From nobody Sun Dec 7 15:05:23 2025 Received: from mail-io1-f53.google.com (mail-io1-f53.google.com [209.85.166.53]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A8C6B2236EE for ; Mon, 6 Oct 2025 17:04:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.166.53 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759770255; cv=none; b=ANy/vb8+nKaPUbK0g+aHR28H4Zda8zvBObBmgRBaMAzHNEvUNgqrmQIA83jHof3evneqPK0EL0yPbi4sU/AMZGf7chKoUi6wajeIcy0cFppIka5qiOZ41QNyTRvaUEWWwJaC+AlQca+YtwpZdr3l91WDSYf0+wrecxEtBTUjIkY= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759770255; c=relaxed/simple; bh=WSOhLCP47bTTI2G1Dx59qp5+Llml7tOV3z6rZZ0BA6M=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=I7Gahney4YgnHATdNlroW69Tejc7RihoPzJc8bIZHskEW//vUdxKPf3IqnW+6JN/h849rx22/zVv7ql+ZNdJUFXSnAXMxYeJLyqbUXl7q1q2OfSZRXbLzzCB4G1++9RBq3LR02kG7uObgW9RaZlgd7IOc5TKcQPx7M4k9u01Kv0= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=PGRc2JTz; arc=none smtp.client-ip=209.85.166.53 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="PGRc2JTz" Received: by mail-io1-f53.google.com with SMTP id ca18e2360f4ac-8ca2e53c37bso462176239f.3 for ; Mon, 06 Oct 2025 10:04:13 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1759770252; x=1760375052; darn=vger.kernel.org; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date :message-id:reply-to; bh=hP5lbLel6FeQQkI2Vhrf17nln8NYIgCIUzjg9SwJbmA=; b=PGRc2JTz4/uwn9Snt+morS+HXTKzi4ghcClcysJmClJsJQI0YI/i7AcdBqByvR7EgR 6+mw4O+4Ixk13SH0YZTVA/xjtM1Fx3Go8vG9xxcWJAgIqBC7Scq9yUzCZz+/7TLJX2uu ltSJ2eCJ0phfYWlcRI1kHVQ+N4colNNqI6yE9YiCs+KOTpCXRVVM0p3/6eusEzuA4zk9 fJdd2CANPSSPdupY4E+BCJMaW6ZfFrXNJ4DZQeVQtLlrCIpVHOSTKsVinNDoT8ye1BBZ LSbOWIhqUpF2xUOVhMVcw7fCw5MFmKeT5aFoxilLdnZ0GjFhzaMfawcGs87JduJxYoww aOJQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1759770252; x=1760375052; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=hP5lbLel6FeQQkI2Vhrf17nln8NYIgCIUzjg9SwJbmA=; b=nnWSYu1fMJ6/7qMIc4WGuI6y5yL60hv7JkDsgahxybf/WkT1mXm8uBlLmjuvDCU3jh l9DYVQpSve0rbQpEZ077fZsfYGhfpNFETpu4ZthOzoGb2nqqc8OTRvtRdh/Rve2dpGVg M+oW+5bpp6V0a5xVPJEiHHO1ESsgrfHvszGWjzmYRtIsYAcumQycr+WtgWHSf3jUDkpV nsFD3t17g2tMzUBkMtwTH0yuJCbCjO+pSVTsBoBnmTiih092MNpZq1ijuXjgX4nuSEy7 mdC5yxKYuPt/fD/fSGaN3r7lKgQB3cpav4ocP2YPEc+buAnJDKiJzMtA5IwD+ZQ5D04T fMdg== X-Gm-Message-State: AOJu0Yyi6gsL2gBIGE/yFFMALzh2GTjkdaUCIaz6utZRNd7/bv9Y2Ne2 jKPNkRY7k5cij5GO0LOlaMvuyu6qPQcd83wJk2ebMq9OsXU0uuYA1pE+HyUuBiNx X-Gm-Gg: ASbGncugOB0zKsgjPIfXSSpX9fk+xnTHAgY/Pn73w7i8VldJwG9ju9zB9hlUQafJJRn N2pzO+XherQIu4WH5ytJiRLwWsS94zqaaHCRPA1NdyTDGzsHHq2d2Rq03aiE6febYeuFBwrbm5o 1gPdZqzn2NtwxIp4t3nYx3PMtKiRog8D17CVQdvu/6sMX9+bIr/7MRFPwily4h8q/8PIpYb+r7C LCa8+AThOliROF5JSQF24kcapfRFJtj53p3wzldiSLSc/BWHkamt/qBzmD5STB/zxkFzr+9FrRd B6wJAepejdQJDpe2mJlqKJNLayAbEU+ljmkoi+zRu45gxzizFqOavTPlE+GcUqZyj35TqqCUV1i R5g1Zhc2VSIKto4NsLHpODJTLuXmnjIPFSH1vGRrcb+UoJFqXIVKvITm5H1mFSpUbbeShgKHOet MBybkBKnAtwLuezGg0pTf9rZbQpII6H55pUB9kk5E9tgMyqPXjI7tK2IJ0 X-Google-Smtp-Source: AGHT+IHtXrzkafzb/xJHKEdIZvSUMa4i5nvKxN/qFCoBRPJYFlRlsqk1i6a2OetdRygINP8sROrbkw== X-Received: by 2002:a05:6602:740e:b0:937:6516:62e9 with SMTP id ca18e2360f4ac-93b96a4fba2mr1685764439f.9.1759770251943; Mon, 06 Oct 2025 10:04:11 -0700 (PDT) Received: from newton-fedora-MZ01GC9H (c-68-45-22-229.hsd1.in.comcast.net. [68.45.22.229]) by smtp.gmail.com with ESMTPSA id ca18e2360f4ac-93a87bb84b4sm488589539f.18.2025.10.06.10.04.