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Fri, 13 Dec 2024 09:55:35 +0000 (UTC) Received: from gmonaco-thinkpadt14gen3.rmtit.com (unknown [10.39.192.43]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 9C471195394B; Fri, 13 Dec 2024 09:55:30 +0000 (UTC) From: Gabriele Monaco To: Mathieu Desnoyers , Ingo Molnar , Peter Zijlstra , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: Juri Lelli , Vincent Guittot , Mel Gorman , Shuah Khan , linux-kselftest@vger.kernel.org, Marco Elver , Ingo Molnar , Gabriele Monaco Subject: [PATCH v2 3/4] sched: Compact RSEQ concurrency IDs with reduced threads and affinity Date: Fri, 13 Dec 2024 10:54:06 +0100 Message-ID: <20241213095407.271357-4-gmonaco@redhat.com> In-Reply-To: <20241213095407.271357-1-gmonaco@redhat.com> References: <20241213095407.271357-1-gmonaco@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 279C1C0008 X-Stat-Signature: 6msd6ao9z48xrmpwfwnnbtjo4tsy1ag1 X-Rspam-User: X-HE-Tag: 1734083729-719833 X-HE-Meta: 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 IqERyhUU 03wPO7BaQ0/hTMWTVKu+be9Pz0ugo3qtzRsi8rmqrcQqX/ZF2yswgonlynvQDQ4YrWyWgIKinUhllOr942Adg5Al13I5FQDaqtoUixXDPJI840UytEX6WmA7X4Lui+ZMcMBX1XNkE7gjpfnDGOGWjcNd+M38y2oNct3/+aL9XnhsbbocJjv5iyPWiFIE3ckjL2to1woA2IwNXlptSDW0x5MfwnSCBmdLvGgIDRkvLY6mRaD8BLnpdhCvglVRUhO1P8xnyydxD4Wbh8pB3tlsxleOszt/53mQScPNz6RKKrH8bo+Q7oPi6/S0s2+ILPWatkKGTJZRRFmjlmtIn8YMfgpP2JP50ef4LET45+8pVRNCZzZazu9ILj9ZIikScPSOBBsIwlb9vpMGhG84PWUm5rWesacMg5G47AtKrzb6sUDzsBtBj/z2S9nf8BDkW7v+NFR5V3a5vN36kd9C7u+ki4K/7NmSpGIeHYF8U95/gHbCkPRc= 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 8a76a1c09234..16076e70a6b9 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -837,10 +837,11 @@ 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 21be461ff913..f3b0d1d86622 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -3652,10 +3652,28 @@ 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 @@ -3663,8 +3681,9 @@ 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))