On Thu 06-09-18 08:41:52, Alexander Duyck wrote: > On Thu, Sep 6, 2018 at 8:13 AM Michal Hocko <mhocko@xxxxxxxxxx> 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
