Re: [External] Re: [RFC] mm: add new syscall pidfd_set_mempolicy()

Michal Hocko
2022-10-13T13:17:49Z

On Thu 13-10-22 20:50:48, Zhongkun He wrote:
> > > Hi Michal
> > >
> > > Could we try to change the MPOL_F_SHARED flag to MPOL_F_STATIC to
> > > mark static mempolicy which cannot be freed, and mpol_needs_cond_ref
> > > can use MPOL_F_STATIC to avoid freeing the static mempolicy.
> >
> > Wouldn't it make more sense to get rid of a different treatment and
> > treat all memory policies the same way?
>
> I found a case, not sure if it makes sense. If there is no policy
> in task->mempolicy, the use of atomic_{inc,dec} can be skiped
> according to MPOL_F_STATIC. Atomic_{inc,dec} in hot path may reduces
> performance.

I would start with a simple conversion and do any potential
optimizations on top of that based on actual numbers. Maybe we can
special case default_policy to avoid reference counting a default (no
policy case). A simple check for pol == &default_policy should be
negligible.
--
Michal Hocko
SUSE Labs