From nobody Thu Dec 18 01:36:41 2025 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 791BF24BCE9 for ; Mon, 10 Feb 2025 15:33:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739201605; cv=none; b=ClN5Z3fMZpv3H0xNvLLYMh+8MpsqSERzAhG142fUhJ1B+Ssv3FlXXrMofEzKMJjpgz7AHhCeWd+JUtp/3KU/JSf7XiwKnKBdfMKYqIRObOhqUNcU4tIn4crCM9NmrjaL3jP3L7y5qrxOdRrk2x+Miqu+TZfNwuhSsJ+IZ5d+bQs= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739201605; c=relaxed/simple; bh=hXKMFjUxmniwLPmXl/zrhJcXY2dfco4gVE9XyWgbOhs=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=ei40++OPW4gTav4fJkH9e0QDdL5PL7+BT6/0Mcm6vdtf09Tt521gJN+uPzgI4Buo00XMDSNtIZZ3TNiv4T5JoHYk3NMqDPsOUbUc0yNG5GBd/RHRydwlqqNQfLYQEsfRFKW+fRJdnAK5b+HjF3Duf7nhmp0+ze9zAwVUw9a6J7E= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=AZcGtqO8; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="AZcGtqO8" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1739201602; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=5B84Awu6JUpstdhEM0yzA51Ej2LPUV/BX8OiM6ds56M=; b=AZcGtqO8LiLbCqFqYoP1gnQohLd+I8NzM7srrv15WS2iMYc++IKASArPQij/54ILIVOqFw i4Lmc8Ho6HH7sQcQfbJm0aiBn6TYs1VciGjm1VRWTRhoAKgd8CU4Fb4m6mwCo5U+2qhAuf EjN7hqtWuRpqLhOyJ5FKE2Q0voayQ8w= Received: from mx-prod-mc-04.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-377-b9b0u06MOzOGh5KpIVY7hQ-1; Mon, 10 Feb 2025 10:33:16 -0500 X-MC-Unique: b9b0u06MOzOGh5KpIVY7hQ-1 X-Mimecast-MFC-AGG-ID: b9b0u06MOzOGh5KpIVY7hQ Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id F40821955D5C; Mon, 10 Feb 2025 15:33:14 +0000 (UTC) Received: from gmonaco-thinkpadt14gen3.rmtit.com (unknown [10.44.34.65]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 73CDE18004A7; Mon, 10 Feb 2025 15:33:10 +0000 (UTC) From: Gabriele Monaco To: linux-kernel@vger.kernel.org, Andrew Morton , Ingo Molnar , Peter Zijlstra , linux-mm@kvack.org Cc: Mathieu Desnoyers , Marco Elver , Ingo Molnar , Gabriele Monaco , "Paul E. McKenney" , Shuah Khan Subject: [PATCH v6 1/3] sched: Compact RSEQ concurrency IDs with reduced threads and affinity Date: Mon, 10 Feb 2025 16:32:50 +0100 Message-ID: <20250210153253.460471-2-gmonaco@redhat.com> In-Reply-To: <20250210153253.460471-1-gmonaco@redhat.com> References: <20250210153253.460471-1-gmonaco@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 Content-Type: text/plain; charset="utf-8" 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_stru= ct *t, struct mm_struct *mm) { struct cpumask *cidmask =3D mm_cidmask(mm); struct mm_cid __percpu *pcpu_cid =3D mm->pcpu_cid; - int cid =3D __this_cpu_read(pcpu_cid->recent_cid); + int cid, max_nr_cid, allowed_max_nr_cid; =20 + /* + * 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 =3D atomic_read(&mm->max_nr_cid); + while ((allowed_max_nr_cid =3D 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 =3D 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 =3D __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 =3D atomic_read(&mm->max_nr_cid); + cid =3D 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)) --=20 2.48.1