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Mon, 20 Jan 2025 06:14:51 -0800 (PST) Received: from [132.69.243.144] ([132.69.243.144]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-ab384d2d3dcsm615041866b.81.2025.01.20.06.14.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 20 Jan 2025 06:14:51 -0800 (PST) Message-ID: <2975f6a4-58ef-4499-b177-4b73ead4c670@gmail.com> Date: Mon, 20 Jan 2025 16:14:49 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 10/12] x86,tlb: do targeted broadcast flushing from tlbbatch code To: Rik van Riel , x86@kernel.org Cc: linux-kernel@vger.kernel.org, bp@alien8.de, peterz@infradead.org, dave.hansen@linux.intel.com, zhengqi.arch@bytedance.com, thomas.lendacky@amd.com, kernel-team@meta.com, linux-mm@kvack.org, akpm@linux-foundation.org, jannh@google.com, mhklinux@outlook.com, andrew.cooper3@citrix.com References: <20250116023127.1531583-1-riel@surriel.com> <20250116023127.1531583-11-riel@surriel.com> <13bc0c49-09a4-434e-bd35-1ea50be38e25@gmail.com> <4dcf2b4ecaede883e2c7f6af3db58a4f6afaf4ad.camel@surriel.com> Content-Language: en-US From: Nadav Amit Autocrypt: addr=nadav.amit@gmail.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 20/01/2025 16:02, Rik van Riel wrote: > On Mon, 2025-01-20 at 11:56 +0200, Nadav Amit wrote: >> >>> @@ -1670,12 +1668,62 @@ void arch_tlbbatch_flush(struct >>> arch_tlbflush_unmap_batch *batch) >>>    local_irq_enable(); >>>    } >>> >>> + /* >>> + * If we issued (asynchronous) INVLPGB flushes, wait for >>> them here. >>> + * The cpumask above contains only CPUs that were running >>> tasks >>> + * not using broadcast TLB flushing. >>> + */ >>> + if (cpu_feature_enabled(X86_FEATURE_INVLPGB) && batch- >>>> used_invlpgb) { >>> + tlbsync(); >>> + migrate_enable(); >> >> Maybe someone mentioned it before, but I would emphasize that I do >> not >> think that preventing migration for potentially long time is that >> great. >> >> One alternative solution would be to set a bit on cpu_tlbstate, that >> when set, you'd issue a tlbsync on context switch. >> >> (I can think about other solutions, but I think the one I just >> mentioned >> is the cleanest one). > > It is clean, but I'm not convinced it is good enough. > > We need to guarantee that the INVLPGBs have finished > before we free the pages. > > Running a TLBSYNC at the next context switch could > mean that TLBSYNC won't run until after the pages > have been freed. > > In practice it is probably good enough, since it > would be simpler for TLBSYNC to return once all > pending (older) INVLPGBs have finished, but it's > not architecturally guaranteed. > > We could send an IPI to remote CPUs in order for > them to call TLBSYNC, but is that really better? > I am not sure we are on the same page. What I suggested is: 1. arch_tlbbatch_flush() would still do tlbsync() 2. No migrate_enable() in arch_tlbbatch_flush() 3. No migrate_disable() in arch_tlbbatch_add_pending() 4. arch_tlbbatch_add_pending() sets cpu_tlbstate.pending_tlb_broadcast 5. switch_mm_irqs_off() checks cpu_tlbstate.pending_tlb_broadcast and if it is set performs tlbsync and clears it.