mm: move pcp and lru-pcp draining into single wq

We currently have 2 specific WQ_RECLAIM workqueues in the mm code.
vmstat_wq for updating pcp stats and lru_add_drain_wq dedicated to drain
per cpu lru caches.  This seems more than necessary because both can run
on a single WQ.  Both do not block on locks requiring a memory
allocation nor perform any allocations themselves.  We will save one
rescuer thread this way.

On the other hand drain_all_pages() queues work on the system wq which
doesn't have rescuer and so this depend on memory allocation (when all
workers are stuck allocating and new ones cannot be created).

Initially we thought this would be more of a theoretical problem but
Hugh Dickins has reported:

: 4.11-rc has been giving me hangs after hours of swapping load.  At
: first they looked like memory leaks ("fork: Cannot allocate memory");
: but for no good reason I happened to do "cat /proc/sys/vm/stat_refresh"
: before looking at /proc/meminfo one time, and the stat_refresh stuck
: in D state, waiting for completion of flush_work like many kworkers.
: kthreadd waiting for completion of flush_work in drain_all_pages().

This worker should be using WQ_RECLAIM as well in order to guarantee a
forward progress.  We can reuse the same one as for lru draining and
vmstat.

Link: http://lkml.kernel.org/r/20170307131751.24936-1-mhocko@kernel.org
Signed-off-by: Michal Hocko <mhocko@suse.com>
Suggested-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Mel Gorman <mgorman@suse.de>
Tested-by: Yang Li <pku.leo@gmail.com>
Tested-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 6 deletions

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 89f95396ec46..809025ed97ea 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1552,7 +1552,6 @@ static const struct file_operations proc_vmstat_file_operations = {
 #endif /* CONFIG_PROC_FS */
 
 #ifdef CONFIG_SMP
-static struct workqueue_struct *vmstat_wq;
 static DEFINE_PER_CPU(struct delayed_work, vmstat_work);
 int sysctl_stat_interval __read_mostly = HZ;
 
@@ -1623,7 +1622,7 @@ static void vmstat_update(struct work_struct *w)
                  * to occur in the future. Keep on running the
                  * update worker thread.
                  */
-                queue_delayed_work_on(smp_processor_id(), vmstat_wq,
+                queue_delayed_work_on(smp_processor_id(), mm_percpu_wq,
                                 this_cpu_ptr(&vmstat_work),
                                 round_jiffies_relative(sysctl_stat_interval));
         }
@@ -1702,7 +1701,7 @@ static void vmstat_shepherd(struct work_struct *w)
                 struct delayed_work *dw = &per_cpu(vmstat_work, cpu);
 
                 if (!delayed_work_pending(dw) && need_update(cpu))
-                        queue_delayed_work_on(cpu, vmstat_wq, dw, 0);
+                        queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0);
         }
         put_online_cpus();
 
@@ -1718,7 +1717,6 @@ static void __init start_shepherd_timer(void)
                 INIT_DEFERRABLE_WORK(per_cpu_ptr(&vmstat_work, cpu),
                         vmstat_update);
 
-        vmstat_wq = alloc_workqueue("vmstat", WQ_FREEZABLE|WQ_MEM_RECLAIM, 0);
         schedule_delayed_work(&shepherd,
                 round_jiffies_relative(sysctl_stat_interval));
 }
@@ -1764,11 +1762,16 @@ static int vmstat_cpu_dead(unsigned int cpu)
 
 #endif
 
+struct workqueue_struct *mm_percpu_wq;
+
 void __init init_mm_internals(void)
 {
-#ifdef CONFIG_SMP
-        int ret;
+        int ret __maybe_unused;
 
+        mm_percpu_wq = alloc_workqueue("mm_percpu_wq",
+                                       WQ_FREEZABLE|WQ_MEM_RECLAIM, 0);
+
+#ifdef CONFIG_SMP
         ret = cpuhp_setup_state_nocalls(CPUHP_MM_VMSTAT_DEAD, "mm/vmstat:dead",
                                         NULL, vmstat_cpu_dead);
         if (ret < 0)