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Message-ID: <7aa0ba1c-bcd4-4ac6-9699-98e2526fb15d@arm.com>
Date: Fri, 1 Aug 2025 15:58:27 +0100
From: Christian Loehle <christian.loehle@....com>
To: Andrea Righi <arighi@...dia.com>
Cc: linux-kernel@...r.kernel.org, sched-ext@...ts.linux.dev, tj@...nel.org,
void@...ifault.com, mingo@...hat.com, peterz@...radead.org
Subject: Re: [PATCH 2/3] sched_ext: Provide scx_bpf_remote_curr()
On 8/1/25 15:47, Andrea Righi wrote:
> On Fri, Aug 01, 2025 at 03:17:40PM +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
>> or acquiring any.
>>
>> 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 | 15 +++++++++++++++
>> tools/sched_ext/include/scx/common.bpf. | 1 +
>> 2 files changed, 16 insertions(+)
>>
>> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
>> index b734f55f3318..92e66bb0b5f2 100644
>> --- a/kernel/sched/ext.c
>> +++ b/kernel/sched/ext.c
>> @@ -7436,6 +7436,20 @@ __bpf_kfunc struct rq *scx_bpf_cpu_rq(s32 cpu)
>> return cpu_rq(cpu);
>> }
>>
>> +/**
>> + * scx_bpf_remote_curr - Fetch the curr of a rq without acquiring its rq lock
>> + * @cpu: CPU of the rq
>> + *
>> + * Neither a rq lock nor a task reference is acquired.
>> + */
>> +__bpf_kfunc struct task_struct *scx_bpf_remote_curr(s32 cpu)
>> +{
>> + if (!kf_cpu_valid(cpu, NULL))
>> + return NULL;
>> +
>> + return cpu_rq(cpu)->curr;
>> +}
>
> As mentioned in my previou comment, this should be something like:
>
> 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;
Alright, that's actually what I had at first, but went with the
drop-in-replacement that doesn't acquire.
I'll resend with an acquire.
Thanks, Andrea!
>
> We may still race with CPU hotplugging, but I think it's not always
> possible to use cpus_read_lock/unlock() here. Also, most of the scx
> schedulers are restarted on CPU hotplugging events, so... one thing at a
> time. :)
>
>> +
>> /**
>> * scx_bpf_task_cgroup - Return the sched cgroup of a task
>> * @p: task of interest
>> @@ -7600,6 +7614,7 @@ 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)
>> #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
>>
>
> -Andrea
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