Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

Paul E. McKenney
2020-08-18T16:13:58Z

On Tue, Aug 18, 2020 at 04:43:14PM +0200, Thomas Gleixner wrote:
> On Tue, Aug 18 2020 at 06:53, Paul E. McKenney wrote:
> > On Tue, Aug 18, 2020 at 09:43:44AM +0200, Michal Hocko wrote:
> >> Thomas had a good point that it doesn't really make much sense to
> >> optimize for flooders because that just makes them more effective.
> >
> > The point is not to make the flooders go faster, but rather for the
> > system to be robust in the face of flooders. Robust as in harder for
> > a flooder to OOM the system.
> >
> > And reducing the number of post-grace-period cache misses makes it
> > easier for the callback-invocation-time memory freeing to keep up with
> > the flooder, thus avoiding (or at least delaying) the OOM.
>
> Throttling the flooder is incresing robustness far more than reducing
> cache misses.

True, but it takes time to identify a flooding event that needs to be
throttled (as in milliseconds). This time cannot be made up.

And in the absence of a flooding event, the last thing you want to do
is to throttle call_rcu(), kfree_rcu(), and kvfree_rcu().

Thanx, Paul