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

[Michal Hocko]

On Wed 01-04-20 12:09:29, Daniel Jordan wrote:
> On Wed, Apr 01, 2020 at 06:00:48PM +0200, Michal Hocko wrote:
> > On Wed 01-04-20 17:50:22, David Hildenbrand wrote:
> > > On 01.04.20 17:42, Michal Hocko wrote:
> > > > I am sorry but I have completely missed this patch.
> > > > 
> > > > On Wed 11-03-20 20:38:48, Shile Zhang wrote:
> > > >> When 'CONFIG_DEFERRED_STRUCT_PAGE_INIT' is set, 'pgdatinit' kthread will
> > > >> initialise the deferred pages with local interrupts disabled. It is
> > > >> introduced by commit 3a2d7fa8a3d5 ("mm: disable interrupts while
> > > >> initializing deferred pages").
> > > >>
> > > >> On machine with NCPUS <= 2, the 'pgdatinit' kthread could be bound to
> > > >> the boot CPU, which could caused the tick timer long time stall, system
> > > >> jiffies not be updated in time.
> > > >>
> > > >> The dmesg shown that:
> > > >>
> > > >>     [    0.197975] node 0 initialised, 32170688 pages in 1ms
> > > >>
> > > >> Obviously, 1ms is unreasonable.
> > > >>
> > > >> Now, fix it by restore in the pending interrupts for every 32*1204 pages
> > > >> (128MB) initialized, give the chance to update the systemd jiffies.
> > > >> The reasonable demsg shown likes:
> > > >>
> > > >>     [    1.069306] node 0 initialised, 32203456 pages in 894ms
> > > >>
> > > >> Fixes: 3a2d7fa8a3d5 ("mm: disable interrupts while initializing deferred pages").
> > > > 
> > > > I dislike this solution TBH. It effectivelly conserves the current code
> > > > and just works around the problem. Why do we hold the IRQ lock here in
> > > > the first place? This is an early init code and a very limited code is
> > > > running at this stage. Certainly nothing memory hotplug related which
> > > > should be the only path really interested in the resize lock AFAIR.
> > > 
> > > Yeah, I don't think ACPI and friends are up yet.
> > 
> > Just to save somebody time to check. The deferred initialization blocks
> > the further boot until all workders are done - see page_alloc_init_late
> > (kernel_init path).
> 
> Ha, I just finished following all the hotplug paths to check this out, and as
> you all know there are a *lot* :-) Well at least we're in agreement.

Good to have it double checked!
 
> > > > This needs a double checking but I strongly believe that the lock can be
> > > > simply dropped in this path.
> 
> This is what my fix does, it limits the time the resize lock is held.

Just remove it from the deferred intialization and add a comment that we
deliberately not taking the lock here because abc

-- 
Michal Hocko
SUSE Labs