Re: [PATCH net-next] net: reduce RFS/ARFS flow updates by checking LLC affinity

From: chuang

Date: Tue Mar 31 2026 - 03:52:42 EST

Hi,

Any thoughts or suggestions on the patch "reduce RFS/ARFS flow updates
by checking LLC affinity"? It’s been a while since the last update.

On Sun, Mar 8, 2026 at 4:20 PM chuang <nashuiliang@xxxxxxxxx> wrote:
>
> Hi,
>
> On Sun, Mar 8, 2026 at 3:19 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >
> > On Sun, Mar 8, 2026 at 8:10 AM Chuang Wang <nashuiliang@xxxxxxxxx> wrote:
> > >
> > > The current implementation of rps_record_sock_flow() updates the flow
> > > table every time a socket is processed on a different CPU. In high-load
> > > scenarios, especially with Accelerated RFS (ARFS), this triggers
> > > frequent flow steering updates via ndo_rx_flow_steer.
> > >
> > > For drivers like mlx5 that implement hardware flow steering, these
> > > constant updates lead to significant contention on internal driver locks
> > > (e.g., arfs_lock). This contention often becomes a performance
> > > bottleneck that outweighs the steering benefits.
> > >
> > > This patch introduces a cache-aware update strategy: the flow record is
> > > only updated if the flow migrates across Last Level Cache (LLC)
> > > boundaries. This minimizes expensive hardware reconfigurations while
> > > preserving cache locality for the application.
> > >
> > > Signed-off-by: Chuang Wang <nashuiliang@xxxxxxxxx>
> > > ---
> > > include/net/rps.h | 17 +--------------
> > > net/core/dev.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++
> > > 2 files changed, 55 insertions(+), 16 deletions(-)
> > >
> > > diff --git a/include/net/rps.h b/include/net/rps.h
> > > index e33c6a2fa8bb..2cd8698a79d5 100644
> > > --- a/include/net/rps.h
> > > +++ b/include/net/rps.h
> > > @@ -55,22 +55,7 @@ struct rps_sock_flow_table {
> > >
> > > #define RPS_NO_CPU 0xffff
> > >
> > > -static inline void rps_record_sock_flow(rps_tag_ptr tag_ptr, u32 hash)
> > > -{
> > > - unsigned int index = hash & rps_tag_to_mask(tag_ptr);
> > > - u32 val = hash & ~net_hotdata.rps_cpu_mask;
> > > - struct rps_sock_flow_table *table;
> > > -
> > > - /* We only give a hint, preemption can change CPU under us */
> > > - val |= raw_smp_processor_id();
> > > -
> > > - table = rps_tag_to_table(tag_ptr);
> > > - /* The following WRITE_ONCE() is paired with the READ_ONCE()
> > > - * here, and another one in get_rps_cpu().
> > > - */
> > > - if (READ_ONCE(table[index].ent) != val)
> > > - WRITE_ONCE(table[index].ent, val);
> > > -}
> > > +void rps_record_sock_flow(rps_tag_ptr tag_ptr, u32 hash);
> > >
> > > static inline void _sock_rps_record_flow_hash(__u32 hash)
> > > {
> > > diff --git a/net/core/dev.c b/net/core/dev.c
> > > index 203dc36aaed5..770cfb6fe06b 100644
> > > --- a/net/core/dev.c
> > > +++ b/net/core/dev.c
> > > @@ -5175,6 +5175,60 @@ static int get_rps_cpu(struct net_device *dev, struct sk_buff *skb,
> > > return cpu;
> > > }
> > >
> > > +/**
> > > + * rps_record_cond - Determine if RPS flow table should be updated
> > > + * @old_val: Previous flow record value
> > > + * @new_val: Target flow record value
> > > + *
> > > + * Returns true if the record needs an update.
> > > + */
> > > +static inline bool rps_record_cond(u32 old_val, u32 new_val)
> > > +{
> > > + u32 old_cpu = old_val & ~net_hotdata.rps_cpu_mask;
> > > + u32 new_cpu = new_val & ~net_hotdata.rps_cpu_mask;
> > > +
> > > + if (old_val == new_val)
> > > + return false;
> > > +
> > > + /* Force update if the recorded CPU is invalid or has gone offline */
> > > + if (old_cpu >= nr_cpu_ids || !cpu_active(old_cpu))
> > > + return true;
> > > +
> > > + /*
> > > + * Force an update if the current task is no longer permitted
> > > + * to run on the old_cpu.
> > > + */
> > > + if (!cpumask_test_cpu(old_cpu, current->cpus_ptr))
> > > + return true;
> > > +
> > > + /*
