Re: [PATCH v2 1/2] mm: Move page struct poisoning to CONFIG_DEBUG_VM_PAGE_INIT_POISON

[Michal Hocko <mhocko@kernel.org>]

On Thu 06-09-18 08:41:52, Alexander Duyck wrote:
> On Thu, Sep 6, 2018 at 8:13 AM Michal Hocko <mhocko@kernel.org> wrote:
> >
> > On Thu 06-09-18 07:59:03, Dave Hansen wrote:
> > > On 09/05/2018 10:47 PM, Michal Hocko wrote:
> > > > why do you have to keep DEBUG_VM enabled for workloads where the boot
> > > > time matters so much that few seconds matter?
> > >
> > > There are a number of distributions that run with it enabled in the
> > > default build.  Fedora, for one.  We've basically assumed for a while
> > > that we have to live with it in production environments.
> > >
> > > So, where does leave us?  I think we either need a _generic_ debug
> > > option like:
> > >
> > >       CONFIG_DEBUG_VM_SLOW_AS_HECK
> > >
> > > under which we can put this an other really slow VM debugging.  Or, we
> > > need some kind of boot-time parameter to trigger the extra checking
> > > instead of a new CONFIG option.
> >
> > I strongly suspect nobody will ever enable such a scary looking config
> > TBH. Besides I am not sure what should go under that config option.
> > Something that takes few cycles but it is called often or one time stuff
> > that takes quite a long but less than aggregated overhead of the former?
> >
> > Just consider this particular case. It basically re-adds an overhead
> > that has always been there before the struct page init optimization
> > went it. The poisoning just returns it in a different form to catch
> > potential left overs. And we would like to have as many people willing
> > to running in debug mode to test for those paths because they are
> > basically impossible to review by the code inspection. More importantnly
> > the major overhead is boot time so my question still stands. Is this
> > worth a separate config option almost nobody is going to enable?
> >
> > Enabling DEBUG_VM by Fedora and others serves us a very good testing
> > coverage and I appreciate that because it has generated some useful bug
> > reports. Those people are paying quite a lot of overhead in runtime
> > which can aggregate over time is it so much to ask about one time boot
> > overhead?
> 
> The kind of boot time add-on I saw as a result of this was about 170
> seconds, or 2 minutes and 50 seconds on a 12TB system.

Just curious. How long does it take to get from power on to even reaach
boot loader on that machine... ;)

> I spent a
> couple minutes wondering if I had built a bad kernel or not as I was
> staring at a dead console the entire time after the grub prompt since
> I hit this so early in the boot. That is the reason why I am so eager
> to slice this off and make it something separate. I could easily see
> this as something that would get in the way of other debugging that is
> going on in a system.

But you would get the same overhead a kernel release ago when the
memmap init optimization was merged. So you are basically back to what
we used to have for years. Unless I misremember.

> If we don't want to do a config option, then what about adding a
> kernel parameter to put a limit on how much memory we will initialize
> like this before we just start skipping it. We could put a default
> limit on it like 256GB and then once we cross that threshold we just
> don't bother poisoning any more memory. With that we would probably be
> able to at least cover most of the early memory init, and that value
> should cover most systems without getting into delays on the order of
> minutes.

No, this will defeat the purpose of the check.
-- 
Michal Hocko
SUSE Labs