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

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Message-ID: <20191220151239.GE2914998@devbig004.ftw2.facebook.com>
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