> > > + * If CPUs do not share a cache, allow the update to prevent
> > > + * expensive remote memory accesses and cache misses.
> > > + */
> > > + if (!cpus_share_cache(old_cpu, new_cpu))
> > > + return true;
> > > +
> > > + return false;
> > > +}
> > > +
> > > +void rps_record_sock_flow(rps_tag_ptr tag_ptr, u32 hash)
> > > +{
> > > + unsigned int index = hash & rps_tag_to_mask(tag_ptr);
> > > + u32 val = hash & ~net_hotdata.rps_cpu_mask;
> > > + struct rps_sock_flow_table *table;
> > > +
> > > + /* We only give a hint, preemption can change CPU under us */
> > > + val |= raw_smp_processor_id();
> > > +
> > > + table = rps_tag_to_table(tag_ptr);
> > > + /* The following WRITE_ONCE() is paired with the READ_ONCE()
> > > + * here, and another one in get_rps_cpu().
> > > + */
> > > + if (rps_record_cond(READ_ONCE(table[index].ent), val))
> > > + WRITE_ONCE(table[index].ent, val);
> > > +}
> > > +EXPORT_SYMBOL(rps_record_sock_flow);
> > > +
> > > #ifdef CONFIG_RFS_ACCEL
> > >
> > > /**
> > > --
> > > 2.47.3
> > >
> >
> > Interesting idea but:
> >
> > 1) Some of us do not use CONFIG_RFS_ACCEL yet.
>
> Points 1 and 3 are noted. If this approach is acceptable, I can
> implement it as a selectable option (e.g., using a static key) to
> toggle the LLC-aware strategy.
>
> > 2) You put a very fast path function out-of-line, why ?
>
> The reason is that tun uses sock_rps_record_flow_hash(). When I moved
> all rps_record_sock_flow and rps_record_cond modifications into
> include/net/rps.h, it triggered the following compilation errors due
> to symbol visibility:
>
> ERROR: modpost: "cpus_share_cache" [drivers/net/tun.ko] undefined!
> ERROR: modpost: "cpus_share_cache" [net/sctp/sctp.ko] undefined!
> make[2]: *** [scripts/Makefile.modpost:147: Module.symvers] Error 1
>
> This arises because the patch uses cpus_share_cache() to limit the
> RFS/ARFS update frequency at the LLC level. To keep this in the fast
> path, I could move cpus_share_cache() to
> include/linux/sched/topology.h.
>
> > 3) I think the behavior should be selectable with a static key or
> > something like that.
>
> Please refer to my response to point 1.
>
> >
> > 4) Please provide benchmark results.
>
> The scenario is similar to the one described in "[RFC] problems with
> RFS on bRPC applications"[1].
>
> I attempted to enable ARFS on a Mellanox CX-6 NIC. While it performs
> well for simple workloads, performance degrades significantly when
> running a bRPC[2] workload on a 2-node NUMA machine. After tracing, I
> identified patterns that ARFS/RFS fails to handle efficiently:
>
> - Multiple threads use epoll to read from the same socket, causing
> frequent flow updates in sock_flow_table.
> - Threads reading from the socket migrate frequently between CPUs.
>
> I tested a PoC version using a bRPC service, utilizing funccount [3]
> to monitor execution frequency and perf top to observe hotspots:
>
> Before Patch
>
> The mlx5e_rx_flow_steer frequency is over 380k/s, and queued_spin_lock
> is a major hotspot (6.30% in perf top). The application also suffers
> from a noticeable drop.
>
> FUNC COUNT
> mlx5e_rx_flow_steer 387594
>
> FUNC COUNT
> mlx5e_rx_flow_steer 390142
>
> FUNC COUNT
> mlx5e_rx_flow_steer 386694
>
> FUNC COUNT
> mlx5e_rx_flow_steer 389094
>
> # perf top hotspot:
> queued_spin_lock 6.30%
>
> After Patch
>
> The ARFS update frequency is significantly reduced. queued_spin_lock
> is no longer a hotspot in perf top, and the application's overall
> performance has improved.
>
> FUNC COUNT
> mlx5e_rx_flow_steer 43
>
> FUNC COUNT
> mlx5e_rx_flow_steer 9
>
> FUNC COUNT
> mlx5e_rx_flow_steer 207
>
> FUNC COUNT
> mlx5e_rx_flow_steer 26
>
> 1: https://lore.kernel.org/netdev/CAHCEFEwToeQe_Ey8e=sf8fOmoobvrDCPsxw+hfUSoRawPX03+Q@xxxxxxxxxxxxxx/t/#u
> 2: https://github.com/apache/brpc
> 3: https://github.com/iovisor/bcc/blob/master/tools/funccount.py
>
>
> >
> > Thanks.