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Thu, 15 Jun 2023 16:26:08 -0700 (PDT) Date: Thu, 15 Jun 2023 18:26:05 -0500 From: David Vernet To: Aaron Lu Cc: Peter Zijlstra , linux-kernel@vger.kernel.org, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, rostedt@goodmis.org, dietmar.eggemann@arm.com, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com, joshdon@google.com, roman.gushchin@linux.dev, tj@kernel.org, kernel-team@meta.com Subject: Re: [RFC PATCH 3/3] sched: Implement shared wakequeue in CFS Message-ID: <20230615232605.GB2915572@maniforge> References: <20230613052004.2836135-1-void@manifault.com> <20230613052004.2836135-4-void@manifault.com> <20230613083203.GR4253@hirez.programming.kicks-ass.net> <20230614043529.GA1942@ziqianlu-dell> <20230615000103.GC2883716@maniforge> <20230615044917.GA109334@ziqianlu-dell> <20230615073153.GA110814@ziqianlu-dell> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230615073153.GA110814@ziqianlu-dell> User-Agent: Mutt/2.2.10 (2023-03-25) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 15, 2023 at 03:31:53PM +0800, Aaron Lu wrote: > On Thu, Jun 15, 2023 at 12:49:17PM +0800, Aaron Lu wrote: > > I'll see if I can find a smaller machine and give it a run there too. > > Found a Skylake with 18cores/36threads on each socket/LLC and with > netperf, the contention is still serious. > > " > $ netserver > $ sudo sh -c "echo SWQUEUE > /sys/kernel/debug/sched/features" > $ for i in `seq 72`; do netperf -l 60 -n 72 -6 -t UDP_RR & done > " > > 53.61% 53.61% [kernel.vmlinux] [k] native_queued_spin_lock_slowpath - - > | > |--27.93%--sendto > | entry_SYSCALL_64 > | do_syscall_64 > | | > | --27.93%--__x64_sys_sendto > | __sys_sendto > | sock_sendmsg > | inet6_sendmsg > | udpv6_sendmsg > | udp_v6_send_skb > | ip6_send_skb > | ip6_local_out > | ip6_output > | ip6_finish_output > | ip6_finish_output2 > | __dev_queue_xmit > | __local_bh_enable_ip > | do_softirq.part.0 > | __do_softirq > | net_rx_action > | __napi_poll > | process_backlog > | __netif_receive_skb > | __netif_receive_skb_one_core > | ipv6_rcv > | ip6_input > | ip6_input_finish > | ip6_protocol_deliver_rcu > | udpv6_rcv > | __udp6_lib_rcv > | udp6_unicast_rcv_skb > | udpv6_queue_rcv_skb > | udpv6_queue_rcv_one_skb > | __udp_enqueue_schedule_skb > | sock_def_readable > | __wake_up_sync_key > | __wake_up_common_lock > | | > | --27.85%--__wake_up_common > | receiver_wake_function > | autoremove_wake_function > | default_wake_function > | try_to_wake_up > | | > | --27.85%--ttwu_do_activate > | enqueue_task > | enqueue_task_fair > | | > | --27.85%--_raw_spin_lock_irqsave > | | > | --27.85%--native_queued_spin_lock_slowpath > | > --25.67%--recvfrom > entry_SYSCALL_64 > do_syscall_64 > __x64_sys_recvfrom > __sys_recvfrom > sock_recvmsg > inet6_recvmsg > udpv6_recvmsg > __skb_recv_udp > | > --25.67%--__skb_wait_for_more_packets > schedule_timeout > schedule > __schedule > | > --25.66%--pick_next_task_fair > | > --25.65%--swqueue_remove_task > | > --25.65%--_raw_spin_lock_irqsave > | > --25.65%--native_queued_spin_lock_slowpath > > I didn't aggregate the throughput(Trans. Rate per sec) from all these > clients, but a glimpse from the result showed that the throughput of > each client dropped from 4xxxx(NO_SWQUEUE) to 2xxxx(SWQUEUE). > > Thanks, > Aaron Ok, it seems that the issue is that I wasn't creating enough netperf clients. I assumed that -n $(nproc) was sufficient. I was able to repro the contention on my 26 core / 52 thread skylake client as well: 41.01% netperf [kernel.vmlinux] [k] queued_spin_lock_slowpath | --41.01%--queued_spin_lock_slowpath | --40.63%--_raw_spin_lock_irqsave | |--21.18%--enqueue_task_fair | | | --21.09%--default_wake_function | | | --21.09%--autoremove_wake_function | | | --21.09%--__wake_up_sync_key | sock_def_readable | __udp_enqueue_schedule_skb | udpv6_queue_rcv_one_skb | __udp6_lib_rcv | ip6_input | ipv6_rcv | process_backlog | net_rx_action | | | --21.09%--__softirqentry_text_start | __local_bh_enable_ip | ip6_output | ip6_local_out | ip6_send_skb | udp_v6_send_skb | udpv6_sendmsg | __sys_sendto | __x64_sys_sendto | do_syscall_64 | entry_SYSCALL_64 | --19.44%--swqueue_remove_task | --19.42%--pick_next_task_fair | --19.42%--schedule | --19.21%--schedule_timeout __skb_wait_for_more_packets __skb_recv_udp udpv6_recvmsg inet6_recvmsg __x64_sys_recvfrom do_syscall_64 entry_SYSCALL_64 40.87% netserver [kernel.vmlinux] [k] queued_spin_lock_slowpath | --40.87%--queued_spin_lock_slowpath | --40.51%--_raw_spin_lock_irqsave | |--21.03%--enqueue_task_fair | | | --20.94%--default_wake_function | | | --20.94%--autoremove_wake_function | | | --20.94%--__wake_up_sync_key | sock_def_readable | __udp_enqueue_schedule_skb | udpv6_queue_rcv_one_skb | __udp6_lib_rcv | ip6_input | ipv6_rcv | process_backlog | net_rx_action | | | --20.94%--__softirqentry_text_start | __local_bh_enable_ip | ip6_output | ip6_local_out | ip6_send_skb | udp_v6_send_skb | udpv6_sendmsg | __sys_sendto | __x64_sys_sendto | do_syscall_64 | entry_SYSCALL_64 | --19.48%--swqueue_remove_task | --19.47%--pick_next_task_fair schedule | --19.38%--schedule_timeout __skb_wait_for_more_packets __skb_recv_udp udpv6_recvmsg inet6_recvmsg __x64_sys_recvfrom do_syscall_64 entry_SYSCALL_64 Thanks for the help in getting the repro on my end. So yes, there is certainly a scalability concern to bear in mind for swqueue for LLCs with a lot of cores. If you have a lot of tasks quickly e.g. blocking and waking on futexes in a tight loop, I expect a similar issue would be observed. On the other hand, the issue did not occur on my 7950X. I also wasn't able to repro the contention on the Skylake if I ran with the default netperf workload rather than UDP_RR (even with the additional clients). I didn't bother to take the mean of all of the throughput results between NO_SWQUEUE and SWQUEUE, but they looked roughly equal. So swqueue isn't ideal for every configuration, but I'll echo my sentiment from [0] that this shouldn't on its own necessarily preclude it from being merged given that it does help a large class of configurations and workloads, and it's disabled by default. [0]: https://lore.kernel.org/all/20230615000103.GC2883716@maniforge/ Thanks, David