Re: [PATCH v2 0/6] RCU tasks fixes for v6.9

Paul E. McKenney
2024-02-26T14:37:44Z

On Mon, Feb 26, 2024 at 02:56:06PM +0100, Frederic Weisbecker wrote:
> Le Fri, Feb 23, 2024 at 04:43:04PM -0800, Paul E. McKenney a écrit :
> > I do indeed mean doing cond_resched() mid-stream.
> >
> > One way to make this happen would be to do something like this:
> >
> > struct task_struct_anchor {
> > struct list_head tsa_list;
> > struct list_head tsa_adjust_list;
> > atomic_t tsa_ref; // Or use an appropriate API.
> > bool tsa_is_anchor;
> > }
> >
> > Each task structure would contain one of these, though there are a
> > number of ways to conserve space if needed.
> >
> > These anchors would be placed perhaps every 1,000 tasks or so. When a
> > traversal encountered one, it could atomic_inc_not_zero() the reference
> > count, and if that succeeded, exit the RCU read-side critical section and
> > do a cond_resched(). It could then enter a new RCU read-side critical
> > section, drop the reference, and continue.
> >
> > A traveral might container_of() its way from ->tsa_list to the
> > task_struct_anchor structure, then if ->tsa_is_anchor is false,
> > container_of() its way to the enclosing task structure.
> >
> > How to maintain proper spacing of the anchors?
> >
> > One way is to make the traversals do the checking. If the space between a
> > pair of anchors was to large or too small, it could add the first of the
> > pair to a list to be adjusted. This list could periodically be processed,
> > perhaps with more urgency if a huge gap had opened up.
> >
> > Freeing an anchor requires decrementing the reference count, waiting for
> > it to go to zero, removing the anchor, waiting for a grace period (perhaps
> > asynchronously), and only then freeing the anchor.
> >
> > Anchors cannot be moved, only added or removed.
> >
> > So it is possible. But is it reasonable? ;-)
>
> Wow! And this will need to be done both for process leaders (p->tasks)
> and for threads (p->thread_node) :-)

True enough! Your point being? ;-)

Thanx, Paul