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

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Message-ID: <6fb206de-0185-4026-a6f5-1d150752d8d0@suse.cz>
Date: Mon, 16 Dec 2024 16:44:41 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Uladzislau Rezki <urezki@...il.com>
Cc: 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 12/16/24 16:41, Uladzislau Rezki wrote:
> 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.

Maybe we could change it to use CONFIG_SLUB_TINY as that has similar use
case (less memory usage on low memory system, tradeoff for worse performance).

> 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