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Message-ID: <9e128547a586f1ee122879c616941340455c2f51.camel@redhat.com> Date: Thu, 25 May 2023 13:16:46 +0200 From: Paolo Abeni <pabeni@...hat.com> To: Kurt Kanzenbach <kurt@...utronix.de>, "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org> Cc: netdev@...r.kernel.org, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, juri.lelli@...hat.com Subject: Re: [PATCH net-next] net/core: Enable socket busy polling on -RT Hi, Tue, 2023-05-23 at 13:15 +0200, Kurt Kanzenbach wrote: > Busy polling is currently not allowed on PREEMPT_RT, because it disables > preemption while invoking the NAPI callback. It is not possible to acquire > sleeping locks with disabled preemption. For details see commit > 20ab39d13e2e ("net/core: disable NET_RX_BUSY_POLL on PREEMPT_RT"). > > However, strict cyclic and/or low latency network applications may prefer busy > polling e.g., using AF_XDP instead of interrupt driven communication. > > The preempt_disable() is used in order to prevent the poll_owner and NAPI owner > to be preempted while owning the resource to ensure progress. Netpoll performs > busy polling in order to acquire the lock. NAPI is locked by setting the > NAPIF_STATE_SCHED flag. There is no busy polling if the flag is set and the > "owner" is preempted. Worst case is that the task owning NAPI gets preempted and > NAPI processing stalls. This is can be prevented by properly prioritising the > tasks within the system. > > Allow RX_BUSY_POLL on PREEMPT_RT if NETPOLL is disabled. Don't disable > preemption on PREEMPT_RT within the busy poll loop. > > Tested on x86 hardware with v6.1-RT and v6.3-RT on Intel i225 (igc) with > AF_XDP/ZC sockets configured to run in busy polling mode. > > Suggested-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de> > Signed-off-by: Kurt Kanzenbach <kurt@...utronix.de> The patch looks reasonable to me, but it would be great to hear a second opinion from someone from RT side. CC: Juri Thanks! Paolo
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