[PATCH AUTOSEL 4.19 05/15] xen/balloon: use a kernel thread instead a workqueue

Sasha Levin
2021-09-23T03:41:28Z

From: Juergen Gross <jgross@suse.com>

[ Upstream commit 8480ed9c2bbd56fc86524998e5f2e3e22f5038f6 ]

Today the Xen ballooning is done via delayed work in a workqueue. This
might result in workqueue hangups being reported in case of large
amounts of memory are being ballooned in one go (here 16GB):

BUG: workqueue lockup - pool cpus=6 node=0 flags=0x0 nice=0 stuck for 64s!
Showing busy workqueues and worker pools:
workqueue events: flags=0x0
pwq 12: cpus=6 node=0 flags=0x0 nice=0 active=2/256 refcnt=3
in-flight: 229:balloon_process
pending: cache_reap
workqueue events_freezable_power_: flags=0x84
pwq 12: cpus=6 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
pending: disk_events_workfn
workqueue mm_percpu_wq: flags=0x8
pwq 12: cpus=6 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
pending: vmstat_update
pool 12: cpus=6 node=0 flags=0x0 nice=0 hung=64s workers=3 idle: 2222 43

This can easily be avoided by using a dedicated kernel thread for doing
the ballooning work.

Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20210827123206.15429-1-jgross@suse.com
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/xen/balloon.c | 62 +++++++++++++++++++++++++++++++------------
1 file changed, 45 insertions(+), 17 deletions(-)

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index b23edf64c2b2..643dbe5620e8 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -43,6 +43,8 @@
#include <linux/sched.h>
#include <linux/cred.h>
#include <linux/errno.h>
+#include <linux/freezer.h>
+#include <linux/kthread.h>
#include <linux/mm.h>
#include <linux/bootmem.h>
#include <linux/pagemap.h>
@@ -120,7 +122,7 @@ static struct ctl_table xen_root[] = {
#define EXTENT_ORDER (fls(XEN_PFN_PER_PAGE) - 1)

/*
- * balloon_process() state:
+ * balloon_thread() state:
*
* BP_DONE: done or nothing to do,
* BP_WAIT: wait to be rescheduled,
@@ -135,6 +137,8 @@ enum bp_state {
BP_ECANCELED
};

+/* Main waiting point for xen-balloon thread. */
+static DECLARE_WAIT_QUEUE_HEAD(balloon_thread_wq);

static DEFINE_MUTEX(balloon_mutex);

@@ -149,10 +153,6 @@ static xen_pfn_t frame_list[PAGE_SIZE / sizeof(xen_pfn_t)];
static LIST_HEAD(ballooned_pages);
static DECLARE_WAIT_QUEUE_HEAD(balloon_wq);

-/* Main work function, always executed in process context. */
-static void balloon_process(struct work_struct *work);
-static DECLARE_DELAYED_WORK(balloon_worker, balloon_process);
-
/* When ballooning out (allocating memory to return to Xen) we don't really
want the kernel to try too hard since that can trigger the oom killer. */
#define GFP_BALLOON \
@@ -383,7 +383,7 @@ static void xen_online_page(struct page *page)
static int xen_memory_notifier(struct notifier_block *nb, unsigned long val, void *v)
{
if (val == MEM_ONLINE)
- schedule_delayed_work(&balloon_worker, 0);
+ wake_up(&balloon_thread_wq);

return NOTIFY_OK;
}
@@ -508,18 +508,43 @@ static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
}

/*
- * As this is a work item it is guaranteed to run as a single instance only.
+ * Stop waiting if either state is not BP_EAGAIN and ballooning action is
+ * needed, or if the credit has changed while state is BP_EAGAIN.
+ */
+static bool balloon_thread_cond(enum bp_state state, long credit)
+{
+ if (state != BP_EAGAIN)
+ credit = 0;
+
+ return current_credit() != credit || kthread_should_stop();
+}
+
+/*
+ * As this is a kthread it is guaranteed to run as a single instance only.
* We may of course race updates of the target counts (which are protected
* by the balloon lock), or with changes to the Xen hard limit, but we will
* recover from these in time.
*/
-static void balloon_process(struct work_struct *work)
+static int balloon_thread(void *unused)
{
enum bp_state state = BP_DONE;
long credit;
+ unsigned long timeout;
+
+ set_freezable();
+ for (;;) {
+ if (state == BP_EAGAIN)
+ timeout = balloon_stats.schedule_delay * HZ;
+ else
+ timeout = 3600 * HZ;
+ credit = current_credit();

+ wait_event_interruptible_timeout(balloon_thread_wq,
+ balloon_thread_cond(state, credit), timeout);
+
+ if (kthread_should_stop())
+ return 0;

- do {
mutex_lock(&balloon_mutex);

credit = current_credit();
@@ -546,12 +571,7 @@ static void balloon_process(struct work_struct *work)
mutex_unlock(&balloon_mutex);

cond_resched();
-
- } while (credit && state == BP_DONE);
-
- /* Schedule more work if there is some still to be done. */
- if (state == BP_EAGAIN)
- schedule_delayed_work(&balloon_worker, balloon_stats.schedule_delay * HZ);
+ }
}

/* Resets the Xen limit, sets new target, and kicks off processing. */
@@ -559,7 +579,7 @@ void balloon_set_new_target(unsigned long target)
{
/* No need for lock. Not read-modify-write updates. */
balloon_stats.target_pages = target;
- schedule_delayed_work(&balloon_worker, 0);
+ wake_up(&balloon_thread_wq);
}
EXPORT_SYMBOL_GPL(balloon_set_new_target);

@@ -664,7 +684,7 @@ void free_xenballooned_pages(int nr_pages, struct page **pages)

/* The balloon may be too large now. Shrink it if needed. */
if (current_credit())
- schedule_delayed_work(&balloon_worker, 0);
+ wake_up(&balloon_thread_wq);

mutex_unlock(&balloon_mutex);
}
@@ -698,6 +718,8 @@ static void __init balloon_add_region(unsigned long start_pfn,

static int __init balloon_init(void)
{
+ struct task_struct *task;
+
if (!xen_domain())
return -ENODEV;

@@ -741,6 +763,12 @@ static int __init balloon_init(void)
}
#endif

+ task = kthread_run(balloon_thread, NULL, "xen-balloon");
+ if (IS_ERR(task)) {
+ pr_err("xen-balloon thread could not be started, ballooning will not work!\n");
+ return PTR_ERR(task);
+ }
+
/* Init the xen-balloon driver. */
xen_balloon_init();

--
2.30.2