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 06 Oct 2025 10:04:11 -0700 (PDT) From: Ryan Newton To: linux-kernel@vger.kernel.org Cc: sched-ext@lists.linux.dev, tj@kernel.org, arighi@nvidia.com, rrnewton@gmail.com, newton@meta.com Subject: [PATCH v3 1/2] sched_ext: Add lockless peek operation for DSQs Date: Mon, 6 Oct 2025 13:04:02 -0400 Message-ID: <20251006170403.3584204-2-rrnewton@gmail.com> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20251006170403.3584204-1-rrnewton@gmail.com> References: <20251006170403.3584204-1-rrnewton@gmail.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 Content-Type: text/plain; charset="utf-8" From: Ryan Newton The builtin DSQ queue data structures are meant to be used by a wide range of different sched_ext schedulers with different demands on these data structures. They might be per-cpu with low-contention, or high-contention shared queues. Unfortunately, DSQs have a coarse-grained lock around the whole data structure. Without going all the way to a lock-free, more scalable implementation, a small step we can take to reduce lock contention is to allow a lockless, small-fixed-cost peek at the head of the queue. This change allows certain custom SCX schedulers to cheaply peek at queues, e.g. during load balancing, before locking them. But it represents a few extra memory operations to update the pointer each time the DSQ is modified, including a memory barrier on ARM so the write appears correctly ordered. This commit adds a first_task pointer field which is updated atomically when the DSQ is modified, and allows any thread to peek at the head of the queue without holding the lock. Signed-off-by: Ryan Newton Reviewed-by: Andrea Righi --- include/linux/sched/ext.h | 1 + kernel/sched/ext.c | 54 +++++++++++++++++++++++- tools/sched_ext/include/scx/common.bpf.h | 1 + tools/sched_ext/include/scx/compat.bpf.h | 19 +++++++++ 4 files changed, 73 insertions(+), 2 deletions(-) diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h index d82b7a9b0658..81478d4ae782 100644 --- a/include/linux/sched/ext.h +++ b/include/linux/sched/ext.h @@ -58,6 +58,7 @@ enum scx_dsq_id_flags { */ struct scx_dispatch_q { raw_spinlock_t lock; + struct task_struct __rcu *first_task; /* lockless peek at head */ struct list_head list; /* tasks in dispatch order */ struct rb_root priq; /* used to order by p->scx.dsq_vtime */ u32 nr; diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index 2b0e88206d07..6d3537e65001 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -944,8 +944,11 @@ static void dispatch_enqueue(struct scx_sched *sch, st= ruct scx_dispatch_q *dsq, container_of(rbp, struct task_struct, scx.dsq_priq); list_add(&p->scx.dsq_list.node, &prev->scx.dsq_list.node); + /* first task unchanged - no update needed */ } else { list_add(&p->scx.dsq_list.node, &dsq->list); + /* not builtin and new task is at head - use fastpath */ + rcu_assign_pointer(dsq->first_task, p); } } else { /* a FIFO DSQ shouldn't be using PRIQ enqueuing */ @@ -953,10 +956,19 @@ static void dispatch_enqueue(struct scx_sched *sch, s= truct scx_dispatch_q *dsq, scx_error(sch, "DSQ ID 0x%016llx already had PRIQ-enqueued tasks", dsq->id); =20 - if (enq_flags & (SCX_ENQ_HEAD | SCX_ENQ_PREEMPT)) + if (enq_flags & (SCX_ENQ_HEAD | SCX_ENQ_PREEMPT)) { list_add(&p->scx.dsq_list.node, &dsq->list); - else + /* new task inserted at head - use fastpath */ + if (!(dsq->id & SCX_DSQ_FLAG_BUILTIN)) + rcu_assign_pointer(dsq->first_task, p); + } else { + bool was_empty; + + was_empty =3D list_empty(&dsq->list); list_add_tail(&p->scx.dsq_list.node, &dsq->list); + if (was_empty && !(dsq->id & SCX_DSQ_FLAG_BUILTIN)) + rcu_assign_pointer(dsq->first_task, p); + } } =20 /* seq records the order tasks are queued, used by BPF DSQ iterator */ @@ -1011,6 +1023,13 @@ static void task_unlink_from_dsq(struct task_struct = *p, p->scx.dsq_flags &=3D ~SCX_TASK_DSQ_ON_PRIQ; } =20 + if (!