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? -- Michal Hocko SUSE Labs
