Re: [PATCH net 1/1] net: stmmac: add fsleep() in HW Rx timestamp checking loop

From: Tan Tee Min <tee.min.tan@linux.intel.com>
To: Jakub Kicinski <kuba@kernel.org>
Cc: Richard Cochran <richardcochran@gmail.com>,
	Tan Tee Min <tee.min.tan@intel.com>,
	Giuseppe Cavallaro <peppe.cavallaro@st.com>,
	Alexandre Torgue <alexandre.torgue@st.com>,
	Jose Abreu <joabreu@synopsys.com>,
	"David S . Miller" <davem@davemloft.net>,
	Paolo Abeni <pabeni@redhat.com>,
	Maxime Coquelin <mcoquelin.stm32@gmail.com>,
	Rayagond Kokatanur <rayagond@vayavyalabs.com>,
	netdev@vger.kernel.org, linux-stm32@st-md-mailman.stormreply.com,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, stable@vger.kernel.org,
	Voon Wei Feng <weifeng.voon@intel.com>,
	Wong Vee Khee <vee.khee.wong@intel.com>,
	Song Yoong Siang <yoong.siang.song@intel.com>,
	Alexandre Torgue <alexandre.torgue@foss.st.com>
Subject: Re: [PATCH net 1/1] net: stmmac: add fsleep() in HW Rx timestamp checking loop
Date: Tue, 19 Apr 2022 08:52:20 +0800	[thread overview]
Message-ID: <20220419005220.GA17634@linux.intel.com> (raw)
In-Reply-To: <20220414104259.0b928249@kernel.org>

On Thu, Apr 14, 2022 at 10:42:59AM +0200, Jakub Kicinski wrote:
> On Thu, 14 Apr 2022 15:29:34 +0800 Tan Tee Min wrote:
> > > > --- a/drivers/net/ethernet/stmicro/stmmac/dwmac4_descs.c
> > > > +++ b/drivers/net/ethernet/stmicro/stmmac/dwmac4_descs.c
> > > > @@ -279,10 +279,11 @@ static int dwmac4_wrback_get_rx_timestamp_status(void *desc, void *next_desc,
> > > >  			/* Check if timestamp is OK from context descriptor */
> > > >  			do {
> > > >  				ret = dwmac4_rx_check_timestamp(next_desc);
> > > > -				if (ret < 0)
> > > > +				if (ret <= 0)
> > > >  					goto exit;
> > > >  				i++;
> > > >  
> > > > +				fsleep(1);  
> > > 
> > > This is nutty.  Why isn't this code using proper deferral mechanisms
> > > like work or kthread?  
> > 
> > Appreciate your comment.
> > The dwmac4_wrback_get_rx_timestamp_status() is called by stmmac_rx()
> > function which is scheduled by NAPI framework.
> > Do we still need to create deferred work inside NAPI work?
> > Would you mind to explain it more in detail?
> 
> fsleep() is a big hammer, can you try cpu_relax() and bumping the max
> loop count a little?

Thanks for the suggestion!
I tried cpu_relax(), unfortunately the issue still happens when
the system is in a high-load situation.

I agree that the fsleep(1) (=1us) is a big hammer.
Thus in order to improve this, I’ve figured out a smaller delay
time that is enough for the context descriptor to be ready which
is ndelay(500) (=500ns).

Would you think this is more acceptable?

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