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imf05.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b=IEGzDoIL; spf=pass (imf05.hostedemail.com: domain of lance.yang@linux.dev designates 91.218.175.174 as permitted sender) smtp.mailfrom=lance.yang@linux.dev; dmarc=pass (policy=none) header.from=linux.dev ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1770305314; a=rsa-sha256; cv=none; b=vjE9CiqU/4ztCDvFMX8xe1BldkoCsSQ1gBQlAnlK8Sj5721vQneJlDiYsFTVjI8maXYtuB eN5ExRjZDrXIu9OoXAUQsbwqL+Spl6aAyBxeNGuvKb7ZxN1a/F4vWhwmzlPvt62cs6flSK ZxRibW7Z28Y3SZIvoV0BA/y3YIXdv3w= Message-ID: <8f5f7617-7a35-4cb2-97b2-0a73c6d9622c@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1770305311; 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=/SL5gP8QX8DLMYTu036KBvNCXId/b3s1citx/YK+obQ=; b=IEGzDoILmn6RaziZQkRjyJvvzWlaFO58iWze8rY0jh1kOJLRhJO6HZpFljbyPC+X0L84Av Cj3u8KxpQa88wJ2ZLpjRYTRBp6+7l5b63JFvgtpcdm+Vg8KJhKoywh+RxTy+/vns8wVrSm I/WRZ4sW+DSRbVjCNuvHzj//KzAkgjE= Date: Thu, 5 Feb 2026 23:28:14 +0800 MIME-Version: 1.0 Subject: Re: [PATCH v4 0/3] targeted TLB sync IPIs for lockless page table Content-Language: en-US To: "David Hildenbrand (Arm)" , 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> <13205d28-7478-4578-8bf3-ead18b4dc6b0@kernel.org> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: <13205d28-7478-4578-8bf3-ead18b4dc6b0@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Stat-Signature: zms6gng1wfi41r8dnxgezpsf4c4u6oen X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 9C936100012 X-HE-Tag: 1770305314-205659 X-HE-Meta: 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 5ywV2ndY +FCoRgL9jSPxcojhVvb0Cse/v/Cmjr34P4LDC/4y6692oTXp/IJUZgNhnCL2pVEMMbUoFCmOCPoRZo8AOVmmGVFp1tRmr+oedjj7D86UkzskFLhUX2ghwciTKrytqADjKBTwzrRfa1ywo8AMqGmH4nRU1r6gOFvdwY1YHYtfxvB17/wJMcWGmaxbI5gGZF3VzqMV2PbOWUWsP/NPr0KSuSqQlcQ6FQ18Ik1OmQ2pXrgp639I= 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 2026/2/5 23:05, David Hildenbrand (Arm) wrote: > 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? :) Yes, GUP-fast is ~3%. I'll keep trying to do that, but first getting the low-hanging fruit done :)