Re: [PATCH v3] mm: fix tick timer stall during deferred page init

[Shile Zhang]

On 2020/3/27 03:36, Pavel Tatashin wrote:
> I agree with Daniel, we should look into approach where
> pgdat_resize_lock is taken only for the duration of updating tracking
> values such as pgdat->first_deferred_pfn (perhaps we would need to add
> another tracker that would show chunks that are currently being worked
> on).
>
> The vast duration of struct page initialization process should happen
> outside of this lock, and only be taken when we update globally seen
> data structures: lists, tracking variables. This way we can solve
> several problems: 1. allow interrupt threads to grow zones if
> required. 2. keep jiffies happy. 3. allow future scaling when we will
> add inner node threads to initialize struct pages (i.e. ktasks from
> Daniel).

It make sense, looking forward to the inner node parallel init.

@Daniel
Is there schedule about ktasks?

Thanks!

> Pasha
>
> On Thu, Mar 26, 2020 at 2:58 PM Daniel Jordan
> <daniel.m.jordan@xxxxxxxxxx> wrote:
> > On Thu, Mar 19, 2020 at 03:05:12PM -0400, Daniel Jordan wrote:
> > > Regardless,
> > > Reviewed-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
> > Darn, I spoke too soon.
> >
> > On a two-socket Xeon, smaller values of TICK_PAGE_COUNT caused the deferred
> > init timestamp to grow by over 25%.  This was with pgdatinit0 bound to the
> > timer interrupt CPU to make sure the issue always reproduces.
> >
> >                 TICK_PAGE_COUNT     node 0 deferred
> >                                     init time (ms)
> >                 ---------------     ---------------
> >                            4096                 610
> >                            8192                 587
> >                           16384                 487
> >                           32768                 480    // used in the patch
> >
> > Instead of trying to find a constant that lets the timer interrupt run often
> > enough, I think a better way forward is to reconsider how we handle the resize
> > lock.  I plan to prototype something and reply back with what I get.

Yes, the time spend of pages init depends on the CPU frequency,
and the jiffies update controlled by HZ, so it's hard to find a general 
constant.

It seems we need a bigger refactors about the lock to get a better solution.