linux-kernel - Re: [PATCH v2 0/5] Move kvfree

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Message-ID: <Z2BKLzOGv_Ki_7fj@pc636>
Date: Mon, 16 Dec 2024 16:41:35 +0100
From: Uladzislau Rezki <urezki@...il.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Uladzislau Rezki <urezki@...il.com>, linux-mm@...ck.org,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	RCU <rcu@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Hyeonggon Yoo <42.hyeyoo@...il.com>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH v2 0/5] Move kvfree_rcu() into SLAB (v2)

On Mon, Dec 16, 2024 at 03:20:44PM +0100, Vlastimil Babka wrote:
> On 12/16/24 12:03, Uladzislau Rezki wrote:
> > On Sun, Dec 15, 2024 at 06:30:02PM +0100, Vlastimil Babka wrote:
> >> On 12/12/24 19:02, Uladzislau Rezki (Sony) wrote:
> >> > Hello!
> >> > 
> >> > This is v2. It is based on the Linux 6.13-rc2. The first version is
> >> > here:
> >> > 
> >> > https://lore.kernel.org/linux-mm/20241210164035.3391747-4-urezki@gmail.com/T/
> >> > 
> >> > The difference between v1 and v2 is that, the preparation process is
> >> > done in original place instead and after that there is one final move.
> >> 
> >> Looks good, will include in slab/for-next
> >> 
> >> I think patch 5 should add more explanation to the commit message - the
> >> subthread started by Christoph could provide content :) Can you summarize so
> >> I can amend the commit log?
> >> 
> > I will :)
> > 
> >> Also how about a followup patch moving the rcu-tiny implementation of
> >> kvfree_call_rcu()?
> >> 
> > As, Paul already noted, it would make sense. Or just remove a tiny
> > implementation.
> 
> AFAICS tiny rcu is for !SMP systems. Do they benefit from the "full"
> implementation with all the batching etc or would that be unnecessary overhead?
> 
Yes, it is for a really small systems with low amount of memory. I see
only one overhead it is about driving objects in pages. For a small
system it can be critical because we allocate.

>From the other hand, for a tiny variant we can modify the normal variant
by bypassing batching logic, thus do not consume memory(for Tiny case)
i.e. merge it to a normal kvfree_rcu() path.

After that we do not depend on CONFIG_RCU_TINY option. Probably we need
also to perform some adaptation of regular kvfree_rcu() for a single CPU
system.

--
Uladzislau Rezki