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

[Yang Li]

On Mon, Mar 13, 2017 at 4:58 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> On Fri 10-03-17 17:31:56, Yang Li wrote:
>> On Tue, Mar 7, 2017 at 7:17 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>> > From: Michal Hocko <mhocko@xxxxxxxx>
>> >
>> > 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)

I did a test on arm64.  This do fix the warnings.

Regards,
Leo