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

[Joe Damato <jdamato@fastly.com>]

On Thu, Oct 31, 2024 at 10:47:05PM -0500, Samudrala, Sridhar wrote:
> 
> 
> 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?

It is not required to use gro_flush_timeout or napi_defer_hard_irqs,
but if they are set they will take over when epoll finds no events.
Their usage is recommended. See the Usage section of the cover
letter for details.

While gro_flush_timeout and napi_defer_hard_irqs are not strictly
required, it is difficult for the polling-based packet delivery loop
to gain control over packet delivery.

Please see a previous email about this from the RFC for more
details:

https://lore.kernel.org/netdev/2bb121dd-3dcd-4142-ab87-02ccf4afd469@uwaterloo.ca/

In the cover letter, you can note the difference in performance when
gro_flush_timeout is set to different values. Note the explanation
of suspendX; each suspend case is testing a different
gro_flush_timeout.

Let us know if you have any other questions; both Martin and I are
happy to help or further explain anything that is not clear.