netdev - Re: [PATCH net 1/3] bnxt_en: Fix accumulation of bp->net_stats

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Date:   Tue, 26 May 2020 15:47:52 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Michael Chan <michael.chan@...adcom.com>
Cc:     David Miller <davem@...emloft.net>, Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net 1/3] bnxt_en: Fix accumulation of
 bp->net_stats_prev.

On Tue, 26 May 2020 15:43:52 -0700 Michael Chan wrote:
> On Tue, May 26, 2020 at 3:04 PM Jakub Kicinski <kuba@...nel.org> wrote:
> > On Mon, 25 May 2020 17:41:17 -0400 Michael Chan wrote:  
> > > We have logic to maintain network counters across resets by storing
> > > the counters in bp->net_stats_prev before reset.  But not all resets
> > > will clear the counters.  Certain resets that don't need to change
> > > the number of rings do not clear the counters.  The current logic
> > > accumulates the counters before all resets, causing big jumps in
> > > the counters after some resets, such as ethtool -G.
> > >
> > > Fix it by only accumulating the counters during reset if the irq_re_init
> > > parameter is set.  The parameter signifies that all rings and interrupts
> > > will be reset and that means that the counters will also be reset.
> > >
> > > Reported-by: Vijayendra Suman <vijayendra.suman@...cle.com>
> > > Fixes: b8875ca356f1 ("bnxt_en: Save ring statistics before reset.")
> > > Signed-off-by: Michael Chan <michael.chan@...adcom.com>  
> >
> > Hi Michael!
> >
> > Could you explain why accumulating counters causes a jump?  
> 
> Yes, during chip reset, we free most hardware resources including
> possibly hardware counter resources.  After freeing the hardware
> counters, the counters will go to zero.  To preserve the counters, we
> take a snapshot of the hardware counters and add them to the
> bp->net_stats_prev.  The counters in bp->net_stats_prev are always
> added to the current hardware counters to provide the true counters.
> 
> The problem is that not all resets will free the hardware counters.
> The old code is unconditionally taking the snapshot during reset.  So
> when we don't free the hardware counters, the snapshot will cause us
> to effectively double the hardware counters after the reset.

Aw! I see what you mean now, thanks for the explanation!