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Wed, 9 Apr 2025 15:53:07 +0000 Message-ID: Date: Wed, 9 Apr 2025 11:53:05 -0400 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v12 2/3] sched: Move task_mm_cid_work to mm work_struct To: Peter Zijlstra Cc: Gabriele Monaco , linux-kernel@vger.kernel.org, Andrew Morton , Ingo Molnar , "Paul E. McKenney" , linux-mm@kvack.org, Ingo Molnar , Shuah Khan References: <20250311062849.72083-1-gmonaco@redhat.com> <20250311062849.72083-3-gmonaco@redhat.com> <20250409140303.GA9833@noisy.programming.kicks-ass.net> <20250409152025.GK9833@noisy.programming.kicks-ass.net> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: <20250409152025.GK9833@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: YQBPR01CA0084.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:c01:3::20) To YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:be::5) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: YT2PR01MB9175:EE_|YQBPR0101MB9199:EE_ X-MS-Office365-Filtering-Correlation-Id: 2a2b2a22-a2d3-444b-a9b6-08dd777e9ea4 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|366016|1800799024; X-Microsoft-Antispam-Message-Info: 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 y+qWVwIR 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 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 2025-04-09 11:20, Peter Zijlstra wrote: > On Wed, Apr 09, 2025 at 10:15:42AM -0400, Mathieu Desnoyers wrote: >> On 2025-04-09 10:03, Peter Zijlstra wrote: >>> On Tue, Mar 11, 2025 at 07:28:45AM +0100, Gabriele Monaco wrote: >>>> +static inline void rseq_preempt_from_tick(struct task_struct *t) >>>> +{ >>>> + u64 rtime = t->se.sum_exec_runtime - t->se.prev_sum_exec_runtime; >>>> + >>>> + if (rtime > RSEQ_UNPREEMPTED_THRESHOLD) >>>> + rseq_preempt(t); >>>> +} >>> >>> This confused me. >>> >>> The goal seems to be to tickle __rseq_handle_notify_resume() so it'll >>> end up queueing that work thing. But why do we want to set PREEMPT_BIT >>> here? >> >> In that scenario, we trigger (from tick) the fact that we may recompact the >> mm_cid, and thus need to update the rseq mm_cid field before returning to >> userspace. >> >> Changing the value of the mm_cid field while userspace is within a rseq >> critical section should abort the critical section, because the rseq >> critical section should be able to expect the mm_cid to be invariant >> for the whole c.s.. > > But, if we run that compaction in a worker, what guarantees the > compaction is done and mm_cid is stable, but the time this task returns > to userspace again? So let's say we have a task which is running and not preempted by any other task on a cpu for a long time. The idea is to have the tick do two things: A) trigger the mm_cid recompaction, B) trigger an update of the task's rseq->mm_cid field at some point after recompaction, so it can get a mm_cid value closer to 0. So in its current form this patch will indeed trigger rseq_preempt() for *every tick* after the task has run for more than 100ms, which I don't think is intended. This should be fixed. Also, doing just an rseq_preempt() is not the correct approach, as AFAIU it won't force the long running task to release the currently held mm_cid value. I think we need something that looks like the following based on the current patch: - rename rseq_preempt_from_tick() to rseq_tick(), - modify rseq_tick() to ensure it calls rseq_set_notify_resume(t) rather than rseq_preempt(). - modify rseq_tick() to ensure it only calls it once every RSEQ_UNPREEMPTED_THRESHOLD, rather than every tick after RSEQ_UNPREEMPTED_THRESHOLD. - modify rseq_tick() so at some point after the work has compacted mm_cids, we do the same things as switch_mm_cid() does, namely to release the currently held cid and get a likely smaller one (closer to 0). If the value changes, then we should trigger rseq_preempt() so the task updates the mm_cid field before returning to userspace and restarts ongoing rseq critical section. Thoughts ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com