(dsq->id & SCX_DSQ_FLAG_BUILTIN) && dsq->first_task =3D=3D p) { + struct task_struct *first_task; + + first_task =3D nldsq_next_task(dsq, NULL, false); + rcu_assign_pointer(dsq->first_task, first_task); + } + list_del_init(&p->scx.dsq_list.node); dsq_mod_nr(dsq, -1); } @@ -6084,6 +6103,36 @@ __bpf_kfunc void bpf_iter_scx_dsq_destroy(struct bpf= _iter_scx_dsq *it) kit->dsq =3D NULL; } =20 +/** + * scx_bpf_dsq_peek - Lockless peek at the first element. + * @dsq_id: DSQ to examine. + * + * Read the first element in the DSQ. This is semantically equivalent to u= sing + * the DSQ iterator, but is lockfree. + * + * Returns the pointer, or NULL indicates an empty queue OR internal error. + */ +__bpf_kfunc struct task_struct *scx_bpf_dsq_peek(u64 dsq_id) +{ + struct scx_sched *sch; + struct scx_dispatch_q *dsq; + + sch =3D rcu_dereference(scx_root); + if (unlikely(!sch)) + return NULL; + if (unlikely(dsq_id & SCX_DSQ_FLAG_BUILTIN)) { + scx_error(sch, "peek disallowed on builtin DSQ 0x%llx", dsq_id); + return NULL; + } + + dsq =3D find_user_dsq(sch, dsq_id); + if (unlikely(!dsq)) { + scx_error(sch, "peek on non-existent DSQ 0x%llx", dsq_id); + return NULL; + } + return rcu_dereference(dsq->first_task); +} + __bpf_kfunc_end_defs(); =20 static s32 __bstr_format(struct scx_sched *sch, u64 *data_buf, char *line_= buf, @@ -6641,6 +6690,7 @@ BTF_KFUNCS_START(scx_kfunc_ids_any) BTF_ID_FLAGS(func, scx_bpf_kick_cpu) BTF_ID_FLAGS(func, scx_bpf_dsq_nr_queued) BTF_ID_FLAGS(func, scx_bpf_destroy_dsq) +BTF_ID_FLAGS(func, scx_bpf_dsq_peek, KF_RCU_PROTECTED | KF_RET_NULL) BTF_ID_FLAGS(func, bpf_iter_scx_dsq_new, KF_ITER_NEW | KF_RCU_PROTECTED) BTF_ID_FLAGS(func, bpf_iter_scx_dsq_next, KF_ITER_NEXT | KF_RET_NULL) BTF_ID_FLAGS(func, bpf_iter_scx_dsq_destroy, KF_ITER_DESTROY) diff --git a/tools/sched_ext/include/scx/common.bpf.h b/tools/sched_ext/inc= lude/scx/common.bpf.h index 06e2551033cb..fbf3e7f9526c 100644 --- a/tools/sched_ext/include/scx/common.bpf.h +++ b/tools/sched_ext/include/scx/common.bpf.h @@ -75,6 +75,7 @@ u32 scx_bpf_reenqueue_local(void) __ksym; void scx_bpf_kick_cpu(s32 cpu, u64 flags) __ksym; s32 scx_bpf_dsq_nr_queued(u64 dsq_id) __ksym; void scx_bpf_destroy_dsq(u64 dsq_id) __ksym; +struct task_struct *scx_bpf_dsq_peek(u64 dsq_id) __ksym __weak; int bpf_iter_scx_dsq_new(struct bpf_iter_scx_dsq *it, u64 dsq_id, u64 flag= s) __ksym __weak; struct task_struct *bpf_iter_scx_dsq_next(struct bpf_iter_scx_dsq *it) __k= sym __weak; void bpf_iter_scx_dsq_destroy(struct bpf_iter_scx_dsq *it) __ksym __weak; diff --git a/tools/sched_ext/include/scx/compat.bpf.h b/tools/sched_ext/inc= lude/scx/compat.bpf.h index dd9144624dc9..97b10c184b2c 100644 --- a/tools/sched_ext/include/scx/compat.bpf.h +++ b/tools/sched_ext/include/scx/compat.bpf.h @@ -130,6 +130,25 @@ int bpf_cpumask_populate(struct cpumask *dst, void *sr= c, size_t src__sz) __ksym false; \ }) =20 + +/* + * v6.19: Introduce lockless peek API for user DSQs. + * + * Preserve the following macro until v6.21. + */ +static inline struct task_struct *__COMPAT_scx_bpf_dsq_peek(u64 dsq_id) +{ + struct task_struct *p =3D NULL; + struct bpf_iter_scx_dsq it; + + if (bpf_ksym_exists(scx_bpf_dsq_peek)) + return scx_bpf_dsq_peek(dsq_id); + if (!bpf_iter_scx_dsq_new(&it, dsq_id, 0)) + p =3D bpf_iter_scx_dsq_next(&it); + bpf_iter_scx_dsq_destroy(&it); + return p; +} + /** * __COMPAT_is_enq_cpu_selected - Test if SCX_ENQ_CPU_SELECTED is on * in a compatible way. We will preserve this __COMPAT helper until v6.16. --=20 2.51.0