Re: [PATCH net-next v3 7/7] docs: networking: Describe irq suspension

["Samudrala, Sridhar" <sridhar.samudrala@intel.com>]

On 10/31/2024 7:48 PM, Joe Damato wrote:
> Describe irq suspension, the epoll ioctls, and the tradeoffs of using
> different gro_flush_timeout values.
> 
> Signed-off-by: Joe Damato <jdamato@fastly.com>
> Co-developed-by: Martin Karsten <mkarsten@uwaterloo.ca>
> Signed-off-by: Martin Karsten <mkarsten@uwaterloo.ca>
> Acked-by: Stanislav Fomichev <sdf@fomichev.me>
> Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
> ---
<snip>


> +
> +IRQ suspension
> +--------------
> +
> +IRQ suspension is a mechanism wherein device IRQs are masked while epoll
> +triggers NAPI packet processing.
> +
> +While application calls to epoll_wait successfully retrieve events, the kernel will
> +defer the IRQ suspension timer. If the kernel does not retrieve any events
> +while busy polling (for example, because network traffic levels subsided), IRQ
> +suspension is disabled and the IRQ mitigation strategies described above are
> +engaged.
> +
> +This allows users to balance CPU consumption with network processing
> +efficiency.
> +
> +To use this mechanism:
> +
> +  1. The per-NAPI config parameter ``irq_suspend_timeout`` should be set to the
> +     maximum time (in nanoseconds) the application can have its IRQs
> +     suspended. This is done using netlink, as described above. This timeout
> +     serves as a safety mechanism to restart IRQ driver interrupt processing if
> +     the application has stalled. This value should be chosen so that it covers
> +     the amount of time the user application needs to process data from its
> +     call to epoll_wait, noting that applications can control how much data
> +     they retrieve by setting ``max_events`` when calling epoll_wait.
> +
> +  2. The sysfs parameter or per-NAPI config parameters ``gro_flush_timeout``
> +     and ``napi_defer_hard_irqs`` can be set to low values. They will be used
> +     to defer IRQs after busy poll has found no data.

Is it required to set gro_flush_timeout and napi_defer_hard_irqs when 
irq_suspend_timeout is set? Doesn't it override any smaller 
gro_flush_timeout value?


> +
> +  3. The ``prefer_busy_poll`` flag must be set to true. This can be done using
> +     the ``EPIOCSPARAMS`` ioctl as described above.
> +
> +  4. The application uses epoll as described above to trigger NAPI packet
> +     processing.
> +
> +As mentioned above, as long as subsequent calls to epoll_wait return events to
> +userland, the ``irq_suspend_timeout`` is deferred and IRQs are disabled. This
> +allows the application to process data without interference.
> +
> +Once a call to epoll_wait results in no events being found, IRQ suspension is
> +automatically disabled and the ``gro_flush_timeout`` and
> +``napi_defer_hard_irqs`` mitigation mechanisms take over.
> +
> +It is expected that ``irq_suspend_timeout`` will be set to a value much larger
> +than ``gro_flush_timeout`` as ``irq_suspend_timeout`` should suspend IRQs for
> +the duration of one userland processing cycle.
>   
>   .. _threaded:
>