---------- Forwarded message ---------
From: Yan Zhai <[email protected]>
Date: Tue, Mar 19, 2024 at 4:44 PM
Subject: [PATCH v5 net 0/3] Report RCU QS for busy network kthreads
To: <[email protected]>
Cc: David S. Miller <[email protected]>, Eric Dumazet
<[email protected]>, Jakub Kicinski <[email protected]>, Paolo Abeni
<[email protected]>, Jiri Pirko <[email protected]>, Simon Horman
<[email protected]>, Daniel Borkmann <[email protected]>, Lorenzo
Bianconi <[email protected]>, Coco Li <[email protected]>, Wei
Wang <[email protected]>, Alexander Duyck <[email protected]>,
<[email protected]>, <[email protected]>,
<[email protected]>, <[email protected]>, Joel Fernandes
<[email protected]>, Paul E. McKenney <[email protected]>, Toke
Høiland-Jørgensen <[email protected]>, Alexei Starovoitov
<[email protected]>, Steven Rostedt <[email protected]>,
<[email protected]>, Jesper Dangaard Brouer <[email protected]>,
Sebastian Andrzej Siewior <[email protected]>
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/[email protected]/
V3: https://lore.kernel.org/lkml/[email protected]/t/
V2: https://lore.kernel.org/bpf/[email protected]/
V1: https://lore.kernel.org/lkml/[email protected]/#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(+)
--
2.30.2
--
https://www.youtube.com/watch?v=N0Tmvv5jJKs Epik Mellon Podcast
Dave Täht CSO, LibreQos
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