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b=syUPtY6w30APzE75pzBQSESfdtrxIxyU+Wb4bl5STSoesRAQgZAS+B+O+5/jpmJ/U dwYjT+SAXdf+ZZni8ujBAnmCclEaVM5dBvrXYnIhbsKNPU3gtHi5r4sGtLP1weUGYy hzyqogeLtiOPBgN0XJ/SYMRW08nXrLGWMBCvKNhLMHwPDQKpEarWGo6DTaRt4zpimH crT8snBDtzAZtyRImNdKI6ysnP2zgsFZlir3ohopfVbsA1xQ2YaEQWoVE5ZzRzzz2A lSg5p4stvb3eFmwiaADNk+wNfUJ/QbF67gqJowQTAQ+JAgHWbYza83XTiqoRQKKqkH HNeqrs7dTvAhA== Message-ID: <06d48a52-e4ec-47cd-b3fb-0fccd4dc49f4@kernel.org> Date: Thu, 5 Feb 2026 14:25:04 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table To: Lance Yang , Peter Zijlstra Cc: Liam.Howlett@oracle.com, akpm@linux-foundation.org, aneesh.kumar@kernel.org, arnd@arndb.de, baohua@kernel.org, baolin.wang@linux.alibaba.com, boris.ostrovsky@oracle.com, bp@alien8.de, dave.hansen@intel.com, dave.hansen@linux.intel.com, dev.jain@arm.com, hpa@zytor.com, hughd@google.com, ioworker0@gmail.com, jannh@google.com, jgross@suse.com, kvm@vger.kernel.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, mingo@redhat.com, npache@redhat.com, npiggin@gmail.com, pbonzini@redhat.com, riel@surriel.com, ryan.roberts@arm.com, seanjc@google.com, shy828301@gmail.com, tglx@linutronix.de, virtualization@lists.linux.dev, will@kernel.org, x86@kernel.org, ypodemsk@redhat.com, ziy@nvidia.com References: <20260202095414.GE2995752@noisy.programming.kicks-ass.net> <20260202110329.74397-1-lance.yang@linux.dev> <20260202125030.GB1395266@noisy.programming.kicks-ass.net> <4700e7ba-8456-4a93-9e28-7e5a3ca2a1be@linux.dev> <20260202133713.GF1395266@noisy.programming.kicks-ass.net> <540adec9-c483-460a-a682-f2076cf015c2@linux.dev> <20260202150957.GD1282955@noisy.programming.kicks-ass.net> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam12 X-Stat-Signature: 85fqs4o6h44koqegu4wyimh9ekpnziop X-Rspamd-Queue-Id: B8C3FC000C X-Rspam-User: X-HE-Tag: 1770297916-706751 X-HE-Meta: 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 hniSsfrd PZFLfC7JNYkZ5kcyYYHkOlNIOE3HAAxTe0KaSegv0Thfhe3bs7rY9rvFuZbzDnpC/HZfVqbAWW1aUvW4MlCizELeUcC1vOMGzpnPnMOHvN6fjAvOQq+J8s7+vf/hmnS8mwpm0cCMXjDjvTUHWc6Av8nlRnBDnxu1kNBRSDQB0oj7apPEPfLs/2VU92MKpa8EW8ODG94sqE4G7scB4s/vhH8LwCOf+U1FtLBTZj1tr1fcqDgTvr+WCrvA17JtoKF1+1ghd9arry3KvDdBRNgYCINc/kl7eA9vdFBBk 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 2/2/26 16:52, Lance Yang wrote: > > > On 2026/2/2 23:09, Peter Zijlstra wrote: >> On Mon, Feb 02, 2026 at 10:37:39PM +0800, Lance Yang wrote: >>> >>> >>> >>> PT_RECLAIM=y does have IPI for unshare/collapse — those paths call >>> tlb_flush_unshared_tables() (for hugetlb unshare) and >>> collapse_huge_page() >>> (in khugepaged collapse), which already send IPIs today (broadcast to >>> all >>> CPUs via tlb_remove_table_sync_one()). >>> >>> What PT_RECLAIM=y doesn't need IPI for is table freeing ( >>> __tlb_remove_table_one() uses call_rcu() instead). But table >>> modification >>> (unshare, collapse) still needs IPI to synchronize with lockless >>> walkers, >>> regardless of PT_RECLAIM. >>> >>> So PT_RECLAIM=y is not broken; it already has IPI where needed. This >>> series >>> just makes those IPIs targeted instead of broadcast. Does that clarify? >> >> Oh bah, reading is hard. I had missed they had more table_sync_one() >> calls, >> rather than remove_table_one(). >> >> So you *can* replace table_sync_one() with rcu_sync(), that will provide >> the same guarantees. Its just a 'little' bit slower on the update side, >> but does not incur the read side cost. > > Yep, we could replace the IPI with synchronize_rcu() on the sync side: > > - Currently: TLB flush → send IPI → wait for walkers to finish > - With synchronize_rcu(): TLB flush → synchronize_rcu() -> waits for > grace period > > Lockless walkers (e.g. GUP-fast) use local_irq_disable(); > synchronize_rcu() also > waits for regions with preemption/interrupts disabled, so it should > work, IIUC. > > And then, the trade-off would be: > - Read side: zero cost (no per-CPU tracking) > - Write side: wait for RCU grace period (potentially slower) > > For collapse/unshare, that write-side latency might be acceptable :) > > @David, what do you think? Given that we just fixed the write-side latency from breaking Oracle's databases completely, we have to be a bit careful here :) The thing is: on many x86 configs we don't need *any* TLB flushed or RCU syncs. So "how much slower" are we talking about, especially on bigger/loaded systems? -- Cheers, David