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SubjectRe: scheduler ignores need_resched flag in 2.2.x and 2.3.x?
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Alan Cox writes:

>> This is what makes Ingo's latency patch less dangerous: need_resched is hard
>> to set. Your scenario requires that an interrupt routine, interrupting
>> kernel mode, set needs_resched. Where does this happen?
>
>A process wake from interrupt state - eg a network event. Its not actually
>that uncommon. The window is small however so it isnt likely to get hit
>much.

Lets talk about audio interfaces too. With a 1.5ms fragment size, we
potentially have a SCHED_FIFO task that wants to get running again 13
times per timer interrupt. If the task only does a few 100usec of work
when it wakes up (not uncommon) and then goes back to sleep, the
window here is big, if I understand Alan's use of the term.

Ditto for a task using the async I/O capabilities of the RTC driver,
and a moderate-to-high interrupt frequency from the RTC.

Ditto for a task waiting for data from a MIDI interface, woken during
the interrupt handler for the device.

Basically, ditto for any situation where a task sleeps on the
condition of an external device/interface, wants to respond in a soft
real time fashion to the change, and rarely, if ever, is still
runnable before the next such interrupt shows up. This is true for a
lot of audio apps.

--p

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