From mboxrd@z Thu Jan 1 00:00:00 1970 From: Florian Fainelli Subject: Re: [PATCH net-next] net: dsa: User per-cpu 64-bit statistics Date: Fri, 4 Aug 2017 08:51:54 -0700 Message-ID: <455c39d3-302e-68ab-3812-112c48ed48c2@gmail.com> References: <20170804043328.4730-1-f.fainelli@gmail.com> <1501824968.25002.35.camel@edumazet-glaptop3.roam.corp.google.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8bit Cc: netdev@vger.kernel.org, davem@davemlot.net, Andrew Lunn , Vivien Didelot , "David S. Miller" , open list To: Eric Dumazet Return-path: In-Reply-To: <1501824968.25002.35.camel@edumazet-glaptop3.roam.corp.google.com> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 08/03/2017 10:36 PM, Eric Dumazet wrote: > On Thu, 2017-08-03 at 21:33 -0700, Florian Fainelli wrote: >> During testing with a background iperf pushing 1Gbit/sec worth of >> traffic and having both ifconfig and ethtool collect statistics, we >> could see quite frequent deadlocks. Convert the often accessed DSA slave >> network devices statistics to per-cpu 64-bit statistics to remove these >> deadlocks and provide fast efficient statistics updates. >> > > This seems to be a bug fix, it would be nice to get a proper tag like : > > Fixes: f613ed665bb3 ("net: dsa: Add support for 64-bit statistics") Right, should have been added, thanks! > > Problem here is that if multiple cpus can call dsa_switch_rcv() at the > same time, then u64_stats_update_begin() contract is not respected. This is really where I struggled understanding what is wrong in the non-per CPU version, my understanding is that we have: - writers for xmit executes in process context - writers for receive executes from NAPI (from the DSA's master network device through it's own NAPI doing netif_receive_skb -> netdev_uses_dsa -> netif_receive_skb) readers should all execute in process context. The test scenario that led to a deadlock involved running iperf in the background, having a while loop with both ifconfig and ethtool reading stats, and somehow when iperf exited, either reader would just be locked. So I guess this leaves us with the two writers not being mutually excluded then, right? > > include/linux/u64_stats_sync.h states : > > * Usage : > * > * Stats producer (writer) should use following template granted it already got > * an exclusive access to counters (a lock is already taken, or per cpu > * data is used [in a non preemptable context]) > * > * spin_lock_bh(...) or other synchronization to get exclusive access > * ... > * u64_stats_update_begin(&stats->syncp); > > > -- Florian