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

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Message-ID: <ZdyYdhZtNuJp3h-a@localhost.localdomain>
Date: Mon, 26 Feb 2024 14:56:06 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
	rcu@...r.kernel.org, Neeraj Upadhyay <Neeraj.Upadhyay@....com>
Subject: Re: [PATCH v2 0/6] RCU tasks fixes for v6.9

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) :-)