Re: [PATCH] mm: move pcp and lru-pcp drainging into single wq

[Michal Hocko <mhocko@kernel.org>]

On Fri 10-03-17 17:31:56, Yang Li wrote:
> On Tue, Mar 7, 2017 at 7:17 AM, Michal Hocko <mhocko@kernel.org> wrote:
> > From: Michal Hocko <mhocko@suse.com>
> >
> > 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). This is
> > not critical as there should be somebody invoking the OOM killer (e.g.
> > the forking worker) and get the situation unstuck and eventually
> > performs the draining. Quite annoying though. This worker should be
> > using WQ_RECLAIM as well. We can reuse the same one as for lru draining
> > and vmstat.
> >
> > Changes since v1
> > - rename vmstat_wq to mm_percpu_wq - per Mel
> > - make sure we are not trying to enqueue anything while the WQ hasn't
> >   been intialized yet. This shouldn't happen because the initialization
> >   is done from an init code but some init section might be triggering
> >   those paths indirectly so just warn and skip the draining in that case
> >   per Vlastimil
> 
> So what's the plan if this really happens?  Shall we put the
> initialization of the mm_percpu_wq earlier?

yes

> Or if it is really harmless we can probably remove the warnings.

Yeah, it is harmless but if we can move it earlier then it would be
prefferable to fix this.

> 
> I'm seeing this on arm64 with a linux-next tree:
[...]
> [    0.279000] [<ffffff80081636bc>] drain_all_pages+0x244/0x25c
> [    0.279065] [<ffffff80081c675c>] start_isolate_page_range+0x14c/0x1f0
> [    0.279137] [<ffffff8008166a48>] alloc_contig_range+0xec/0x354
> [    0.279203] [<ffffff80081c6c5c>] cma_alloc+0x100/0x1fc
> [    0.279263] [<ffffff8008481714>] dma_alloc_from_contiguous+0x3c/0x44
> [    0.279336] [<ffffff8008b25720>] atomic_pool_init+0x7c/0x208
> [    0.279399] [<ffffff8008b258f0>] arm64_dma_init+0x44/0x4c
> [    0.279461] [<ffffff8008083144>] do_one_initcall+0x38/0x128
> [    0.279525] [<ffffff8008b20d30>] kernel_init_freeable+0x1a0/0x240
> [    0.279596] [<ffffff8008807778>] kernel_init+0x10/0xfc
> [    0.279654] [<ffffff8008082b70>] ret_from_fork+0x10/0x20

The following should address this. I didn't get to test it yet though.
---
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 21ee5503c702..8362dca071cb 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -32,6 +32,8 @@ struct user_struct;
 struct writeback_control;
 struct bdi_writeback;
 
+void init_mm_internals(void);
+
 #ifndef CONFIG_NEED_MULTIPLE_NODES	/* Don't use mapnrs, do it properly */
 extern unsigned long max_mapnr;
 
diff --git a/init/main.c b/init/main.c
index 51aa8f336819..c72d35250e84 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1023,6 +1023,8 @@ static noinline void __init kernel_init_freeable(void)
 
 	workqueue_init();
 
+	init_mm_internals();
+
 	do_pre_smp_initcalls();
 	lockup_detector_init();
 
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 4bbc775f9d08..d0871fc1aeca 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1762,7 +1762,7 @@ static int vmstat_cpu_dead(unsigned int cpu)
 
 struct workqueue_struct *mm_percpu_wq;
 
-static int __init setup_vmstat(void)
+void __init init_mm_internals(void)
 {
 	int ret __maybe_unused;
 
@@ -1792,9 +1792,7 @@ static int __init setup_vmstat(void)
 	proc_create("vmstat", S_IRUGO, NULL, &proc_vmstat_file_operations);
 	proc_create("zoneinfo", S_IRUGO, NULL, &proc_zoneinfo_file_operations);
 #endif
-	return 0;
 }
-module_init(setup_vmstat)
 
 #if defined(CONFIG_DEBUG_FS) && defined(CONFIG_COMPACTION)
 

-- 
Michal Hocko
SUSE Labs