From: "Emil Tsalapatis" <emil@etsalapatis•com>
To: "Tejun Heo" <tj@kernel•org>, "David Vernet" <void@manifault•com>,
"Andrea Righi" <arighi@nvidia•com>,
"Changwoo Min" <changwoo@igalia•com>,
"Alexei Starovoitov" <ast@kernel•org>,
"Andrii Nakryiko" <andrii@kernel•org>,
"Daniel Borkmann" <daniel@iogearbox•net>,
"Martin KaFai Lau" <martin.lau@linux•dev>,
"Kumar Kartikeya Dwivedi" <memxor@gmail•com>
Cc: "Peter Zijlstra" <peterz@infradead•org>,
"Catalin Marinas" <catalin.marinas@arm•com>,
"Will Deacon" <will@kernel•org>,
"Thomas Gleixner" <tglx@kernel•org>,
"Ingo Molnar" <mingo@redhat•com>,
"Borislav Petkov" <bp@alien8•de>,
"Dave Hansen" <dave.hansen@linux•intel.com>,
"Andrew Morton" <akpm@linux-foundation•org>,
"David Hildenbrand" <david@kernel•org>,
"Mike Rapoport" <rppt@kernel•org>,
"Emil Tsalapatis" <emil@etsalapatis•com>,
<sched-ext@lists•linux.dev>, <bpf@vger•kernel.org>,
<x86@kernel•org>, <linux-arm-kernel@lists•infradead.org>,
<linux-mm@kvack•org>, <linux-kernel@vger•kernel.org>
Subject: Re: [PATCH 6/8] sched_ext: Require an arena for cid-form schedulers
Date: Thu, 21 May 2026 00:15:26 -0400 [thread overview]
Message-ID: <DIO2ARBBUOJX.1OJ5RVION2UH1@etsalapatis.com> (raw)
In-Reply-To: <20260520235052.4180316-7-tj@kernel.org>
On Wed May 20, 2026 at 7:50 PM EDT, Tejun Heo wrote:
> Upcoming patches will let the kernel place arena-resident scratch shared
> with the BPF program (e.g. per-CPU set_cmask cmask) so the BPF side can
> dereference it directly via __arena pointers, replacing the current
> cmask_copy_from_kernel() probe-read loop. That requires each cid-form
> scheduler to expose its arena to the kernel. Kernel- side accesses are
> recovered by the per-arena scratch-page mechanism.
>
> bpf_scx_reg_cid() walks the struct_ops member progs via
> bpf_struct_ops_for_each_prog() and reads each prog's arena via
> bpf_prog_arena(). The verifier enforces one arena per program, so each
> member prog contributes at most one arena. All non-NULL contributions must
> match and at least one member prog must use an arena. The map ref is held on
> scx_sched and dropped on sched destroy. cpu-form schedulers (bpf_scx_reg)
> are unchanged - no arena requirement.
>
> Signed-off-by: Tejun Heo <tj@kernel•org>
> ---
> kernel/sched/ext.c | 56 ++++++++++++++++++++++++++++++++++++-
> kernel/sched/ext_internal.h | 8 ++++++
> 2 files changed, 63 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 9c458552d14f..56f94ac32ba0 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -5003,6 +5003,8 @@ static void scx_sched_free_rcu_work(struct work_struct *work)
>
> rhashtable_free_and_destroy(&sch->dsq_hash, NULL, NULL);
> free_exit_info(sch->exit_info);
> + if (sch->arena_map)
> + bpf_map_put(sch->arena_map);
> kfree(sch);
> }
>
> @@ -6746,6 +6748,7 @@ struct scx_enable_cmd {
> struct sched_ext_ops_cid *ops_cid;
> };
> bool is_cid_type;
> + struct bpf_map *arena_map; /* arena ref to transfer to sch */
> int ret;
> };
>
> @@ -6913,6 +6916,15 @@ static struct scx_sched *scx_alloc_and_add_sched(struct scx_enable_cmd *cmd,
> return ERR_PTR(ret);
> }
> #endif /* CONFIG_EXT_SUB_SCHED */
> +
> + /*
> + * Consume the arena_map ref bpf_scx_reg_cid() took. Defer to here so
> + * earlier failure paths leave cmd->arena_map set and bpf_scx_reg_cid
> + * drops the ref. After this point, sch owns the ref and any cleanup
> + * runs through scx_sched_free_rcu_work() which puts it.
> + */
> + sch->arena_map = cmd->arena_map;
> + cmd->arena_map = NULL;
> return sch;
>
> #ifdef CONFIG_EXT_SUB_SCHED
> @@ -7898,11 +7910,53 @@ static int bpf_scx_reg(void *kdata, struct bpf_link *link)
> return scx_enable(&cmd, link);
> }
>
> +struct scx_arena_scan {
> + struct bpf_map *arena;
> + int err;
Can we skip the int err here...
> +};
> +
> +/*
> + * The verifier enforces one arena per BPF program, so each struct_ops
> + * member prog contributes at most one arena via bpf_prog_arena().
> + * Require all non-NULL contributions to match.
