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b=Nnxsi5HUZSnSL0xRsB5HIqzymZ2WU5Q/JBJ1FWvlgD8f9Lf/AVQex1Zc8QhGgF3DI R7lmPisE2yWGZnoq6iFZJDqbv5+mx0ONEx6i4GL+pcsC3YBx1mustgCqN1AwvF2d4C NTmEmcBL2COI1YAebiZ003vRsEnEGjtKX0EndcECnyp50BdIDHtPW9iTQzc210OsgR FrIfZT5QL6SPqN1QDGYyTQNzTHLZoUO8Wolg/RsAIIGoxBftIyhVrqKHtt8FA0pRQ9 9ZINY/D+Miq//aTKcVNhoPESi8R9eIKvURNUJmA0lPtJ0o/8y0TEXOx+kzwxWNy/zy w0o3UTmd7ykOw== Message-ID: <13205d28-7478-4578-8bf3-ead18b4dc6b0@kernel.org> Date: Thu, 5 Feb 2026 16:05:37 +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 , dave.hansen@intel.com 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@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> <06d48a52-e4ec-47cd-b3fb-0fccd4dc49f4@kernel.org> <3026ad8d-92ad-4683-8c3e-733d4070d033@linux.dev> 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: rspam09 X-Rspamd-Queue-Id: 428141A0007 X-Stat-Signature: udpudx8e3deym53se7sn5f9rmu116x3x X-Rspam-User: X-HE-Tag: 1770303951-414429 X-HE-Meta: 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 pXRHCwgq 6lMg3aimXCknT+Ht8hXodHM6eXJCNe2+W04VL1y2C+uvPGcpzrfVIk4TAgaWM85SqN1XBwuv1gwPlpqjUL+Ljj153os6lwVFU1HTXmMW/YIQTVs+Xlu9dnkk+b/oclc5JKQfW8EUenezsibcT/E228hB5Eqb4Y9ZGLdsU7TxcPtIKotK+13s2955gLPpiVH4MdiiYVXiso+OW4tPuHHbdpiggW/vCbUsHgCxds66E1sE/lJyJsuaPaxOb5ahJULhhnhd7wSDxgQuWy0EP719wIDUW589Xc7ASHec4 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 2/5/26 16:01, Lance Yang wrote: > > > On 2026/2/5 21:25, David Hildenbrand (Arm) wrote: >> On 2/2/26 16:52, Lance Yang wrote: >>> >>> >>> >>> 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 :) > > Yep, agreed. > >> >> The thing is: on many x86 configs we don't need *any* TLB flushed or >> RCU syncs. > > Right. Looks like that is low-hanging fruit. I'll send that out > separately :) > >> >> So "how much slower" are we talking about, especially on bigger/loaded >> systems? > > Unfortunately the numbers are pretry bad. On an x86-64 64-core system > under high load, each synchronize_rcu() is about *22.9* ms on average ... > > So for now, neither approach looks good: tracking on the read side adss > cost to GUP-fast, and syncing on the write side e.g. synchronize_rcu() > is too slow on large systems. GUP-fast is 3%, right? Any way we can reduce that to 1% and call it noise? :) -- Cheers, David