance