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Mon, 10 Feb 2025 07:57:14 +0000 (UTC) Received: from gmonaco-thinkpadt14gen3.rmtit.com (unknown [10.44.32.69]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 861531800358; Mon, 10 Feb 2025 07:57:10 +0000 (UTC) From: Gabriele Monaco To: Mathieu Desnoyers , Andrew Morton , Ingo Molnar , Peter Zijlstra , linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: Marco Elver , Ingo Molnar , Gabriele Monaco Subject: [PATCH v5 1/3] sched: Compact RSEQ concurrency IDs with reduced threads and affinity Date: Mon, 10 Feb 2025 08:57:00 +0100 Message-ID: <20250210075703.79125-2-gmonaco@redhat.com> In-Reply-To: <20250210075703.79125-1-gmonaco@redhat.com> References: <20250210075703.79125-1-gmonaco@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 8E8571C0003 X-Stat-Signature: roi1pnpna1w7e6xmw4pkjywnd6mgyhxo X-Rspam-User: X-HE-Tag: 1739174241-180962 X-HE-Meta: U2FsdGVkX1+oV1YTAgFjL+aomeQLnHzhDGNKVU25J+nh4kAtmPNHk/zZF3YkQ+PzG5/XpGUFjrrEQHEuWKT2Ri6GyvAV91+D4n8cp5QAMWyO8PLLE6vG31x3XnCfDdGn0rbrunGvxdOCuj0c5TIoFUXqBQBTrE4hwX6C+9nVx2SOHif9hQZ1AG/qycrx05GOanUjqbaEdee9I2s578+h0kfCjxHQWbPOqtAiD/38mzON3n1q0aEuPxeqyKjaeYm3JMt8Ixl6s6xctvC7qOXmDQ68EabbeSgRRMrPGzT018JLKc81efi2nDStvtkYZCV3KtLL16RinnSO5/z4dGtIJBJW4BSxJwxM27KGJnizz1hWK/7OxtDfj/IVjwParA6yeaejWkIjm3LrCRl1WaA+bNCzVwt/A+G2Nc/ubUx2LyiXwPdj02ulWwYc+6URGExvQfLxNRPiy6sAS0uq/R7oJGpmnwix2E2rbUNxTfCdWqfBUJG2v3gJUSlJ/yVaNTsTqpz7R5zHOQXb9YL2FerucbaDAWGdbGRu0dQttEE+ubYfHa0i/ODA08eE15aJBL6/D0hwpsudpwhCyZD9UCsQwWSmoJxxQ6qikYv0lAAor/Bo7grsLaMdGaPEhdT1xPprTPCZttrngG2G+VUkW9Q1jLkEaDa+lyUZMODo3qv+vktCxZDYTDf2vvvCN7D/JUeHbetilRg36mPCuLtsJ5MVDv3yAiDm/0zRxqdL0X8XB/3ddjNov421vuyHXVxgrcq5ypjwOaDWY9ZYQ8PVSvhjdlAVw65ACB1w+W85k7+0vJOcrtDUHvPrinXxVy50oX8mEYGPHRSorvNKzKsgpa3HsLJWY4bKGo2q+PaIWp4Wvw2oxKJgyWYaR7l7oXs4AmtRs1qZ+Vk02n0ljSKYja4rTq2ylC9/F7l0BbMsmdat5yPxHgZdtE9sB2KOairJqtaC4zsZAdhuhU6CguCRxvo Id35yfbV AYJwDjxba60700rcOHwKPTjtm6RLvQfC6KobD7aVIxvkC8AzOhmxccpp09UkhHUEqxJVdin/HS4u4WvexSTN0dAEKiA4z3D0lfw02cZQzTmOXpk/gwdMG8OM5+RgkLYo6VOOStHjolSAvIJ7ic29yzQqVW9WfTZRZc2hZALFwqROrZ0epjiIJCC0oeXCO/Ac3ftJWBoxLVjDI2ILUP0BotM94frjZfcVkxt8sMj4dt1u6Aq5vdelJ1B3t4DNgcqDoxkBZ/GPYLanAB34OQlLIdQlbcm9KOcwpiUdd4BUpaVdZuOTHrWI6YWfBueqS1ghRh13U+ffF8V7YhIh3LtePYXG37nxpyXVdZfttRvS4CSfnXm+rXSxHM1elj3PF5r9bQHQFcdSxQ+KsjxZtZ570CKS5chCiIgw+QAiGeflqfnglaArTylaRJU3JOAu/tm9V70MrI3fFDGDRjru/J1x8PbcWDC6fRUihARu3u7D+0sgjlAI= 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: From: Mathieu Desnoyers When a process reduces its number of threads or clears bits in its CPU affinity mask, the mm_cid allocation should eventually converge towards smaller values. However, the change introduced by: commit 7e019dcc470f ("sched: Improve cache locality of RSEQ concurrency IDs for intermittent workloads") adds a per-mm/CPU recent_cid which is never unset unless a thread migrates. This is a tradeoff between: A) Preserving cache locality after a transition from many threads to few threads, or after reducing the hamming weight of the allowed CPU mask. B) Making the mm_cid upper bounds wrt nr threads and allowed CPU mask easy to document and understand. C) Allowing applications to eventually react to mm_cid compaction after reduction of the nr threads or allowed CPU mask, making the tracking of mm_cid compaction easier by shrinking it back towards 0 or not. D) Making sure applications