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

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Message-ID: <alpine.DEB.2.21.1912201536120.16819@www.lameter.com>
Date:   Fri, 20 Dec 2019 15:36:51 +0000 (UTC)
From:   Christopher Lameter <cl@...ux.com>
To:     Tejun Heo <tj@...nel.org>
cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Björn Töpel <bjorn.topel@...il.com>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Dennis Zhou <dennis@...nel.org>
Subject: Re: Percpu variables, benchmarking, and performance weirdness

On Fri, 20 Dec 2019, Tejun Heo wrote:

> 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?

And there are some accesses from remote processors to per cpu ares of
other cpus. If those are in the same cacheline then those will cause
additional latencies.