Date: Wed, 29 May 2013 11:56:28 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-drivers@freebsd.org Cc: Orit Moskovich <oritm@mellanox.com> Subject: Re: taskqueues Message-ID: <201305291156.29230.jhb@freebsd.org> In-Reply-To: <981733489AB3BD4DB24B48340F53E0A55B0D5206@MTLDAG01.mtl.com> References: <981733489AB3BD4DB24B48340F53E0A55B0D5206@MTLDAG01.mtl.com>
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On Thursday, May 23, 2013 9:47:14 am Orit Moskovich wrote:
> Hi,
>
> Can you please specify the difference between interrupt threads and regular
kernel threads in the context of the different default taskqueues?
> Meaning, I saw that the taskqueue taskqueue_thread is actually a kernel
thread running the function taskqueue_thread_loop,
> And the 3 other default taskqueues are working with ithreads.
>
> Which of the above preempts the other? What should I use if performance is
critical (something equivalent to Linux softirq...)?
ithreads use the most important priority range:
#define PRI_MIN (0) /* Highest priority. */
#define PRI_MAX (255) /* Lowest priority. */
#define PRI_MIN_ITHD (PRI_MIN)
#define PRI_MAX_ITHD (PRI_MIN_REALTIME - 1)
#define PI_REALTIME (PRI_MIN_ITHD + 0)
#define PI_AV (PRI_MIN_ITHD + 4)
#define PI_NET (PRI_MIN_ITHD + 8)
#define PI_DISK (PRI_MIN_ITHD + 12)
#define PI_TTY (PRI_MIN_ITHD + 16)
#define PI_DULL (PRI_MIN_ITHD + 20)
#define PI_SOFT (PRI_MIN_ITHD + 24)
#define PI_SWI(x) (PI_SOFT + (x) * RQ_PPQ)
For example, an INTR_TYPE_NET interrupt will generally run in an ithread with
the PI_NET priority. For software interrupt threads the priority is generally
PI_SWI(SWI_xxx). The SWI_xxx constants are:
/*
* Software interrupt numbers in priority order. The priority determines
* the priority of the corresponding interrupt thread.
*/
#define SWI_TTY 0
#define SWI_NET 1
#define SWI_CAMBIO 2
#define SWI_VM 3
#define SWI_CLOCK 4
#define SWI_TQ_FAST 5
#define SWI_TQ 6
#define SWI_TQ_GIANT 6
For taskqueues you have three global taskqueues to choose from:
taskqueue_fast: runs at PI_SWI(SWI_TQ_FAST) = 44
taskqueue_swi: runs at PI_SWI(SWI_TQ) = 48
taskqueue_thread: runs at PWAIT = 108
However, you can create your own thread-backed taskqueue that can run at
whatever priority you choose. There is nothing really special about having
the taskqueue be an SWI vs a normal thread. If you wanted a taskqueue that
ran at PI_NET you could do:
TASKQUEUE_DEFINE(foo, taskqueue_thread_enqueue, &taskqueue_foo,
taskqueue_start_threads(&taskqueue_foo, 1, PI_NET, "foo taskq");
If you needed to queue tasks from an interrupt filter you would want to use
a fast taskqueue instead:
TASKQUEUE_FAST_DEFINE(< same args as above >)
--
John Baldwin
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