netdev - Re: [PATCH net] r8169: fix NAPI handling under high load

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Message-ID: <e7607b0f-86ed-6784-638c-37d68d910fb7@applied-asynchrony.com>
Date:   Wed, 17 Oct 2018 21:11:12 +0200
From:   Holger Hoffstätte <holger@...lied-asynchrony.com>
To:     Heiner Kallweit <hkallweit1@...il.com>,
        David Miller <davem@...emloft.net>,
        Realtek linux nic maintainers <nic_swsd@...ltek.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net] r8169: fix NAPI handling under high load

On 10/17/18 20:12, Heiner Kallweit wrote:
> On 16.10.2018 23:17, Holger Hoffstätte wrote:
>> On 10/16/18 22:37, Heiner Kallweit wrote:
>>> rtl_rx() and rtl_tx() are called only if the respective bits are set
>>> in the interrupt status register. Under high load NAPI may not be
>>> able to process all data (work_done == budget) and it will schedule
>>> subsequent calls to the poll callback.
>>> rtl_ack_events() however resets the bits in the interrupt status
>>> register, therefore subsequent calls to rtl8169_poll() won't call
>>> rtl_rx() and rtl_tx() - chip interrupts are still disabled.
>>
>> Very interesting! Could this be the reason for the mysterious
>> hangs & resets we experienced when enabling BQL for r8169?
>> They happened more often with TSO/GSO enabled and several people
>> attempted to fix those hangs unsuccessfully; it was later reverted
>> and has been since then (#87cda7cb43).
>> If this bug has been there "forever" it might be tempting to
>> re-apply BQL and see what happens. Any chance you could give that
>> a try? I'll gladly test patches, just like I'll run this one.
>>
> After reading through the old mail threads regarding BQL on r8169
> I don't think the fix here is related.
> It seems that BQL on r8169 worked fine for most people, just one
> had problems on one of his systems. I assume the issue was specific

I continued to use the BQL patch in my private tree after it was reverted
and also had occasional timeouts, but *only* after I started playing
with ethtool to change offload settings. Without offloads or the BQL patch
everything has been rock-solid since then.
The other weird problem was that timeouts would occur on an otherwise
*completely idle* system. Since that occasionally borked my NFS server
over night I ultimately removed BQL as well. Rock-solid since then.

> I will apply the old BQL patch and see how it's on my system
> (with GRO and SG enabled).

I don't think it still applies cleanly, but if you cook up an updated
version I'll gladly test it.

Thanks! :)
Holger