linux-kernel - Re: [PATCH v2 2/3] sched_ext: Provide scx_bpf_remote

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Message-ID: <c8b71208-b8da-4f64-9550-13ddeca9b225@arm.com>
Date: Mon, 4 Aug 2025 14:27:14 +0100
From: Christian Loehle <christian.loehle@....com>
To: Andrea Righi <arighi@...dia.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 8/4/25 13:51, Andrea Righi wrote:
> 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.
> 

At least currently <linux/btf.h> doesn't declare bpf_task_acquire.
I'm not quite sure what's most elegant here, none of the bpf helper
kfuncs seem to be used from kernel code / kernel/sched/ext.c


>> +
>> +/**
>> + * 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().

Sure.

> 
> 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()?

Will change it to
bpf_task_acquire_remote_curr()?
..on_cpu() seems like a bit of a jump semantically.

> 
>> + */
>> +__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