From nobody Tue Dec 23 16:24:43 2025 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 DF19D232378 for ; Mon, 13 Jan 2025 07:42:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1736754179; cv=none; b=mjg9b8YjxzhqYcVltI4euIXJI0GqELgGfQGHJ8mfq/nzchAQLzu18yFeF9ndacCDiemMOa+DMUvDazV90znCe9fSnkSi7+4R2NA3AQEt7FtWpk0F2UewjPUA8g6WxJl+XTItqHDZZnSJjXkwW4ORZVeA89V5pwMmwGcu/5BmSnM= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1736754179; c=relaxed/simple; bh=eM7V3bNyBIptHHYXlzwgnAIte13dhG+zHJmtV3j1mmU=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=YUpe+BtJBlmhjD50sq1R35bMmjcicOnbQYAcfw1ZjTZr/KKUpkrQVnyLGrHFze4crKuW3Wxl/2m4TgFklyEZ8rSn3Xqx/eJjlIJB07RfW7vYpsMiOqmRCZvii0pjfyJCaP28lc+srDVMY2FfpSPJBJrKn3tgkDbVkU4g6GDZuxU= 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=Kaj+t23t; arc=none smtp.client-ip=170.10.133.124 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="Kaj+t23t" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1736754176; 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=GJeI+pmaW4qowU1ejojWx0fMypXcCKUplYgl81NpP58=; b=Kaj+t23twOmkjSfJJykID0I9kK+JQD3pcrjc1+ylz99YWRkUz/Vw9TvjrjPOwVGdyRvSrI zbJ/UeG8dHOsUE7L61xp7I0w9mOraaVbVIxjr4u3aSBbDiOziM8/Oy1cdP2CihJNx4YjbC PUfqYLJL9R/izu/aY5eeNxo7spxPSZQ= Received: from mx-prod-mc-01.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-450-zE0BzDsWNwaOxeq0QooVgw-1; Mon, 13 Jan 2025 02:42:53 -0500 X-MC-Unique: zE0BzDsWNwaOxeq0QooVgw-1 X-Mimecast-MFC-AGG-ID: zE0BzDsWNwaOxeq0QooVgw Received: from mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.15]) (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-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 0D15519560B3; 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> 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 3.0 on 10.30.177.15 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 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_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 @@ -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 =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 2.47.1