Date: Tue, 28 May 2013 15:00:56 -0400 From: John Baldwin <jhb@freebsd.org> To: Ian Lepore <ian@freebsd.org> Cc: Orit Moskovich <oritm@mellanox.com>, freebsd-arch@freebsd.org Subject: Re: interrupt threads Message-ID: <201305281500.56679.jhb@freebsd.org> In-Reply-To: <1369766472.1258.11.camel@revolution.hippie.lan> References: <981733489AB3BD4DB24B48340F53E0A55B0D53AD@MTLDAG01.mtl.com> <201305280841.17685.jhb@freebsd.org> <1369766472.1258.11.camel@revolution.hippie.lan>
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On Tuesday, May 28, 2013 2:41:12 pm Ian Lepore wrote: > On Tue, 2013-05-28 at 08:41 -0400, John Baldwin wrote: > > On Sunday, May 26, 2013 2:16:39 am Orit Moskovich wrote: > > > Hi, > > > > > > I'm trying to understand the difference between using taskqueues defined by > > ithreads (taskqueue_swi, taskqueue_swi_giant, taskqueue_fast) to defer an > > interrupt's work, and defining an interrupt handler to give as ithread > > parameter to bus_setup_intr. > > > Is there a difference in the priority? Which of the methods is preferable > > when writing a network device and performance is important? > > > > There are two types of interrupt handlers somewhat similar to the setup in OS > > X's IOKit: a filter which runs in the borrowed thread context when an > > interrupt happens, and a threaded handler. Currently you can only use one or > > the other. However, filters are more restricted in what they can do (can only > > use spin locks, can call wakeup(), but generally not use many other APIs). > > Most drivers just use a threaded handler that runs in an ithread. However, a > > few drivers use a filter (e.g. the Intel gigabit driver uses a filter to > > workaround an Intel chipset bug where if you mask the interrupt in the I/O > > APIC while the ithread runs, the chipset decides you aren't using the APIC at > > all and routes the NIC's interrupt to an alternate pin triggering a duplicate > > interrupt on a pin shared with a USB controller). When using a filter a > > driver needs to manually schedule a thread context to perform the actions it > > cannot perform in the filter (e.g. passing received packets up the network > > stack). The taskqueue_fast is used to manage those threads. Typically the > > threads backing such a taskqueue will have the same priority as ithreads as > > they basically function as ithreads. > > > > Eventually we will support having both a filter and a threaded handler for an > > interrupt where the filter's return value decides whether or not the threaded > > handler is scheduled to run. > > > > By "eventually" you must mean "since 2007." At least, that's when the > code defining the filter handler return values was added. I know it > works as documented at least since 8.2 because I use the feature in a > PPS driver (the pps capture is done in the filter handler and the rest > of the pps event processing is done in the threaded handler). Eh, it only works if you build a kernel with the INTR_FILTER option (not on by default) IIRC. I started on cleaning that up and ended up rototilling much of the ithread code in a p4 branch. I hope to merge that into HEAD at which point filters + handlers will always work (along with several other changes aimed at removing the need for taskqueue_fast for most cases). -- John Baldwin
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