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Mon, 16 Dec 2024 13:09:20 +0000 (UTC) Received: from gmonaco-thinkpadt14gen3.rmtit.com (unknown [10.39.193.102]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 665EA1955F43; Mon, 16 Dec 2024 13:09:17 +0000 (UTC) From: Gabriele Monaco To: Mathieu Desnoyers , Peter Zijlstra , Ingo Molnar , linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: Juri Lelli , Marco Elver , Gabriele Monaco Subject: [PATCH v3 1/3] sched: Compact RSEQ concurrency IDs with reduced threads and affinity Date: Mon, 16 Dec 2024 14:09:07 +0100 Message-ID: <20241216130909.240042-2-gmonaco@redhat.com> In-Reply-To: <20241216130909.240042-1-gmonaco@redhat.com> References: <20241216130909.240042-1-gmonaco@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 0D6D11C0003 X-Stat-Signature: mu7gxbxqiqcigphexhhm6t7kgstyqkab X-Rspam-User: X-HE-Tag: 1734354536-787009 X-HE-Meta: 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 JiSqCM0/ vKNGVpZ1VYMJAmw7FTWNXtIrJaPQCJh+YAPvEXCZL2phVwkv2mJEtVLtaRLNdiQMgPlyNxUoljp2tE5ztOqSk4PM069cmV4bNjfhPbbuTzkBc1QLy3Kqjl7PvdwGiQjYlc0p3iKqPQifDwSPwMYWByzOiuOmAToQ2g+4AyA1wmsApmfPRmE+wpyvzYYyLy5PnDTNrOXbsJYN7ed7f0KrrNjZhQLXsFYgB/5o2s5TN8+ooFkhMoC9ieJw2x3OFLyTRWZShgMTRVfEeriWcO9OIlWY+ns/itZR8y3NhTLdd8YbnMI4s1TMwUrMyQyFBsC/No06ZtafVGqJ/o4BPVKOiUYFr4Rqxtss9d5foBpA5oIO+TxES0ZXAKok+P3FWZjaX5Xorh9OnvobeVFktxu/MvaMFHNU6ITtu2Va7BjybEteCOce9Ko9wybsjBb15I0NDubtITj6y/mF/UhigZCis8B7m7lxXTlP3uy7RpMr8lEPR4II= 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 7361a8f3ab68..d56948a74254 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -843,10 +843,11 @@ 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 76f5f53a645f..b50dcd908702 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -3657,10 +3657,28 @@ 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 @@ -3668,8 +3686,9 @@ 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))