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From: Heiner Kallweit <hkallweit1@gmail.com>
To: Vladimir Oltean <vladimir.oltean@nxp.com>
Cc: "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"andrew@lunn.ch" <andrew@lunn.ch>,
	"f.fainelli@gmail.com" <f.fainelli@gmail.com>,
	"vivien.didelot@gmail.com" <vivien.didelot@gmail.com>,
	"kuba@kernel.org" <kuba@kernel.org>,
	Christian Eggers <ceggers@arri.de>,
	Kurt Kanzenbach <kurt@linutronix.de>
Subject: Re: [RFC PATCH 01/13] net: dsa: add plumbing for custom netdev statistics
Date: Sun, 18 Oct 2020 16:13:28 +0200
Message-ID: <2ae30988-5918-3d02-87f1-e65942acc543@gmail.com> (raw)
In-Reply-To: <20201018134843.emustnvgyby32cm4@skbuf>

On 18.10.2020 15:48, Vladimir Oltean wrote:
> On Sun, Oct 18, 2020 at 03:09:32PM +0200, Heiner Kallweit wrote:
>> On 18.10.2020 14:16, Vladimir Oltean wrote:
>>> On Sun, Oct 18, 2020 at 02:02:46PM +0200, Heiner Kallweit wrote:
>>>> Wouldn't a simple unsigned long (like in struct net_device_stats) be
>>>> sufficient here? This would make handling the counter much simpler.
>>>> And as far as I understand we talk about a packet counter that is
>>>> touched in certain scenarios only.
>>>
>>> I don't understand, in what sense 'sufficient'? This counter is exported
>>> to ethtool which works with u64 values, how would an unsigned long,
>>> which is u32 on 32-bit systems, help?
>>>
>> Sufficient for me means that it's unlikely that a 32 bit counter will
>> overflow. Many drivers use the 32 bit counters (on a 32bit system) in
>> net_device_stats for infrequent events like rx/tx errors, and 64bit
>> counters only for things like rx/tx bytes, which are more likely to
>> overflow.
> 
> 2^32 = 4,294,967,296 = 4 billion packets
> Considering that every packet that needs TX timestamping must be
> reallocated, protocols such as IEEE 802.1AS will trigger 5 reallocs per
> second. So time synchronization alone (background traffic, by all
> accounts) will make this counter overflow in 27 years.
> Every packet flooded or multicast by the bridge will also trigger
> reallocs. In this case it is not difficult to imagine that overflows
> might occur sooner.
> 
> Even if the above wasn't true. What becomes easier when I make the
> counter an unsigned long? I need to make arch-dependent casts between an
> unsigned long an an u64 when I expose the counter to ethtool, and there
> it ends up being u64 too, doesn't it?
> 

Access to unsigned long should be atomic, so you could avoid the
following and access the counter directly (like other drivers do it
with the net_device_stats members). On a side note, because I'm also
just dealing with it: this_cpu_ptr() is safe only if preemption is
disabled. Is this the case here? Else there's get_cpu_ptr/put_cpu_ptr.
Also, if irq's aren't disabled there might be a need to use
u64_stats_update_begin_irqsave() et al. See:
2695578b896a ("net: usbnet: fix potential deadlock on 32bit hosts")
But I don't know dsa good enough to say whether this is applicable
here.

+	e = this_cpu_ptr(p->extra_stats);
+	u64_stats_update_begin(&e->syncp);
+	e->tx_reallocs++;
+	u64_stats_update_end(&e->syncp);

+		e = per_cpu_ptr(p->extra_stats, i);
+		do {
+			start = u64_stats_fetch_begin_irq(&e->syncp);
+			tx_reallocs	= e->tx_reallocs;
+		} while (u64_stats_fetch_retry_irq(&e->syncp, start));


  reply index

Thread overview: 49+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-17 21:35 [RFC PATCH 00/13] Generic TX reallocation for DSA Vladimir Oltean
2020-10-17 21:35 ` [RFC PATCH 01/13] net: dsa: add plumbing for custom netdev statistics Vladimir Oltean
2020-10-17 22:13   ` Florian Fainelli
2020-10-17 23:15   ` Andrew Lunn
2020-10-18  0:23     ` Vladimir Oltean
2020-10-18 12:02   ` Heiner Kallweit
2020-10-18 12:16     ` Vladimir Oltean
2020-10-18 13:09       ` Heiner Kallweit
2020-10-18 13:48         ` Vladimir Oltean
2020-10-18 14:13           ` Heiner Kallweit [this message]
2020-10-18 22:58             ` Vladimir Oltean
2020-10-18 23:11               ` Florian Fainelli
2020-10-19  0:21                 ` Vladimir Oltean
2020-10-19  3:49                   ` Florian Fainelli
2020-10-19 12:05                     ` Andrew Lunn
2020-10-19 16:27                       ` Florian Fainelli
2020-10-18 16:49   ` Jakub Kicinski
2020-10-18 17:36     ` Andrew Lunn
2020-10-18 18:26       ` Jakub Kicinski
2020-10-18 18:33         ` Jakub Kicinski
2020-10-17 21:36 ` [RFC PATCH 02/13] net: dsa: implement a central TX reallocation procedure Vladimir Oltean
2020-10-17 22:01   ` Vladimir Oltean
2020-10-17 22:11     ` Florian Fainelli
2020-10-17 22:17       ` Vladimir Oltean
2020-10-18  0:37         ` Florian Fainelli
2020-10-19  8:33       ` David Laight
2020-10-19 10:30         ` Vladimir Oltean
2020-10-19 11:14           ` David Laight
2020-10-19 11:41             ` Vladimir Oltean
2020-10-19 12:19           ` Andrew Lunn
2020-10-17 22:31     ` Vladimir Oltean
2020-10-18  0:13       ` Vladimir Oltean
2020-10-18 10:36         ` Christian Eggers
2020-10-18 11:42           ` Vladimir Oltean
2020-10-18 11:59             ` Christian Eggers
2020-10-18 12:15               ` Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 03/13] net: dsa: tag_ksz: don't allocate additional memory for padding/tagging Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 04/13] net: dsa: trailer: " Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 05/13] net: dsa: tag_qca: let DSA core deal with TX reallocation Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 06/13] net: dsa: tag_ocelot: " Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 07/13] net: dsa: tag_mtk: " Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 08/13] net: dsa: tag_lan9303: " Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 09/13] net: dsa: tag_edsa: " Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 10/13] net: dsa: tag_brcm: " Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 11/13] net: dsa: tag_dsa: " Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 12/13] net: dsa: tag_gswip: " Vladimir Oltean
2020-10-17 22:19   ` Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 13/13] net: dsa: tag_ar9331: " Vladimir Oltean
2020-10-17 23:07 ` [RFC PATCH 00/13] Generic TX reallocation for DSA Andrew Lunn

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