linux-kernel - Re: Percpu variables, benchmarking, and performance weirdness

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]

Date:   Fri, 20 Dec 2019 07:12:39 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Björn Töpel <bjorn.topel@...il.com>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Christoph Lameter <cl@...ux.com>,
        Dennis Zhou <dennis@...nel.org>
Subject: Re: Percpu variables, benchmarking, and performance weirdness

On Fri, Dec 20, 2019 at 10:34:20AM +0100, Jesper Dangaard Brouer wrote:
> > So, my question to the uarch/percpu folks out there: Why are percpu
> > accesses (%gs segment register) more expensive than regular global
> > variables in this scenario.
> 
> I'm also VERY interested in knowing the answer to above question!?
> (Adding LKML to reach more people)

No idea.  One difference is that percpu accesses are through vmap area
which is mapped using 4k pages while global variable would be accessed
through the fault linear mapping.  Maybe you're getting hit by tlb
pressure?

Thanks.

-- 
tejun