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Message-ID: <aQJ0cPxIokHuX1li@gpd4>
Date: Wed, 29 Oct 2025 21:09:20 +0100
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
sched-ext@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH sched_ext/for-6.19] sched_ext/tools: Restore backward
compat with v6.12 kernels
Hi Tejun,
On Wed, Oct 29, 2025 at 08:35:25AM -1000, Tejun Heo wrote:
> Commit 111a79800aed ("tools/sched_ext: Strip compatibility macros for
> cgroup and dispatch APIs") removed the compat layer for v6.12-v6.13 kfunc
> renaming, but v6.12 is the current LTS kernel and will remain supported
> through 2026. Restore backward compatibility so schedulers built with v6.19+
> headers can run on v6.12 kernels.
>
> The restored compat differs from the original in two ways:
>
> 1. Uses ___new/___old suffixes instead of ___compat for clarity. The new
> macros check for v6.13+ names (scx_bpf_dsq_move*), fall back to v6.12
> names (scx_bpf_dispatch_from_dsq*, scx_bpf_consume), then return safe
> no-ops for missing symbols.
>
> 2. Integrates with the args-struct-packing changes added in c0d630ba347c
> ("sched_ext: Wrap kfunc args in struct to prepare for aux__prog").
> scx_bpf_dsq_insert_vtime() now tries __scx_bpf_dsq_insert_vtime (args
> struct), then scx_bpf_dsq_insert_vtime___compat (v6.13-v6.18), then
> scx_bpf_dispatch_vtime___compat (pre-v6.13).
>
> Forward compatibility is not restored - binaries built against v6.13 or
> earlier headers won't run on v6.19+ kernels, as the old kfunc names are not
> exported. This is acceptable since the priority is new binaries running on
> older kernels.
>
> Also add missing compat checks for ops.cgroup_set_bandwidth() (added v6.17)
> and ops.cgroup_set_idle() (added v6.19). These need to be NULLed out in
> userspace on older kernels.
>
> Reported-by: Andrea Righi <arighi@...dia.com>
> Signed-off-by: Tejun Heo <tj@...nel.org>
Looks good to me (and tested with v6.12).
Acked-and-tested-by: Andrea Righi <arighi@...dia.com>
Thanks,
-Andrea
> ---
> tools/sched_ext/include/scx/common.bpf.h | 6 --
> tools/sched_ext/include/scx/compat.bpf.h | 81 ++++++++++++++++++++++++++++++-
> tools/sched_ext/include/scx/compat.h | 14 +++++
> 3 files changed, 93 insertions(+), 8 deletions(-)
>
> --- a/tools/sched_ext/include/scx/common.bpf.h
> +++ b/tools/sched_ext/include/scx/common.bpf.h
> @@ -65,11 +65,6 @@ s32 __scx_bpf_select_cpu_and(struct task
> bool __scx_bpf_dsq_insert_vtime(struct task_struct *p, struct scx_bpf_dsq_insert_vtime_args *args) __ksym __weak;
> u32 scx_bpf_dispatch_nr_slots(void) __ksym;
> void scx_bpf_dispatch_cancel(void) __ksym;
> -bool scx_bpf_dsq_move_to_local(u64 dsq_id) __ksym __weak;
> -void scx_bpf_dsq_move_set_slice(struct bpf_iter_scx_dsq *it__iter, u64 slice) __ksym __weak;
> -void scx_bpf_dsq_move_set_vtime(struct bpf_iter_scx_dsq *it__iter, u64 vtime) __ksym __weak;
> -bool scx_bpf_dsq_move(struct bpf_iter_scx_dsq *it__iter, struct task_struct *p, u64 dsq_id, u64 enq_flags) __ksym __weak;
> -bool scx_bpf_dsq_move_vtime(struct bpf_iter_scx_dsq *it__iter, struct task_struct *p, u64 dsq_id, u64 enq_flags) __ksym __weak;
> 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;
> @@ -104,7 +99,6 @@ s32 scx_bpf_task_cpu(const struct task_s
> struct rq *scx_bpf_cpu_rq(s32 cpu) __ksym;
> struct rq *scx_bpf_locked_rq(void) __ksym;
> struct task_struct *scx_bpf_cpu_curr(s32 cpu) __ksym __weak;
> -struct cgroup *scx_bpf_task_cgroup(struct task_struct *p) __ksym __weak;
> u64 scx_bpf_now(void) __ksym __weak;
> void scx_bpf_events(struct scx_event_stats *events, size_t events__sz) __ksym __weak;
>
> --- a/tools/sched_ext/include/scx/compat.bpf.h
> +++ b/tools/sched_ext/include/scx/compat.bpf.h
> @@ -15,6 +15,68 @@
> __ret; \
> })
>
> +/* v6.12: 819513666966 ("sched_ext: Add cgroup support") */
> +struct cgroup *scx_bpf_task_cgroup___new(struct task_struct *p) __ksym __weak;
> +
> +#define scx_bpf_task_cgroup(p) \
> + (bpf_ksym_exists(scx_bpf_task_cgroup___new) ? \
> + scx_bpf_task_cgroup___new((p)) : NULL)
> +
> +/*
> + * v6.13: The verb `dispatch` was too overloaded and confusing. kfuncs are
> + * renamed to unload the verb.
