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Mon, 13 Jan 2025 07:42:52 +0000 (UTC) Received: from gmonaco-thinkpadt14gen3.rmtit.com (unknown [10.39.192.91]) by mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id D5FB2195608A; Mon, 13 Jan 2025 07:42:48 +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 v4 1/3] sched: Compact RSEQ concurrency IDs with reduced threads and affinity Date: Mon, 13 Jan 2025 08:42:29 +0100 Message-ID: <20250113074231.61638-2-gmonaco@redhat.com> In-Reply-To: <20250113074231.61638-1-gmonaco@redhat.com> References: <20250113074231.61638-1-gmonaco@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.0 on 10.30.177.15 X-Rspamd-Queue-Id: 64D65100007 X-Rspamd-Server: rspam12 X-Stat-Signature: 5ufdduzcefbi1bijweb1y3qszbubbo6b X-Rspam-User: X-HE-Tag: 1736754178-6368 X-HE-Meta: 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 JZw1dMUb z1Fyr/Fy10d+llFUN3dmjJdTR9t8JpY0o7hLnCAIlv0ZTnkF/n7X4JvVkgIm+1+4/Zw7TqLMeZNsSbmX5wM1Z+MPofuwlvsosxsmfdjqCeLl87Tx2UFhrjSYeqDaYhnAslpqgOWmqwdIxCXSaZw53OnrIJFOwFvQsO8ChqlAItyvRp0Os7Lv8ZVHx5CTfjbcQGsek+2dTH1xevV/ioHUkmKVxmY+zdc8W6IuVtD9IVLHsOIBSfDRPxEmraoxF41CM2u6ggGFVrxru7E1rZcCnGq07sjS9XhcreVfKODbElBxv/XC5UaQ3MwYNowBEP9Gl8ezqlLE2a9eOKlnNbm1tKjy8q0sgsGMwL2r1UJQW8WZvy4jcZ1N7vnnBTFybxROPsnKTmHl87U1CNZ/Ky/SG6ZxZr0Ct4f12BZL8cA9qP85uMWb4esDLtPcqHRAWhRUYUjhA/i9UFIkQAeXLMWxsYS2SF8lUG2xubC4i06igEel+ASQ= 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 332cee2856620..14fc1b39c0cf3 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -873,10 +873,11 @@ 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 c5d67a43fe524..13db32370a381 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -3666,10 +3666,28 @@ 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 @@ -3677,8 +3695,9 @@ 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))