linux-kernel - Re: [PATCH 00/40] Memory allocation profiling

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ZFAsm0XTqC//f4FP@P9FQF9L96D>
Date:   Mon, 1 May 2023 14:18:19 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
        mhocko@...e.com, vbabka@...e.cz, hannes@...xchg.org,
        mgorman@...e.de, dave@...olabs.net, willy@...radead.org,
        liam.howlett@...cle.com, corbet@....net, void@...ifault.com,
        peterz@...radead.org, juri.lelli@...hat.com, ldufour@...ux.ibm.com,
        catalin.marinas@....com, will@...nel.org, arnd@...db.de,
        tglx@...utronix.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
        x86@...nel.org, peterx@...hat.com, david@...hat.com,
        axboe@...nel.dk, mcgrof@...nel.org, masahiroy@...nel.org,
        nathan@...nel.org, dennis@...nel.org, tj@...nel.org,
        muchun.song@...ux.dev, rppt@...nel.org, paulmck@...nel.org,
        pasha.tatashin@...een.com, yosryahmed@...gle.com,
        yuzhao@...gle.com, dhowells@...hat.com, hughd@...gle.com,
        andreyknvl@...il.com, keescook@...omium.org,
        ndesaulniers@...gle.com, gregkh@...uxfoundation.org,
        ebiggers@...gle.com, ytcoode@...il.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        bristot@...hat.com, vschneid@...hat.com, cl@...ux.com,
        penberg@...nel.org, iamjoonsoo.kim@....com, 42.hyeyoo@...il.com,
        glider@...gle.com, elver@...gle.com, dvyukov@...gle.com,
        shakeelb@...gle.com, songmuchun@...edance.com, jbaron@...mai.com,
        rientjes@...gle.com, minchan@...gle.com, kaleshsingh@...gle.com,
        kernel-team@...roid.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
        linux-arch@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-modules@...r.kernel.org,
        kasan-dev@...glegroups.com, cgroups@...r.kernel.org
Subject: Re: [PATCH 00/40] Memory allocation profiling

On Mon, May 01, 2023 at 03:37:58PM -0400, Kent Overstreet wrote:
> On Mon, May 01, 2023 at 11:14:45AM -0700, Roman Gushchin wrote:
> > It's a good idea and I generally think that +25-35% for kmalloc/pgalloc
> > should be ok for the production use, which is great!
> > In the reality, most workloads are not that sensitive to the speed of
> > memory allocation.
> 
> :)
> 
> My main takeaway has been "the slub fast path is _really_ fast". No
> disabling of preemption, no atomic instructions, just a non locked
> double word cmpxchg - it's a slick piece of work.
> 
> > > For kmalloc, the overhead is low because after we create the vector of
> > > slab_ext objects (which is the same as what memcg_kmem does), memory
> > > profiling just increments a lazy counter (which in many cases would be
> > > a per-cpu counter).
> > 
> > So does kmem (this is why I'm somewhat surprised by the difference).
> > 
> > > memcg_kmem operates on cgroup hierarchy with
> > > additional overhead associated with that. I'm guessing that's the
> > > reason for the big difference between these mechanisms but, I didn't
> > > look into the details to understand memcg_kmem performance.
> > 
> > I suspect recent rt-related changes and also the wide usage of
> > rcu primitives in the kmem code. I'll try to look closer as well.
> 
> Happy to give you something to compare against :)

To be fair, it's not an apple-to-apple comparison, because:
1) memcgs are organized in a tree, these days usually with at least 3 layers,
2) memcgs are dynamic. In theory a task can be moved to a different
   memcg while performing a (very slow) allocation, and the original
   memcg can be released. To prevent this we have to perform a lot
   of operations which you can happily avoid.

That said, there is clearly a place for optimization, so thank you
for indirectly bringing this up.

Thanks!