that periodically reduce and then increase again the nr threads or allowed CPU mask still benefit from good cache locality with mm_cid. Introduce the following changes: * After shrinking the number of threads or reducing the number of allowed CPUs, reduce the value of max_nr_cid so expansion of CID allocation will preserve cache locality if the number of threads or allowed CPUs increase again. * Only re-use a recent_cid if it is within the max_nr_cid upper bound, else find the first available CID. Fixes: 7e019dcc470f ("sched: Improve cache locality of RSEQ concurrency IDs for intermittent workloads") Cc: Peter Zijlstra (Intel) Cc: Marco Elver Cc: Ingo Molnar Tested-by: Gabriele Monaco Signed-off-by: Mathieu Desnoyers Signed-off-by: Gabriele Monaco --- include/linux/mm_types.h | 7 ++++--- kernel/sched/sched.h | 25 ++++++++++++++++++++++--- 2 files changed, 26 insertions(+), 6 deletions(-) diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 6b27db7f94963..0234f14f2aa6b 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -875,10 +875,11 @@ struct mm_struct { */ unsigned int nr_cpus_allowed; /** - * @max_nr_cid: Maximum number of concurrency IDs allocated. + * @max_nr_cid: Maximum number of allowed concurrency + * IDs allocated. * - * Track the highest number of concurrency IDs allocated for the - * mm. + * Track the highest number of allowed concurrency IDs + * allocated for the mm. */ atomic_t max_nr_cid; /** diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 38e0e323dda26..606c96b74ebfa 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -3698,10 +3698,28 @@ static inline int __mm_cid_try_get(struct task_struct *t, struct mm_struct *mm) { struct cpumask *cidmask = mm_cidmask(mm); struct mm_cid __percpu *pcpu_cid = mm->pcpu_cid; - int cid = __this_cpu_read(pcpu_cid->recent_cid); + int cid, max_nr_cid, allowed_max_nr_cid; + /* + * After shrinking the number of threads or reducing the number + * of allowed cpus, reduce the value of max_nr_cid so expansion + * of cid allocation will preserve cache locality if the number + * of threads or allowed cpus increase again. + */ + max_nr_cid = atomic_read(&mm->max_nr_cid); + while ((allowed_max_nr_cid = min_t(int, READ_ONCE(mm->nr_cpus_allowed), + atomic_read(&mm->mm_users))), + max_nr_cid > allowed_max_nr_cid) { + /* atomic_try_cmpxchg loads previous mm->max_nr_cid into max_nr_cid. */ + if (atomic_try_cmpxchg(&mm->max_nr_cid, &max_nr_cid, allowed_max_nr_cid)) { + max_nr_cid = allowed_max_nr_cid; + break; + } + } /* Try to re-use recent cid. This improves cache locality. */ - if (!mm_cid_is_unset(cid) && !cpumask_test_and_set_cpu(cid, cidmask)) + cid = __this_cpu_read(pcpu_cid->recent_cid); + if (!mm_cid_is_unset(cid) && cid < max_nr_cid && + !cpumask_test_and_set_cpu(cid, cidmask)) return cid; /* * Expand cid allocation if the maximum number of concurrency @@ -3709,8 +3727,9 @@ static inline int __mm_cid_try_get(struct task_struct *t, struct mm_struct *mm) * and number of threads. Expanding cid allocation as much as * possible improves cache locality. */ - cid = atomic_read(&mm->max_nr_cid); + cid = max_nr_cid; while (cid < READ_ONCE(mm->nr_cpus_allowed) && cid < atomic_read(&mm->mm_users)) { + /* atomic_try_cmpxchg loads previous mm->max_nr_cid into cid. */ if (!atomic_try_cmpxchg(&mm->max_nr_cid, &cid, cid + 1)) continue; if (!cpumask_test_and_set_cpu(cid, cidmask))