netdev - Re: [PATCH v4 net-next 0/4] net: Provide SMP threads for backlog NAPI

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Message-ID: <3fpntlz5golidj775wfnlzecd7ksimwutcqg7e6d2efejt6sip@akexo2hmy3hb>
Date: Tue, 5 Mar 2024 09:07:26 -0300
From: Wander Lairson Costa <wander@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Jesper Dangaard Brouer <hawk@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Yan Zhai <yan@...udflare.com>
Subject: Re: [PATCH v4 net-next 0/4] net: Provide SMP threads for backlog NAPI

On Tue, Mar 05, 2024 at 12:53:18PM +0100, Sebastian Andrzej Siewior wrote:
> The RPS code and "deferred skb free" both send IPI/ function call
> to a remote CPU in which a softirq is raised. This leads to a warning on
> PREEMPT_RT because raising softiqrs from function call led to undesired
> behaviour in the past. I had duct tape in RT for the "deferred skb free"
> and Wander Lairson Costa reported the RPS case.
> 
> This series only provides support for SMP threads for backlog NAPI, I
> did not attach a patch to make it default and remove the IPI related
> code to avoid confusion. I can post it for reference it asked.
> 
> The RedHat performance team was so kind to provide some testing here.
> The series (with the IPI code removed) has been tested and no regression
> vs without the series has been found. For testing iperf3 was used on 25G
> interface, provided by mlx5, ix40e or ice driver and RPS was enabled. I
> can provide the individual test results if needed.
> 
> Changes:
> - v3…v4 https://lore.kernel.org/all/20240228121000.526645-1-bigeasy@linutronix.de/
> 
>   - Rebase on top of current net-next, collect Acks.
> 
>   - Add struct softnet_data as an argument to kick_defer_list_purge().
> 
>   - Add sd_has_rps_ipi_waiting() check to napi_threaded_poll_loop() which was
>     accidentally removed.
> 
> - v2…v3 https://lore.kernel.org/all/20240221172032.78737-1-bigeasy@linutronix.de/
> 
>   - Move the "if use_backlog_threads()" case into the CONFIG_RPS block
>     within napi_schedule_rps().
> 
>   - Use __napi_schedule_irqoff() instead of napi_schedule_rps() in
>     kick_defer_list_purge().
> 
> - v1…v2 https://lore.kernel.org/all/20230929162121.1822900-1-bigeasy@linutronix.de/
> 
>   - Patch #1 is new. It ensures that NAPI_STATE_SCHED_THREADED is always
>     set (instead conditional based on task state) and the smboot thread
>     logic relies on this bit now. In v1 NAPI_STATE_SCHED was used but is
>     racy.
> 
>   - The defer list clean up is split out and also relies on
>     NAPI_STATE_SCHED_THREADED. This fixes a different race.
> 
> - RFC…v1 https://lore.kernel.org/all/20230814093528.117342-1-bigeasy@linutronix.de/
> 
>    - Patch #2 has been removed. Removing the warning is still an option.
> 
>    - There are two patches in the series:
>      - Patch #1 always creates backlog threads
>      - Patch #2 creates the backlog threads if requested at boot time,
>        mandatory on PREEMPT_RT.
>      So it is either or and I wanted to show how both look like.
> 
>    - The kernel test robot reported a performance regression with
>      loopback (stress-ng --udp X --udp-ops Y) against the RFC version.
>      The regression is now avoided by using local-NAPI if backlog
>      processing is requested on the local CPU.
> 
> Sebastian
> 

Patch 0002 does not apply for me. I tried torvalds/master and
linux-rt-devel/linux-6.8.y-rt. Which tree should I use?