Re: [PATCH v5 0/7] Global Software Interrupt Moderation (GSIM)

On Wed, Aug 19, 2026 at 12:43:34PM +0000, Luigi Rizzo wrote:

Configuration is easy and robust. System administrators specify the
maximum targets (moderation delay; interrupt rate; percentage of time
spent in hardirq), and which interrupt sources should be moderated.
Independent per-CPU control loops adjust actual delays to try and keep
metrics within the targets.

Can we find a way to autodetect and autoenable this?  A magic go faster
mode that needs very specific tuning is annoying compare to sensible
defaults.

Also how does this interact with adaptive polling code inside drivers
like NAPI or the upcoming nvme variant?

The system is adaptive. Moderation affects only latency and only in
high load scenarios.  Throughput and CPU efficiencly generally benefits
significantly.  Targets don't need to match precisely the platform
limits, and one can make conservative and robust choices. Values like
delay_us=100, target_intr_rate=1000000, hardirq_percent=70 are a very
good starting point.

GSIM does not rely on any special hardware feature.

Global parameters can be modified at runtime with

    echo ${VALUE} | sudo tee /proc/irq/sw_moderation/${NAME}

/proc/irq/sw_moderation/stats exports statistics when enabled.

and moderation on individual interrupts can be turned on/off at runtime with

    echo 1 | sudo tee /proc/irq/NN/allow_moderation  # use 0 to disable

EXAMPLE:
    # global configuration: 50us, target max 1M intr/s, and 70% in intr
    echo 50 | sudo tee /proc/irq/sw_moderation/delay_us
    echo 1000000 | sudo tee /proc/irq/sw_moderation/target_intr_rate
    echo 70 | sudo tee /proc/irq/sw_moderation/hardirq_percent

    # allow moderation on all interrupts that support it
    # remember to periodically check and set the flag for dynamically
    # created interrupts since the default is 0
    echo 1 | sudo tee /proc/irq/*/allow_moderation
    
    #  check the status
    grep -r . /proc/irq/*/*/../allow_moderation

    # look at statistics
    less /proc/irq/sw_moderation/stats

PERFORMANCE BENEFITS:
Below are some experimental results under high load comparing conventional
moderation with GSIM:

- 100Gbps NIC, 32 queues: rx goes from 50 Gbps to 92.8 Gbps (line rate).
- 200Gbps NIC, 10 VMs (total 160 queues): rx goes from 30 Gbps to 190 Gbps (line rate).
- 12 SSD, 96 queues: 4K random read goes from 6M to 20.5M IOPS (device max).

In all cases, with adaptive moderatrion, latency up to p95 is unaffected
at low/moderate load, even if compared with no moderation at all.

Changes in v5:
- refactored the commits based on previous feedback
- various cleanups
- conditionally reverted parent IRQ mask/unmask, which would completely
  defeat the mechanism GSIM is based on.

Changes in v4:
- added irqdesc and irqdata flags as suggested by maintainer
- parameters are only configured via independent procfs entries.
  No module parameters anymore.
- merged control and interrupt functions back into a single header/C files
- applied various annotations (lockdep, data_race())
- formatting and various renaming as suggested by maintainer.
- added performance measurements with adaptive moderation.
- removed the mechanism to conditionally enable moderation at interrupt
  creation. This can be done in userspace and suitable udev extensions
  will be handled separately.

Changes in v3:
- clearly documented architecture in kernel/irq/irq_moderation.c
  including how to handle enable/disable/mask, interrupt migration,
  hotplug and suspend.
- split implementation in 4 files irq_moderation.[ch] and
  irq_moderation_hook.[ch] for better separation of control plane and
  "dataplane" (functions ran on each interrupt)
- limited scope to handle_edge_irq() and handle_fasteoi_irq() which
  have been tested on actual hardware.
- tested on Intel (also with intremap=posted_msi), AMD, ARM, with NIC,
  nvme, vfio

Changes in v2:
- many style fixes (mostly on comments) based on reviewers' comments on v1
- removed background from Documentation/core-api/irq/irq-moderation.rst
- split procfs handlers
- moved internal details to kernel/irq/irq_moderation.h
- use cpu hotplug for per-CPU setup, removed unnecessary arch-specific changes
- select suitable irqs based on !irqd_is_level_type(irqd) && irqd_is_single_target(irqd)
- use a static_key to enable/disable the feature



Luigi Rizzo (7):
  genirq: Add flags for software interrupt moderation.
  genirq: Add GSIM infrastructure
  genirq: Implement core GSIM moderation logic
  genirq: Integrate GSIM into interrupt flow
  genirq: Add GSIM user space configuration (procfs)
  genirq: Adaptive Global Software Interrupt Moderation (GSIM).
  PCI/MSI: re-enable conditional parent mask/unmask with sw moderation

 drivers/irqchip/irq-msi-lib.c |   8 +
 drivers/pci/msi/irqdomain.c   |  20 +
 include/linux/irq.h           |  11 +-
 include/linux/irqdesc.h       |  12 +
 kernel/irq/Kconfig            |  11 +
 kernel/irq/Makefile           |   1 +
 kernel/irq/chip.c             |  14 +
 kernel/irq/debugfs.c          |   3 +
 kernel/irq/internals.h        |  20 +
 kernel/irq/irq_moderation.c   | 959 ++++++++++++++++++++++++++++++++++
 kernel/irq/irq_moderation.h   | 156 ++++++
 kernel/irq/irqdesc.c          |   1 +
 kernel/irq/manage.c           |  10 +
 kernel/irq/proc.c             |   2 +
 kernel/irq/settings.h         |  17 +
 15 files changed, 1244 insertions(+), 1 deletion(-)
 create mode 100644 kernel/irq/irq_moderation.c
 create mode 100644 kernel/irq/irq_moderation.h

-- 
2.55.0.737.g08866a6d13-goog

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