linux-kernel - Re: [PATCH RFC 4/4] UNFINISHED mm, fs: use kmem_cache_charge() in path

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Date: Tue, 12 Mar 2024 10:22:54 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Roman Gushchin <roman.gushchin@...ux.dev>,
 Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>, Jeff Layton <jlayton@...nel.org>,
 Chuck Lever <chuck.lever@...cle.com>, Kees Cook <kees@...nel.org>,
 Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
 David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Hyeonggon Yoo <42.hyeyoo@...il.com>, Johannes Weiner <hannes@...xchg.org>,
 Michal Hocko <mhocko@...nel.org>, Shakeel Butt <shakeelb@...gle.com>,
 Muchun Song <muchun.song@...ux.dev>, Alexander Viro
 <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>,
 Jan Kara <jack@...e.cz>, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 cgroups@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH RFC 4/4] UNFINISHED mm, fs: use kmem_cache_charge() in
 path_openat()

On 3/1/24 19:53, Roman Gushchin wrote:
> On Fri, Mar 01, 2024 at 09:51:18AM -0800, Linus Torvalds wrote:
>> What I *think* I'd want for this case is
>> 
>>  (a) allow the accounting to go over by a bit
>> 
>>  (b) make sure there's a cheap way to ask (before) about "did we go
>> over the limit"
>> 
>> IOW, the accounting never needed to be byte-accurate to begin with,
>> and making it fail (cheaply and early) on the next file allocation is
>> fine.
>> 
>> Just make it really cheap. Can we do that?
>> 
>> For example, maybe don't bother with the whole "bytes and pages"
>> stuff. Just a simple "are we more than one page over?" kind of
>> question. Without the 'stock_lock' mess for sub-page bytes etc
>> 
>> How would that look? Would it result in something that can be done
>> cheaply without locking and atomics and without excessive pointer
>> indirection through many levels of memcg data structures?
> 
> I think it's possible and I'm currently looking into batching charge,
> objcg refcnt management and vmstats using per-task caching. It should
> speed up things for the majority of allocations.
> For allocations from an irq context and targeted allocations
> (where the target memcg != memcg of the current task) we'd probably need to
> keep the old scheme. I hope to post some patches relatively soon.

Do you think this will work on top of this series, i.e. patches 1+2 could be
eventually put to slab/for-next after the merge window, or would it
interfere with your changes?

> I tried to optimize the current implementation but failed to get any
> significant gains. It seems that the overhead is very evenly spread across
> objcg pointer access, charge management, objcg refcnt management and vmstats.
> 
> Thanks!