> + *
> + * Build error is triggered if old names are used. New binaries work with both
> + * new and old names. The compat macros will be removed on v6.15 release.
> + *
> + * scx_bpf_dispatch_from_dsq() and friends were added during v6.12 by
> + * 4c30f5ce4f7a ("sched_ext: Implement scx_bpf_dispatch[_vtime]_from_dsq()").
> + */
> +bool scx_bpf_dsq_move_to_local___new(u64 dsq_id) __ksym __weak;
> +void scx_bpf_dsq_move_set_slice___new(struct bpf_iter_scx_dsq *it__iter, u64 slice) __ksym __weak;
> +void scx_bpf_dsq_move_set_vtime___new(struct bpf_iter_scx_dsq *it__iter, u64 vtime) __ksym __weak;
> +bool scx_bpf_dsq_move___new(struct bpf_iter_scx_dsq *it__iter, struct task_struct *p, u64 dsq_id, u64 enq_flags) __ksym __weak;
> +bool scx_bpf_dsq_move_vtime___new(struct bpf_iter_scx_dsq *it__iter, struct task_struct *p, u64 dsq_id, u64 enq_flags) __ksym __weak;
> +
> +bool scx_bpf_consume___old(u64 dsq_id) __ksym __weak;
> +void scx_bpf_dispatch_from_dsq_set_slice___old(struct bpf_iter_scx_dsq *it__iter, u64 slice) __ksym __weak;
> +void scx_bpf_dispatch_from_dsq_set_vtime___old(struct bpf_iter_scx_dsq *it__iter, u64 vtime) __ksym __weak;
> +bool scx_bpf_dispatch_from_dsq___old(struct bpf_iter_scx_dsq *it__iter, struct task_struct *p, u64 dsq_id, u64 enq_flags) __ksym __weak;
> +bool scx_bpf_dispatch_vtime_from_dsq___old(struct bpf_iter_scx_dsq *it__iter, struct task_struct *p, u64 dsq_id, u64 enq_flags) __ksym __weak;
> +
> +#define scx_bpf_dsq_move_to_local(dsq_id) \
> + (bpf_ksym_exists(scx_bpf_dsq_move_to_local___new) ? \
> + scx_bpf_dsq_move_to_local___new((dsq_id)) : \
> + scx_bpf_consume___old((dsq_id)))
> +
> +#define scx_bpf_dsq_move_set_slice(it__iter, slice) \
> + (bpf_ksym_exists(scx_bpf_dsq_move_set_slice___new) ? \
> + scx_bpf_dsq_move_set_slice___new((it__iter), (slice)) : \
> + (bpf_ksym_exists(scx_bpf_dispatch_from_dsq_set_slice___old) ? \
> + scx_bpf_dispatch_from_dsq_set_slice___old((it__iter), (slice)) : \
> + (void)0))
> +
> +#define scx_bpf_dsq_move_set_vtime(it__iter, vtime) \
> + (bpf_ksym_exists(scx_bpf_dsq_move_set_vtime___new) ? \
> + scx_bpf_dsq_move_set_vtime___new((it__iter), (vtime)) : \
> + (bpf_ksym_exists(scx_bpf_dispatch_from_dsq_set_vtime___old) ? \
> + scx_bpf_dispatch_from_dsq_set_vtime___old((it__iter), (vtime)) : \
> + (void)0))
> +
> +#define scx_bpf_dsq_move(it__iter, p, dsq_id, enq_flags) \
> + (bpf_ksym_exists(scx_bpf_dsq_move___new) ? \
> + scx_bpf_dsq_move___new((it__iter), (p), (dsq_id), (enq_flags)) : \
> + (bpf_ksym_exists(scx_bpf_dispatch_from_dsq___old) ? \
> + scx_bpf_dispatch_from_dsq___old((it__iter), (p), (dsq_id), (enq_flags)) : \
> + false))
> +
> +#define scx_bpf_dsq_move_vtime(it__iter, p, dsq_id, enq_flags) \
> + (bpf_ksym_exists(scx_bpf_dsq_move_vtime___new) ? \
> + scx_bpf_dsq_move_vtime___new((it__iter), (p), (dsq_id), (enq_flags)) : \
> + (bpf_ksym_exists(scx_bpf_dispatch_vtime_from_dsq___old) ? \
> + scx_bpf_dispatch_vtime_from_dsq___old((it__iter), (p), (dsq_id), (enq_flags)) : \
> + false))
> +
> /*
> * v6.15: 950ad93df2fc ("bpf: add kfunc for populating cpumask bits")
> *
> @@ -166,12 +228,16 @@ static inline struct task_struct *__COMP
> * replaced with variants that pack scalar arguments in a struct. Wrappers are
> * provided to maintain source compatibility.
