netdev - Re: Page allocator bottleneck

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Date:   Mon, 23 Apr 2018 21:10:33 +0800
From:   Aaron Lu <aaron.lu@...el.com>
To:     Tariq Toukan <tariqt@...lanox.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        David Miller <davem@...emloft.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Alexei Starovoitov <ast@...com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Eran Ben Elisha <eranbe@...lanox.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: Page allocator bottleneck

On Mon, Apr 23, 2018 at 11:54:57AM +0300, Tariq Toukan wrote:
> Hi,
> 
> I ran my tests with your patches.
> Initial BW numbers are significantly higher than I documented back then in
> this mail-thread.
> For example, in driver #2 (see original mail thread), with 6 rings, I now
> get 92Gbps (slightly less than linerate) in comparison to 64Gbps back then.
> 
> However, there were many kernel changes since then, I need to isolate your
> changes. I am not sure I can finish this today, but I will surely get to it
> next week after I'm back from vacation.
> 
> Still, when I increase the scale (more rings, i.e. more cpus), I see that
> queued_spin_lock_slowpath gets to 60%+ cpu. Still high, but lower than it
> used to be.

I wonder if it is on allocation path or free path?

Also, increasing PCP size through vm.percpu_pagelist_fraction would
still help with my patches since it can avoid touching even more cache
lines on allocation path with a higher PCP->batch(which has an upper
limit of 96 though at the moment).

> 
> This should be root solved by the (orthogonal) changes planned in network
> subsystem, which will change the SKB allocation/free scheme so that SKBs are
> released on the originating cpu.