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McKenney" , Boqun Feng , Jonathan Corbet , Prakash Sangappa , Madadi Vineeth Reddy , K Prateek Nayak , Steven Rostedt , Sebastian Andrzej Siewior , Arnd Bergmann , linux-arch@vger.kernel.org, Randy Dunlap , Ron Geva , Waiman Long , "carlos@redhat.com" , Michael Jeanson References: <20251215155615.870031952@linutronix.de> <20251215155708.669472597@linutronix.de> <87jyyjbclh.ffs@tglx> <225b9868-4ab7-4a90-8acb-8d965626f6a7@efficios.com> <87ldi4gjm3.ffs@tglx> <45fd706a-86be-42b8-879e-11bbe262e159@efficios.com> <20260119102138.GQ830755@noisy.programming.kicks-ass.net> <5d2f428d-4fdc-486d-90e1-474f3ee9f54f@efficios.com> <20260119110300.GR830755@noisy.programming.kicks-ass.net> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: <20260119110300.GR830755@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: GVX0EPF00014AEE.SWEP280.PROD.OUTLOOK.COM (2603:10a6:158:401::318) To YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:be::5) Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: YT2PR01MB9175:EE_|YT2PR01MB11256:EE_ X-MS-Office365-Filtering-Correlation-Id: ac563362-fe97-4132-3924-08de574b5ccb X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|7416014|1800799024|366016; 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I recall that doing an additional rseq system call at thread >>>> creation was analyzed thoroughly for performance regressions at the >>>> libc level. I would not want to start requiring libc to issue a >>>> handful of additional prctl system calls per thread creation for no good >>>> reason. >>> >>> A wee something like so? >>> >>> That would allow registering rseq with RSEQ_FLAG_SLICE_EXT_DEFAULT_ON >>> set and if all the stars align, it will then have it on at the end. >> >> That's a very good step in the right direction. I just wonder how >> userspace is expected to learn that it runs on a kernel which >> accepts the RSEQ_FLAG_SLICE_EXT_DEFAULT_ON flag ? >> >> I think it could expect it when getauxval for AT_RSEQ_FEATURE_SIZE >> includes the slice ext field. This gives us a cheap way to know >> from userspace whether this new flag is supported or not. > > struct rseq vs struct rseq_data. I don't think that slice field is > exposed on the user side of things. Yes it is. (unless I'm missing something ?) See the original patch of this thread at https://lore.kernel.org/lkml/20251215155708.669472597@linutronix.de/ --- a/include/uapi/linux/rseq.h +++ b/include/uapi/linux/rseq.h [...] @@ -142,6 +174,12 @@ struct rseq { __u32 mm_cid; /* + * Time slice extension control structure. CPU local updates from + * kernel and user space. + */ + struct rseq_slice_ctrl slice_ctrl; + + /* * Flexible array member at end of structure, after last feature field. */ char end[]; > > I was thinking it could just try with the flag the firs time, and then > record if that worked or not and use the 'correct' value for all future > rseq calls. That would work too, but would waste a system call on process startup in case of failure. Not a big deal, but checking with getauxval would be better because this information is already exported to userspace at program execution and available without doing any system call. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com