netdev - Re: Kernel 4.6.7-rt13: Intel Ethernet driver igb causes huge latencies in cyclictest

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Date:   Sat, 15 Oct 2016 00:06:33 +0200
From:   Richard Cochran <richardcochran@...il.com>
To:     "Koehrer Mathias (ETAS/ESW5)" <mathias.koehrer@...s.com>
Cc:     Julia Cartwright <julia@...com>,
        "Williams, Mitch A" <mitch.a.williams@...el.com>,
        "Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
        "linux-rt-users@...r.kernel.org" <linux-rt-users@...r.kernel.org>,
        Sebastian Andrzej Siewior <sebastian.siewior@...utronix.de>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        Greg <gvrose8192@...il.com>
Subject: Re: Kernel 4.6.7-rt13: Intel Ethernet driver igb causes huge
 latencies in cyclictest

On Fri, Oct 14, 2016 at 08:58:22AM +0000, Koehrer Mathias (ETAS/ESW5) wrote:
> @@ -753,7 +756,9 @@ u32 igb_rd32(struct e1000_hw *hw, u32 re
>  	if (E1000_REMOVED(hw_addr))
>  		return ~value;
>  
> +        trace_igb(801);
>  	value = readl(&hw_addr[reg]);
> +        trace_igb(802);

Nothing prevents this code from being preempted between the two trace
points, and so you can't be sure whether the time delta in the trace
is caused by the PCIe read stalling or not.

Thanks,
Richard