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SubjectRe: [RFC PATCH 0/6] Convert all tasklets to workqueues

* Jeff Garzik <jeff@garzik.org> wrote:

> Tasklets fill a niche not filled by either workqueues (slower,
> requiring context switches, and possibly much latency is all wq's
> processes are active) [...]

... workqueues are also possibly much more scalable (percpu workqueues
are easy without changing anything in your code but the call where you
create the workqueue).

the context-switch argument i'll believe if i see numbers. You'll
probably need in excess of tens of thousands of irqs/sec to even be able
to measure its overhead. (workqueues are driven by nice kernel threads
so there's no TLB overhead, etc.)

the only remaining argument is latency: but workqueues are already
pretty high-prio (with a default priority of nice -5) - and you can
increase it even further. You can make it SCHED_FIFO prio 98 if latency
is so important. Tasklets on the other hand are _unconditionally_
high-priority. So this argument is more of an arms-race argument: "i
want _my_ processing to be done immediately!". The fact that workqueues
can be preempted and that their priorities can be adjusted flexibly is
an optional _bonus_, not a disadvantage. If low-prio workqueues hurts
your workflow, make them high-prio.

> And moving code -back- into hardirq is just the wrong thing to do,
> usually.

agreed - except if the in-tasklet processing is really thin and there's
already a softirq layer in the workflow. (which the case was for the
example that was cited.) In such a case moving either to the hardirq or
to the softirq looks like the right thing - instead of the tasklet
intermediary.

Ingo
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