Merge branch 'bpf-replace-wq-users-and-add-wq_percpu-to-alloc_workqueue-users'

Marco Crivellari says:

====================
Below is a summary of a discussion about the Workqueue API and cpu isolation
considerations. Details and more information are available here:

        "workqueue: Always use wq_select_unbound_cpu() for WORK_CPU_UNBOUND."
        https://lore.kernel.org/all/[email protected]/

=== Current situation: problems ===

Let's consider a nohz_full system with isolated CPUs: wq_unbound_cpumask is
set to the housekeeping CPUs, for !WQ_UNBOUND the local CPU is selected.

This leads to different scenarios if a work item is scheduled on an isolated
CPU where "delay" value is 0 or greater then 0:
        schedule_delayed_work(, 0);

This will be handled by __queue_work() that will queue the work item on the
current local (isolated) CPU, while:

        schedule_delayed_work(, 1);

Will move the timer on an housekeeping CPU, and schedule the work there.

Currently if a user enqueue a work item using schedule_delayed_work() the
used wq is "system_wq" (per-cpu wq) while queue_delayed_work() use
WORK_CPU_UNBOUND (used when a cpu is not specified). The same applies to
schedule_work() that is using system_wq and queue_work(), that makes use
again of WORK_CPU_UNBOUND.

This lack of consistentcy cannot be addressed without refactoring the API.

=== Plan and future plans ===

This patchset is the first stone on a refactoring needed in order to
address the points aforementioned; it will have a positive impact also
on the cpu isolation, in the long term, moving away percpu workqueue in
favor to an unbound model.

These are the main steps:
1)  API refactoring (that this patch is introducing)
    -   Make more clear and uniform the system wq names, both per-cpu and
        unbound. This to avoid any possible confusion on what should be
        used.

    -   Introduction of WQ_PERCPU: this flag is the complement of WQ_UNBOUND,
        introduced in this patchset and used on all the callers that are not
        currently using WQ_UNBOUND.

        WQ_UNBOUND will be removed in a future release cycle.

        Most users don't need to be per-cpu, because they don't have
        locality requirements, because of that, a next future step will be
        make "unbound" the default behavior.

2)  Check who really needs to be per-cpu
    -   Remove the WQ_PERCPU flag when is not strictly required.

3)  Add a new API (prefer local cpu)
    -   There are users that don't require a local execution, like mentioned
        above; despite that, local execution yeld to performance gain.

        This new API will prefer the local execution, without requiring it.

=== Introduced Changes by this series ===

1) [P 1-2] Replace use of system_wq and system_unbound_wq

        system_wq is a per-CPU workqueue, but his name is not clear.
        system_unbound_wq is to be used when locality is not required.

        Because of that, system_wq has been renamed in system_percpu_wq, and
        system_unbound_wq has been renamed in system_dfl_wq.

2) [P 3] add WQ_PERCPU to remaining alloc_workqueue() users

        Every alloc_workqueue() caller should use one among WQ_PERCPU or
        WQ_UNBOUND. This is actually enforced warning if both or none of them
        are present at the same time.

        WQ_UNBOUND will be removed in a next release cycle.

=== For Maintainers ===

There are prerequisites for this series, already merged in the master branch.
The commits are:

128ea9f ("workqueue: Add system_percpu_wq and
system_dfl_wq")

930c2ea ("workqueue: Add new WQ_PERCPU flag")
====================

Acked-by: Tejun Heo <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: Alexei Starovoitov

kernel/bpf/cgroup.c

@@ -27,14 +27,15 @@ EXPORT_SYMBOL(cgroup_bpf_enabled_key);
 /*
  * cgroup bpf destruction makes heavy use of work items and there can be a lot
  * of concurrent destructions.  Use a separate workqueue so that cgroup bpf
- * destruction work items don't end up filling up max_active of system_wq
+ * destruction work items don't end up filling up max_active of system_percpu_wq
  * which may lead to deadlock.
  */
 static struct workqueue_struct *cgroup_bpf_destroy_wq;
 
 static int __init cgroup_bpf_wq_init(void)
 {
-	cgroup_bpf_destroy_wq = alloc_workqueue("cgroup_bpf_destroy", 0, 1);
+	cgroup_bpf_destroy_wq = alloc_workqueue("cgroup_bpf_destroy",
+						WQ_PERCPU, 1);
 	if (!cgroup_bpf_destroy_wq)
 		panic("Failed to alloc workqueue for cgroup bpf destroy.\n");
 	return 0;

kernel/bpf/cpumap.c

@@ -550,7 +550,7 @@ static void __cpu_map_entry_replace(struct bpf_cpu_map *cmap,
 	old_rcpu = unrcu_pointer(xchg(&cmap->cpu_map[key_cpu], RCU_INITIALIZER(rcpu)));
 	if (old_rcpu) {
 		INIT_RCU_WORK(&old_rcpu->free_work, __cpu_map_entry_free);
-		queue_rcu_work(system_wq, &old_rcpu->free_work);
+		queue_rcu_work(system_percpu_wq, &old_rcpu->free_work);
 	}
 }
 

kernel/bpf/helpers.c

@@ -1594,7 +1594,7 @@ void bpf_timer_cancel_and_free(void *val)
 	 * timer callback.
 	 */
 	if (this_cpu_read(hrtimer_running)) {
-		queue_work(system_unbound_wq, &t->cb.delete_work);
+		queue_work(system_dfl_wq, &t->cb.delete_work);
 		return;
 	}
 
@@ -1607,7 +1607,7 @@ void bpf_timer_cancel_and_free(void *val)
 		if (hrtimer_try_to_cancel(&t->timer) >= 0)
 			kfree_rcu(t, cb.rcu);
 		else
-			queue_work(system_unbound_wq, &t->cb.delete_work);
+			queue_work(system_dfl_wq, &t->cb.delete_work);
 	} else {
 		bpf_timer_delete_work(&t->cb.delete_work);
 	}

kernel/bpf/memalloc.c

@@ -736,7 +736,7 @@ static void destroy_mem_alloc(struct bpf_mem_alloc *ma, int rcu_in_progress)
 	/* Defer barriers into worker to let the rest of map memory to be freed */
 	memset(ma, 0, sizeof(*ma));
 	INIT_WORK(&copy->work, free_mem_alloc_deferred);
-	queue_work(system_unbound_wq, &copy->work);
+	queue_work(system_dfl_wq, &copy->work);
 }
 
 void bpf_mem_alloc_destroy(struct bpf_mem_alloc *ma)

kernel/bpf/syscall.c

@@ -905,7 +905,7 @@ static void bpf_map_free_in_work(struct bpf_map *map)
 	/* Avoid spawning kworkers, since they all might contend
 	 * for the same mutex like slab_mutex.
 	 */
-	queue_work(system_unbound_wq, &map->work);
+	queue_work(system_dfl_wq, &map->work);
 }
 
 static void bpf_map_free_rcu_gp(struct rcu_head *rcu)