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Message-ID: <aJCsx7hbCD9f5RK3@gpd4>
Date: Mon, 4 Aug 2025 14:51:19 +0200
From: Andrea Righi <arighi@...dia.com>
To: Christian Loehle <christian.loehle@....com>
Cc: tj@...nel.org, void@...ifault.com, linux-kernel@...r.kernel.org,
sched-ext@...ts.linux.dev, changwoo@...lia.com, hodgesd@...a.com,
mingo@...hat.com, peterz@...radead.org
Subject: Re: [PATCH v2 2/3] sched_ext: Provide scx_bpf_remote_curr()
On Mon, Aug 04, 2025 at 12:27:42PM +0100, Christian Loehle wrote:
> Provide scx_bpf_remote_curr() as a way for scx schedulers to
> check the curr task of a remote rq, without assuming its lock
> is held.
>
> Many scx schedulers make use of scx_bpf_cpu_rq() to check a
> remote curr (e.g. to see if it should be preempted). This is
> problematic because scx_bpf_cpu_rq() provides access to all
> fields of struct rq, most of which aren't safe to use without
> holding the associated rq lock.
>
> Signed-off-by: Christian Loehle <christian.loehle@....com>
> ---
> kernel/sched/ext.c | 24 ++++++++++++++++++++++++
> tools/sched_ext/include/scx/common.bpf.h | 1 +
> 2 files changed, 25 insertions(+)
>
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 3ea3f0f18030..1d9d9cbed0aa 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -7426,6 +7426,29 @@ __bpf_kfunc struct rq *scx_bpf_cpu_rq(s32 cpu)
> return cpu_rq(cpu);
> }
>
> +struct task_struct *bpf_task_acquire(struct task_struct *p);
Can we move include <linux/btf.h> all the way to the top? In this way we
don't have to add this forward declaration.
> +
> +/**
> + * scx_bpf_remote_curr - Fetch the curr of a rq without acquiring its rq lock
> + * @cpu: CPU of the rq
> + *
> + * Increments the refcount of the task_struct which needs to be released later.
Maybe we should mention that the task must be released by calling
bpf_task_release().
While at it, what do you think about renaming this to something like
scx_bpf_task_acquire_on_cpu(), so that it looks similar to
bpf_task_acquire()?
> + */
> +__bpf_kfunc struct task_struct *scx_bpf_remote_curr(s32 cpu)
> +{
> + struct task_struct *p;
> +
> + if (!kf_cpu_valid(cpu, NULL))
> + return NULL;
> +
> + rcu_read_lock();
> + p = cpu_rq(cpu)->curr;
> + if (p)
> + p = bpf_task_acquire(p);
> + rcu_read_unlock();
> + return p;
> +}
> +
> /**
> * scx_bpf_task_cgroup - Return the sched cgroup of a task
> * @p: task of interest
> @@ -7590,6 +7613,7 @@ BTF_ID_FLAGS(func, scx_bpf_put_cpumask, KF_RELEASE)
> BTF_ID_FLAGS(func, scx_bpf_task_running, KF_RCU)
> BTF_ID_FLAGS(func, scx_bpf_task_cpu, KF_RCU)
> BTF_ID_FLAGS(func, scx_bpf_cpu_rq, KF_RET_NULL)
> +BTF_ID_FLAGS(func, scx_bpf_remote_curr, KF_RET_NULL | KF_ACQUIRE)
> #ifdef CONFIG_CGROUP_SCHED
> BTF_ID_FLAGS(func, scx_bpf_task_cgroup, KF_RCU | KF_ACQUIRE)
> #endif
> diff --git a/tools/sched_ext/include/scx/common.bpf.h b/tools/sched_ext/include/scx/common.bpf.h
> index d4e21558e982..e5d4ef124532 100644
> --- a/tools/sched_ext/include/scx/common.bpf.h
> +++ b/tools/sched_ext/include/scx/common.bpf.h
> @@ -91,6 +91,7 @@ s32 scx_bpf_pick_any_cpu(const cpumask_t *cpus_allowed, u64 flags) __ksym;
> bool scx_bpf_task_running(const struct task_struct *p) __ksym;
> s32 scx_bpf_task_cpu(const struct task_struct *p) __ksym;
> struct rq *scx_bpf_cpu_rq(s32 cpu) __ksym;
> +struct task_struct *scx_bpf_remote_curr(s32 cpu) __ksym;
> 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;
> --
> 2.34.1
>
Thanks,
-Andrea
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