> + */
> +static int scx_arena_scan_prog(struct bpf_prog *prog, void *data)
> +{
> + struct scx_arena_scan *s = data;
> + struct bpf_map *arena = bpf_prog_arena(prog);
> +
> + if (!arena)
> + return 0;
> + if (s->arena && s->arena != arena) {
> + s->err = -EINVAL;
...and just directly return -EINVAL here? bpf_struct_ops_for_each_prog
breaks when we return non-zero so do we need the extra scx_arena_scan
struct?
> + return 1;
> + }
> + s->arena = arena;
> + return 0;
> +}
> +
> static int bpf_scx_reg_cid(void *kdata, struct bpf_link *link)
> {
> struct scx_enable_cmd cmd = { .ops_cid = kdata, .is_cid_type = true };
> + struct scx_arena_scan scan = {};
> + int ret;
>
> - return scx_enable(&cmd, link);
> + bpf_struct_ops_for_each_prog(kdata, scx_arena_scan_prog, &scan);
> + if (scan.err) {
> + pr_err("sched_ext: cid-form scheduler uses multiple arena maps\n");
> + return scan.err;
> + }
> + if (!scan.arena) {
> + pr_err("sched_ext: cid-form scheduler must use a BPF arena map\n");
> + return -EINVAL;
> + }
> +
> + bpf_map_inc(scan.arena);
> + cmd.arena_map = scan.arena;
> + ret = scx_enable(&cmd, link);
> + if (cmd.arena_map) /* not consumed by scx_alloc_and_add_sched() */
> + bpf_map_put(cmd.arena_map);
> + return ret;
> }
>
> static void bpf_scx_unreg(void *kdata, struct bpf_link *link)
> diff --git a/kernel/sched/ext_internal.h b/kernel/sched/ext_internal.h
> index 7258aea94b9f..d40cfd29ddaa 100644
> --- a/kernel/sched/ext_internal.h
> +++ b/kernel/sched/ext_internal.h
> @@ -1111,6 +1111,14 @@ struct scx_sched {
> struct sched_ext_ops_cid ops_cid;
> };
> bool is_cid_type; /* true if registered via bpf_sched_ext_ops_cid */
> +
> + /*
> + * Arena map auto-discovered from member progs at struct_ops attach.
> + * cid-form schedulers must use exactly one arena across all member
> + * progs. NULL on cpu-form.
> + */
> + struct bpf_map *arena_map;
> +
> DECLARE_BITMAP(has_op, SCX_OPI_END);
>
> /*
next prev parent reply other threads:[~2026-05-21 4:15 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-20 23:50 [PATCHSET v3 sched_ext/for-7.2] bpf/arena: Direct kernel-side access Tejun Heo
2026-05-20 23:50 ` [PATCH 1/8] mm: Add ptep_try_set() for lockless empty-slot installs Tejun Heo
2026-05-21 7:00 ` Andrea Righi
2026-05-21 17:37 ` [PATCH v3 " Tejun Heo
2026-05-20 23:50 ` [PATCH 2/8] bpf: Recover arena kernel faults with scratch page Tejun Heo
2026-05-21 3:16 ` Emil Tsalapatis
2026-05-21 9:42 ` Alexei Starovoitov
2026-05-21 17:39 ` Tejun Heo
2026-05-20 23:50 ` [PATCH 3/8] bpf: Add sleepable variant of bpf_arena_alloc_pages for kernel callers Tejun Heo
2026-05-21 3:17 ` Emil Tsalapatis
2026-05-20 23:50 ` [PATCH 4/8] bpf: Add bpf_struct_ops_for_each_prog() Tejun Heo
2026-05-21 4:07 ` Emil Tsalapatis
2026-05-20 23:50 ` [PATCH 5/8] bpf/arena: Add bpf_arena_map_kern_vm_start() and bpf_prog_arena() Tejun Heo
2026-05-21 4:08 ` Emil Tsalapatis
2026-05-20 23:50 ` [PATCH 6/8] sched_ext: Require an arena for cid-form schedulers Tejun Heo
2026-05-21 4:15 ` Emil Tsalapatis [this message]
2026-05-20 23:50 ` [PATCH 7/8] sched_ext: Sub-allocator over kernel-claimed BPF arena pages Tejun Heo
2026-05-21 7:56 ` Andrea Righi
2026-05-21 17:22 ` Tejun Heo
2026-05-21 17:37 ` [PATCH v2 " Tejun Heo
2026-05-21 17:54 ` Andrea Righi
2026-05-20 23:50 ` [PATCH 8/8] sched_ext: Convert ops.set_cmask() to arena-resident cmask Tejun Heo
2026-05-21 4:19 ` Emil Tsalapatis
2026-05-22 1:59 ` [PATCH v3 2/8] bpf: Recover arena kernel faults with scratch page Tejun Heo
-- strict thread matches above, loose matches on Subject: below --
2026-05-22 17:22 [PATCHSET v4 sched_ext/for-7.2] bpf/arena: Direct kernel-side access Tejun Heo
2026-05-22 17:22 ` [PATCH 6/8] sched_ext: Require an arena for cid-form schedulers Tejun Heo
2026-05-17 21:12 [PATCHSET v2 sched_ext/for-7.2] bpf/arena: Direct kernel-side access Tejun Heo
2026-05-17 21:12 ` [PATCH 6/8] sched_ext: Require an arena for cid-form schedulers Tejun Heo
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