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

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Date:   Tue, 16 Oct 2018 16:03:55 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Holger Hoffstätte 
        <holger@...lied-asynchrony.com>
Cc:     Heiner Kallweit <hkallweit1@...il.com>,
        David Miller <davem@...emloft.net>,
        Realtek linux nic maintainers <nic_swsd@...ltek.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net] r8169: fix NAPI handling under high load

On Tue, 16 Oct 2018 23:17:31 +0200
Holger Hoffstätte <holger@...lied-asynchrony.com> 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.
> 
> cheers
> Holger

Many drivers have buggy usage of napi_complete_done.

Might even be worth forcing all network drivers to check the return
value. But fixing 150 broken drivers will be a nuisance.

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index dc1d9ed33b31..c38bc66ffe74 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -466,7 +466,8 @@ static inline bool napi_reschedule(struct napi_struct *napi)
        return false;
 }
 
-bool napi_complete_done(struct napi_struct *n, int work_done);
+bool __must_check napi_complete_done(struct napi_struct *n, int work_done);
+
 /**
  *     napi_complete - NAPI processing complete
  *     @n: NAPI context