Re: [PATCH RFC v1] mm: is_mem_section_removable() overhaul

[Michal Hocko <mhocko@kernel.org>]

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