Re: [PATCH] mm: Free per cpu pages async to shorten program exit time

Daniel Jordan
2021-10-13T17:43:13Z

(I'm just back from vacation)

On Mon, Oct 11, 2021 at 11:28:10AM +0200, Michal Hocko wrote:
> On Fri 08-10-21 10:17:50, David Hildenbrand wrote:
> > On 08.10.21 08:39, ultrachin@163.com wrote:
> > > From: chen xiaoguang <xiaoggchen@tencent.com>
> > >
> > > The exit time is long when program allocated big memory and
> > > the most time consuming part is free memory which takes 99.9%
> > > of the total exit time. By using async free we can save 25% of
> > > exit time.
> > >
> > > Signed-off-by: chen xiaoguang <xiaoggchen@tencent.com>
> > > Signed-off-by: zeng jingxiang <linuszeng@tencent.com>
> > > Signed-off-by: lu yihui <yihuilu@tencent.com>
> >
> > I recently discussed with Claudio if it would be possible to tear down the
> > process MM deferred, because for some use cases (secure/encrypted
> > virtualization, very large mmaps) tearing down the page tables is already
> > the much more expensive operation.
> >
> > There is mmdrop_async(), and I wondered if one could reuse that concept when
> > tearing down a process -- I didn't look into feasibility, however, so it's
> > just some very rough idea.
>
> This is not a new problem. Large process tear down can take ages. The
> primary road block has been accounting. This lot of work has to be
> accounted to the proper domain (e.g. cpu cgroup). A deferred and
> properly accounted context implementation is still lacking AFAIK.

Right, still doesn't exist. It's coming though, and there was a session
on it at LPC this year[1].

> I have a vague recollection we have padata framework but I am not sure
> anybody has explored this to be used for the address space shutdown.
> IIRC Daniel Jordan was active in that area.

Yeah, address space teardown is one of the things we want to use padata
for. It's on the list.

[1] https://linuxplumbersconf.org/event/11/contributions/1041/