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

[Michal Hocko]

On Tue 14-03-17 18:07:38, Yang Li wrote:
> 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.

Thanks! Can I assume your
Tested-by: Yang Li <pku.leo@xxxxxxxxx>

-- 
Michal Hocko
SUSE Labs