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Linux 7.3 SMP Improvement To Help Reduce Latency, Improve Real-Time Performance

Written by Michael Larabel in Linux Kernel on 17 August 2026 at 10:09 AM EDT. 1 Comment

Among the pull requests sent out today now that the Linux 7.3 merge window is open are the SMP improvements for this next kernel version. Standing out among the SMP code are some enhancements for helping reduce latency and ultimately enhancing real-time Linux performance.

Due to blocking until remote CPUs complete the IPI function execution, scheduling latency can increase "dramatically" with the number of remote CPUs and other conditions. Especially for x86_64 where Translation Lookaside Buffer (TLB) flushes are performance via IPIs, process exit and process-mapped page reclamation can lead to waiting on multiple IPI operations.

Bytedance engineers thus took to reworking the kernel SMP code to allow preemption during IPI completion waiting in order to avoid scheduling latency spikes. Bytedance engineers found the existing Linux SMP behavior to be particularly bad for latency-sensitive workloads like DPDK. In their end their optimizations paid off big time with a 90% reduction in P99 latency in testing with DPDK:

Thomas Gleixner summed up the situation elegantly in the SMP pull request:<br>"Reduce the preemption disabled sections in smp_call_function*().

The various smp call functions keep preemption disabled across the full operation which includes the wait for completion. Especially the latter can take some time when one of the target CPUs is not immediately responding to the IPI, which can result in large latency spikes.

To improve this provide a per task CPU mask to track the CPUs to wait for. That makes the information required for the wait task local and therefore allows to reenable preemption before the wait. While this comes with moderate extra memory cost this reduces SMP function call induced latency measured in a fleet for high priority tasks from ~17ms to ~1.5ms (~90%)."

The pull is now awaiting action by Linus Torvalds. Barring any code nit picks from Linus Torvalds, it's pretty safe to assume it will land this week as we kick off the Linux 7.3 merge window.

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Michael Larabel is the principal author of Phoronix.com and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 20,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated benchmarking software. He can be followed via Twitter, LinkedIn, or contacted via MichaelLarabel.com.

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