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Fri, 1 Nov 2024 03:47:10 +0000 Message-ID: Date: Thu, 31 Oct 2024 22:47:05 -0500 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net-next v3 7/7] docs: networking: Describe irq suspension To: Joe Damato , CC: , , , , , , , , , , , , , Martin Karsten , Bagas Sanjaya , "David S. 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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: >