Re: [PATCH v2 3/3] mm/page_alloc: Remotely drain per-cpu lists

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Dec 14, 2021 at 12:42:58PM +0100, Christoph Lameter wrote:
> On Tue, 14 Dec 2021, Marcelo Tosatti wrote:
> 
> > > downside is that there will be a performance penalty if an application
> > > running on a NOHZ_FULL CPU is page allocator intensive for whatever
> > > reason.  However, I guess this is unlikely because if there was a lot
> > > of kernel activity for a NOHZ_FULL CPU, the vmstat shepherd would also
> > > cause interference.
> >
> > Yes, it does, and its being fixed:
> >
> > https://lkml.org/lkml/2021/12/8/663
> >
> > Honestly i am not sure whether the association between a nohz_full CPU
> > and "should be mostly in userspace" is desired. The RCU solution
> > would be more generic. As Nicolas mentioned, for the usecases in
> > questions, either solution is OK.
> >
> > Thomas, Frederic, Christoph, do you have any opinion on this ?
> 
> Applications running would ideally have no performance penalty and there
> is no  issue with kernel activity unless the application is in its special
> low latency loop. NOHZ is currently only activated after spinning in that
> loop for 2 seconds or so. Would be best to be able to trigger that
> manually somehow.

Can add a task isolation feature to do that.

> And I would prefer to be able to run the whole system as
> NOHZ and have the ability to selectively enable the quiet mode if a
> process requires it for its processing.

IIRC Frederic has been working on that.

Thanks.