On Wed 22-01-20 19:46:15, David Hildenbrand wrote: > On 22.01.20 19:38, Michal Hocko wrote: [...] > > How exactly is check + offline more optimal then offline which makes > > check as its first step? I will get to your later points after this is > > clarified. > > Scanning (almost) lockless is more efficient than bouncing back and > forth with the device_hotplug_lock, mem_hotplug_lock, cpu_hotplug_lock > and zone locks - as far as I understand. All but the zone lock shouldn't be really contended and as such shouldn't cause any troubles. zone->lock really depends on the page allocator usage of course. But as soon as we have a contention then it is just more likely that the result is less reliable. I would be also really curious about how much actual time could be saved by this - some real numbers - because hotplug operations shouldn't happen so often that this would stand out. At least that is my understanding. > And as far as I understood, that was the whole reason of the original > commit. Well, I have my doubts but it might be just me and I might be wrong. My experience from a large part of the memory hotplug functionality is that it was driven by a good intention but without a due diligence to think behind the most obvious usecase. Having a removable flag on the memblock sounds like a neat idea of course. But an inherently racy flag is just borderline useful. Anyway, I will stop at this moment and wait for real usecases. Thanks! -- Michal Hocko SUSE Labs
