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McKenney" , rcu@vger.kernel.org, linux-kernel@vger.kernel.org Cc: kernel-team@meta.com, Boqun Feng , Steven Rostedt , lkmm@lists.linux.dev, Zqiang , Wang Lian , Kunwu Chan , Bradley Morgan , Bradley Morgan References: <20260919000056.3132131-26-paulmck@kernel.org> <51044724-7c3d-4200-b0d3-540a31d95804@efficios.com> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: YT4PR01CA0494.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:10c::25) To YT6PR01MB491026.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:1d0::10) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: YT6PR01MB491026:EE_|YT3PR01MB9692:EE_ X-MS-Office365-Filtering-Correlation-Id: 8db4d07d-8dc8-432e-086f-08df1b39f337 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|23010399003|366016|376014|7416014|4143699003|10067099003|56012099006|18002099003|22082099003; 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McKenney wrote: >>>> From: Mathieu Desnoyers >>>> >>>> Implement a two-phase wildcard scan to guarantee forward progress of >>>> synchronize_hazptr() even if there is a steady stream of ill-timed >>>> readers which populate wildcards into per-CPU slots. >>> >>> Hmm, I am not sure that I understand the problem here. The per-CPU slot is >>> scanned only once per CPU, and patch 1 already introduces flipping of the >>> overflow list. What prevents the forward progress? >> >> A steady stream of readers acquiring and releasing various hazard >> pointers happening concurrently with the percpu slots checks, being >> unlucky enough that each of the slot is constantly in a "wildcard" >> state, thus preventing forward progress of the synchronize, just with >> a steady stream of individually time-bound readers. > > Oh, so the issue is that we cannot progress over a single slot, because with > ill-timing a new iteration of the inner loop of "smp_cond_load_acquire" could > see a new reader while waiting for the slot to be released? Correct. > > So, in essence, the flipping is used to prevent ABA problem on percpu slots? Yes, specifically an ABA which could theoretically prevent forward progress of synchronize given a steady flow of hazptr acquire/release. > >> >>> >>> I think having a shared global read by all CPUs sounds really undesirable, >>> especially that it gets flipped for each hazptr_synchronize -- this means that >>> in the pathological case where there are a steady stream of hazptr_synchronize >>> calls, each fast-path hazptr_acquire will have a cache miss reading >>> hazptr_wildcard. >> >> There is a straightforward optimization we can do if this happen to >> cause performance issues: only do the flip when the synchronize >> encounters a wildcard retry delay beyond a specified threshold. >> So we ensure synchronize observe the absence of both wildcard >> values in each cpu slots, and only flip the current wildcard on retry >> delay. > > Another option would be avoid using WILDCARD if possible. IIRC the wildcard is > used to ensure forward progress on the reader side, so it avoids the possibility > of READ_ONCE(*addr_p) changing before and after protecting. Using the wildcard has a few benefits: 1) Prevents this retry loop on the read-side. 2) Prevents comparison of a loaded pointer value against a re-load of that value, which causes issues with compiler optimizations (I did a ptr_eq() patch in a prior version of the hazard pointer patches to handle this). It does have a downside though: given a very long preemption by a host VM, the guest VM could technically keep a wildcard present for a long time in a per-cpu slot, which would prevent hazptr synchronize from progressing for a long time in the guest VM kernel. > > One option would be to first use the typical hazard pointer impl that read the > pointer twice, and when that fails, use the wildcard protection. This would mean > that in the common case where the hazptr_acquire does not race with a pointer > update, the WILDCARD protection is not used at all. So your idea is to use the hazptr load+reload approach (with ptr_eq() check preventing the compiler from removing the dependency on the second load), but rather than retry, fallback to the two-phases wildcard. This way, we get the best of both worlds: guaranteed progress for the read-side (with the wildcard fallback), and typically we are immune to long-host-VM preemption delays, because the wildcard fallback would almost never fire. I like it. What do you guys think ? Thanks, Mathieu > > Best, > Gary > >> >> This would prevent the common "frequent" synchronize case you >> mention from causing cache misses on the read-side. >> >> But I preferred to keep it simple and wait until we hit this >> level of synchronize call throughput until adding that extra >> complexity. >> >> Thanks, >> >> Mathieu > -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com