linux-kernel - [PATCH 2/4] kthread: Extract KTHREAD_IS_PER

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Message-ID: <20210112144843.788106541@infradead.org>
Date:   Tue, 12 Jan 2021 15:43:46 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     mingo@...nel.org, tglx@...utronix.de
Cc:     linux-kernel@...r.kernel.org, jiangshanlai@...il.com,
        valentin.schneider@....com, cai@...hat.com,
        vincent.donnefort@....com, decui@...rosoft.com, paulmck@...nel.org,
        vincent.guittot@...aro.org, rostedt@...dmis.org, axboe@...nel.dk,
        tj@...nel.org, peterz@...radead.org
Subject: [PATCH 2/4] kthread: Extract KTHREAD_IS_PER_CPU

There is a need to distinguish geniune per-cpu kthreads from kthreads
that happen to have a single CPU affinity.

Geniune per-cpu kthreads are kthreads that are CPU affine for
correctness, these will obviously have PF_KTHREAD set, but must also
have PF_NO_SETAFFINITY set, lest userspace modify their affinity and
ruins things.

However, these two things are not sufficient, PF_NO_SETAFFINITY is
also set on other tasks that have their affinities controlled through
other means, like for instance workqueues.

Therefore another bit is needed; it turns out kthread_create_per_cpu()
already has such a bit: KTHREAD_IS_PER_CPU, which is used to make
kthread_park()/kthread_unpark() work correctly.

Expose this flag and remove the implicit setting of it from
kthread_create_on_cpu(); the io_uring usage of it seems dubious at
best.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Tested-by: Paul E. McKenney <paulmck@...nel.org>
---
 include/linux/kthread.h |    3 +++
 kernel/kthread.c        |   25 ++++++++++++++++++++++++-
 kernel/sched/core.c     |    2 +-
 kernel/sched/sched.h    |    4 ++--
 kernel/smpboot.c        |    1 +
 kernel/workqueue.c      |   11 +++++++++--
 6 files changed, 40 insertions(+), 6 deletions(-)

--- a/include/linux/kthread.h
+++ b/include/linux/kthread.h
@@ -33,6 +33,9 @@ struct task_struct *kthread_create_on_cp
 					  unsigned int cpu,
 					  const char *namefmt);
 
+void kthread_set_per_cpu(struct task_struct *k, bool set);
+bool kthread_is_per_cpu(struct task_struct *k);
+
 /**
  * kthread_run - create and wake a thread.
  * @threadfn: the function to run until signal_pending(current).
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -493,11 +493,34 @@ struct task_struct *kthread_create_on_cp
 		return p;
 	kthread_bind(p, cpu);
 	/* CPU hotplug need to bind once again when unparking the thread. */
-	set_bit(KTHREAD_IS_PER_CPU, &to_kthread(p)->flags);
 	to_kthread(p)->cpu = cpu;
 	return p;
 }
 
+void kthread_set_per_cpu(struct task_struct *k, bool set)
+{
+	struct kthread *kthread = to_kthread(k);
+	if (!kthread)
+		return;
+
+	if (set) {
+		WARN_ON_ONCE(!(k->flags & PF_NO_SETAFFINITY));
+		WARN_ON_ONCE(k->nr_cpus_allowed != 1);
+		set_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
+	} else {
+		clear_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
+	}
+}
+
+bool kthread_is_per_cpu(struct task_struct *k)
+{
+	struct kthread *kthread = to_kthread(k);
+	if (!kthread)
+		return false;
+
+	return test_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
+}
+
 /**
  * kthread_unpark - unpark a thread created by kthread_create().
  * @k:		thread created by kthread_create().
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -188,6 +188,7 @@ __smpboot_create_thread(struct smp_hotpl
 		kfree(td);
 		return PTR_ERR(tsk);
 	}
+	kthread_set_per_cpu(tsk, true);
 	/*
 	 * Park the thread so that it could start right on the CPU
 	 * when it is available.