linux-kernel - Re: [PATCH RFC] rcu/kfree: Do not request RCU when not needed

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Message-ID: <Y2O8k2U+ACr1N6Fe@pc638.lan>
Date:   Thu, 3 Nov 2022 14:05:23 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] rcu/kfree: Do not request RCU when not needed

On Thu, Nov 03, 2022 at 01:44:51PM +0100, Uladzislau Rezki wrote:
> > 
> > > Though I am thinking, workqueue context is normally used to invoke
> > > code that can block, and would the issue you mentioned affect those as
> > > well, or affect RCU when those non-RCU work items block. So for
> > > example, when other things in the system that can queue things on the
> > > system_wq and block.  (I might be throwing darts in the dark).
> > > 
> > > To be safe, we can implement your suggestion which is basically a form
> > > of my initial patch.
> > > 
> > > Should we add Tejun to the thread?
> > 
> > Let's get organized first, but that would be a good thing.  Or I could
> > reach out to Tejun internally.
> > 
> > For but one thing to get organized about, maybe kfree_rcu() should be
> > using a workqueue with the WQ_MEM_RECLAIM flag set.
> > 
> It can be as an option to consider. Because such workqueue has some
> special priority for better handling of memory releasing. I can have
> a look at it closer to see how kvfree_rcu() works if it goes with WQ_MEM_RECLAIM.
> 
An extra note. It would work well with posted patch because we can
directly queue the reclaim work to the WQ_MEM_RECLAIM queue.

As for now RCU-core kthreads like: rcugp, rcuop use "regular"
queue. I think system_wq one.

--
Uladzislau Rezki