Date: Wed, 20 Mar 2024 11:30:21 +0100
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Yan Zhai <yan@...udflare.com>, netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Jiri Pirko <jiri@...nulli.us>,
 Simon Horman <horms@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Lorenzo Bianconi <lorenzo@...nel.org>, Coco Li <lixiaoyan@...gle.com>,
 Wei Wang <weiwan@...gle.com>, Alexander Duyck <alexanderduyck@...com>,
 linux-kernel@...r.kernel.org, rcu@...r.kernel.org, bpf@...r.kernel.org,
 kernel-team@...udflare.com, Joel Fernandes <joel@...lfernandes.org>,
 "Paul E. McKenney" <paulmck@...nel.org>,
 Toke Høiland-Jørgensen <toke@...hat.com>,
 Alexei Starovoitov <alexei.starovoitov@...il.com>,
 Steven Rostedt <rostedt@...dmis.org>, mark.rutland@....com,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH v5 net 0/3] Report RCU QS for busy network kthreads


On 19/03/2024 21.44, Yan Zhai wrote:
> This changeset fixes a common problem for busy networking kthreads.
> These threads, e.g. NAPI threads, typically will do:
> 
> * polling a batch of packets
> * if there are more work, call cond_resched() to allow scheduling
> * continue to poll more packets when rx queue is not empty
> 
> We observed this being a problem in production, since it can block RCU
> tasks from making progress under heavy load. Investigation indicates
> that just calling cond_resched() is insufficient for RCU tasks to reach
> quiescent states. This also has the side effect of frequently clearing
> the TIF_NEED_RESCHED flag on voluntary preempt kernels. As a result,
> schedule() will not be called in these circumstances, despite schedule()
> in fact provides required quiescent states. This at least affects NAPI
> threads, napi_busy_loop, and also cpumap kthread.
> 
> By reporting RCU QSes in these kthreads periodically before cond_resched, the
> blocked RCU waiters can correctly progress. Instead of just reporting QS for
> RCU tasks, these code share the same concern as noted in the commit
> d28139c4e967 ("rcu: Apply RCU-bh QSes to RCU-sched and RCU-preempt when safe").
> So report a consolidated QS for safety.
> 
> It is worth noting that, although this problem is reproducible in
> napi_busy_loop, it only shows up when setting the polling interval to as high
> as 2ms, which is far larger than recommended 50us-100us in the documentation.
> So napi_busy_loop is left untouched.
> 
> Lastly, this does not affect RT kernels, which does not enter the scheduler
> through cond_resched(). Without the mentioned side effect, schedule() will
> be called time by time, and clear the RCU task holdouts.
> 
> V4: https://lore.kernel.org/bpf/cover.1710525524.git.yan@cloudflare.com/
> V3: https://lore.kernel.org/lkml/20240314145459.7b3aedf1@kernel.org/t/
> V2: https://lore.kernel.org/bpf/ZeFPz4D121TgvCje@debian.debian/
> V1: https://lore.kernel.org/lkml/Zd4DXTyCf17lcTfq@debian.debian/#t
> 
> changes since v4:
>   * polished comments and docs for the RCU helper as Paul McKenney suggested
> 
> changes since v3:
>   * fixed kernel-doc errors
> 
> changes since v2:
>   * created a helper in rcu header to abstract the behavior
>   * fixed cpumap kthread in addition
> 
> changes since v1:
>   * disable preemption first as Paul McKenney suggested
> 
> Yan Zhai (3):
>    rcu: add a helper to report consolidated flavor QS
>    net: report RCU QS on threaded NAPI repolling
>    bpf: report RCU QS in cpumap kthread
> 
>   include/linux/rcupdate.h | 31 +++++++++++++++++++++++++++++++
>   kernel/bpf/cpumap.c      |  3 +++
>   net/core/dev.c           |  3 +++
>   3 files changed, 37 insertions(+)
> 

Acked-by: Jesper Dangaard Brouer <hawk@...nel.org>

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