> *
> + * v6.13: scx_bpf_dsq_insert_vtime() renaming is also handled here. See the
> + * block on dispatch renaming above for more details.
> + *
> * The kernel will carry the compat variants until v6.23 to maintain binary
> * compatibility. After v6.23 release, remove the compat handling and move the
> * wrappers to common.bpf.h.
> */
> s32 scx_bpf_select_cpu_and___compat(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
> const struct cpumask *cpus_allowed, u64 flags) __ksym __weak;
> +void scx_bpf_dispatch_vtime___compat(struct task_struct *p, u64 dsq_id, u64 slice, u64 vtime, u64 enq_flags) __ksym __weak;
> void scx_bpf_dsq_insert_vtime___compat(struct task_struct *p, u64 dsq_id, u64 slice, u64 vtime, u64 enq_flags) __ksym __weak;
>
> /**
> @@ -227,10 +293,14 @@ scx_bpf_dsq_insert_vtime(struct task_str
> };
>
> return __scx_bpf_dsq_insert_vtime(p, &args);
> - } else {
> + } else if (bpf_ksym_exists(scx_bpf_dsq_insert_vtime___compat)) {
> scx_bpf_dsq_insert_vtime___compat(p, dsq_id, slice, vtime,
> enq_flags);
> return true;
> + } else {
> + scx_bpf_dispatch_vtime___compat(p, dsq_id, slice, vtime,
> + enq_flags);
> + return true;
> }
> }
>
> @@ -239,18 +309,25 @@ scx_bpf_dsq_insert_vtime(struct task_str
> * scx_bpf_dsq_insert() decl to common.bpf.h and drop compat helper after v6.22.
> * The extra ___compat suffix is to work around libbpf not ignoring __SUFFIX on
> * kernel side. The entire suffix can be dropped later.
> + *
> + * v6.13: scx_bpf_dsq_insert() renaming is also handled here. See the block on
> + * dispatch renaming above for more details.
> */
> bool scx_bpf_dsq_insert___v2___compat(struct task_struct *p, u64 dsq_id, u64 slice, u64 enq_flags) __ksym __weak;
> void scx_bpf_dsq_insert___v1(struct task_struct *p, u64 dsq_id, u64 slice, u64 enq_flags) __ksym __weak;
> +void scx_bpf_dispatch___compat(struct task_struct *p, u64 dsq_id, u64 slice, u64 enq_flags) __ksym __weak;
>
> static inline bool
> scx_bpf_dsq_insert(struct task_struct *p, u64 dsq_id, u64 slice, u64 enq_flags)
> {
> if (bpf_ksym_exists(scx_bpf_dsq_insert___v2___compat)) {
> return scx_bpf_dsq_insert___v2___compat(p, dsq_id, slice, enq_flags);
> - } else {
> + } else if (bpf_ksym_exists(scx_bpf_dsq_insert___v1)) {
> scx_bpf_dsq_insert___v1(p, dsq_id, slice, enq_flags);
> return true;
> + } else {
> + scx_bpf_dispatch___compat(p, dsq_id, slice, enq_flags);
> + return true;
> }
> }
>
> --- a/tools/sched_ext/include/scx/compat.h
> +++ b/tools/sched_ext/include/scx/compat.h
> @@ -151,6 +151,10 @@ static inline long scx_hotplug_seq(void)
> *
> * ec7e3b0463e1 ("implement-ops") in https://github.com/sched-ext/sched_ext is
> * the current minimum required kernel version.
> + *
> + * COMPAT:
> + * - v6.17: ops.cgroup_set_bandwidth()
> + * - v6.19: ops.cgroup_set_idle()
> */
> #define SCX_OPS_OPEN(__ops_name, __scx_name) ({ \
> struct __scx_name *__skel; \
> @@ -162,6 +166,16 @@ static inline long scx_hotplug_seq(void)
> SCX_BUG_ON(!__skel, "Could not open " #__scx_name); \
> __skel->struct_ops.__ops_name->hotplug_seq = scx_hotplug_seq(); \
> SCX_ENUM_INIT(__skel); \
> + if (__skel->struct_ops.__ops_name->cgroup_set_bandwidth && \
> + !__COMPAT_struct_has_field("sched_ext_ops", "cgroup_set_bandwidth")) { \
> + fprintf(stderr, "WARNING: kernel doesn't support ops.cgroup_set_bandwidth()\n"); \
> + __skel->struct_ops.__ops_name->cgroup_set_bandwidth = NULL; \
> + } \
> + if (__skel->struct_ops.__ops_name->cgroup_set_idle && \
> + !__COMPAT_struct_has_field("sched_ext_ops", "cgroup_set_idle")) { \
> + fprintf(stderr, "WARNING: kernel doesn't support ops.cgroup_set_idle()\n"); \
> + __skel->struct_ops.__ops_name->cgroup_set_idle = NULL; \
> + } \
> __skel; \
> })
>
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