netdev - Re: XDP performance regression due to CONFIG

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Message-ID: <20180417071339.GA24498@infradead.org>
Date:   Tue, 17 Apr 2018 00:13:39 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "xdp-newbies@...r.kernel.org" <xdp-newbies@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        David Woodhouse <dwmw2@...radead.org>,
        William Tu <u9012063@...il.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: XDP performance regression due to CONFIG_RETPOLINE Spectre V2

On Tue, Apr 17, 2018 at 09:07:01AM +0200, Jesper Dangaard Brouer wrote:
> > > number should improve more).  
> > 
> > What is the number for the otherwise comparable setup without repolines?
> 
> Approx 12 Mpps.
> 
> You forgot to handle the dma_direct_mapping_error() case, which still
> used the retpoline in the above 8.9 Mpps measurement, I fixed it up and
> performance increased to 9.6 Mpps.
> 
> Notice, in this test there are still two retpoline/indirect-calls
> left.  The net_device->ndo_xdp_xmit and the invocation of the XDP BPF
> prog.

But that seems like a pretty clear indicator that we want the fast path
direct mapping.  I'll try to find some time over the next weeks to
do a cleaner version of it.