linux-kernel - Re: [PATCH v2 09/16] rcu/tree: Maintain separate array for vmalloc ptrs

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Date:   Thu, 18 Jun 2020 19:35:20 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Joel Fernandes <joel@...lfernandes.org>,
        RCU <rcu@...r.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH v2 09/16] rcu/tree: Maintain separate array for vmalloc
 ptrs

> > 
> > I don't think that replacing direct function calls with indirect function
> > calls is a great suggestion with the current state of play around branch
> > prediction.
> > 
> > I'd suggest:
> > 
> >  			rcu_lock_acquire(&rcu_callback_map);
> > 			trace_rcu_invoke_kfree_bulk_callback(rcu_state.name,
> > 				bkvhead[i]->nr_records, bkvhead[i]->records);
> >  			if (i == 0) {
> >  				kfree_bulk(bkvhead[i]->nr_records,
> >  					bkvhead[i]->records);
> >  			} else {
> >  				for (j = 0; j < bkvhead[i]->nr_records; j++) {
> >  					vfree(bkvhead[i]->records[j]);
> >  				}
> >  			}
> >  			rcu_lock_release(&rcu_callback_map);
> > 
> > But I'd also suggest a vfree_bulk be added.  There are a few things
> > which would be better done in bulk as part of the vfree process
> > (we batch them up already, but i'm sure we could do better).
> 
> I suspect that he would like to keep the tracing.
> 
> It might be worth trying the branches, given that they would be constant
> and indexed by "i".  The compiler might well remove the indirection.
> 
> The compiler guys brag about doing so, which of course might or might
> not have any correlation to a given compiler actually doing so.  :-/
> 
> Having a vfree_bulk() might well be useful, but I would feel more
> confidence in that if there were other callers of kfree_bulk().
>
Hmm... I think replacing that with vfree_bulk() is a good idea though.

> 
> But again, either way, future work as far as this series is concerned.
> 
What do you mean: is concerned?

We are planning to implement kfree_rcu() to be integrated directly into
SLAB: SLAB, SLUB, SLOB. So, there are plenty of future work :)

--
Vlad Rezki