[PATCH 13/13] percpu: queue the balance work on an unbound workqueue

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

From: Chris Mason <mason@kernel.org>

A burst of small atomic per-CPU allocations can fail with -ENOMEM while the
machine has free memory and idle CPUs, as atomic per-CPU allocations are
only served from pages that are already populated, and those can be used up
before a worker gets to populate more.

Consider:

    caller on CPU 1                     kworker bound to CPU 1
    ===============                     ======================
    allocates 8 bytes, which leaves
    one empty page and 4088 bytes
      schedule_work()                   woken, waits for CPU 1
    allocates 8 bytes, 1023 times
    next allocation: "no space left"
    gives up CPU 1                      populates pages

The allocator keeps a few empty populated pages in reserve, and the work
item pcpu_balance_work refills them.

pcpu_alloc_noprof() queues the work item using schedule_work(), so only a
worker bound to the CPU of the caller can run the work item.

The issue is that the caller keeps that CPU - a task keeps it for the rest
of its time slice, and a softirq keeps it until the softirq ends.

In that time the caller can use up the reserve, resulting in an atomic
per-CPU allocation failing.

Fix the issue by queueing the work item on an unbound workqueue so that
another CPU can populate pages while the caller keeps allocating.

Fixes: 1a4d76076cda ("percpu: implement asynchronous chunk population")
Signed-off-by: Chris Mason <mason@kernel.org>
[ ljs: tweaked commit message, comment ]
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 mm/percpu.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index f87813e6ad73..39cba1710383 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -200,8 +200,9 @@ static bool pcpu_atomic_alloc_failed;
 
 static void pcpu_schedule_balance_work(void)
 {
+	/* Unbound, so another CPU can populate pgs if caller keeps this one. */
 	if (pcpu_async_enabled)
-		schedule_work(&pcpu_balance_work);
+		queue_work(system_dfl_wq, &pcpu_balance_work);
 }
 
 /**

-- 
2.56.0