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

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Date:   Fri, 20 Dec 2019 08:22:02 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Tejun Heo <tj@...nel.org>,
        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 12/20/19 7:12 AM, 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?

I definitely seen expensive per-cpu updates in the stack.
(SNMP counters, or per-cpu stats for packets/bytes counters)

It might be nice to have an option to use 2M pages.

(I recall sending some patches in the past about using high-order pages for vmalloc,
but this went nowhere)