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On Fri, 17 Dec 1999, William Montgomery wrote:
> [...] however, there is a related
> problem which can occur. It is possible for a SCHED_OTHER task
> to be running in user mode when a rtc_interrupt occurs which
> sends a SIGIO to a SCHED_FIFO task, the SCHED_FIFO task is put
> on the run queue and the need_resched flag of the SCHED_OTHER task
> is set but the ret_with_resched is not run and the SCHED_OTHER task
> can continue for several milliseconds before schedule is run.
>
> The above scenario can also occur when setitimer is used as a
> timing source and a SIGALRM is sent. Maybe we should *always*
> run ret_with_resched upon ret_from_intr and not only when in
> supervisor mode? Any reasons this would cause problems?
> Maybe also after running bottom halfs (in the case of setitimer)?
are you sure you are describing the right scenario? I do not doubt that
you see some kind of bug, but detecting ->need_resched == 1 after
user-space has been interrupted is fairly common - this is how timer
interrupts work.
there is a bug in this area though, we do not re-run the need_resched
check _after_ running do_signal(). We always run the check even if
user-space is interrupted, but the problem is that it has to be restarted
after delivering a signal too. The preliminary fix for this against
2.3.34-pre1 is attached, does it solve your problems? [patch works here
just fine]
the fix is a bit subtle, because we must not naively rerun the
need_resched-check after do_signal(), because that can cause signal
recursion. The patch does it's own, signal-local need_resched check after
do_signal. [schedule() runs bottom halves so no need to recheck for bottom
halves.]
btw., this patch plus the hlt patch fixes the SysKonnect gigabit ethernet
latency fluctuation i've noticed a couple of days ago - now latency over a
real gigabit network is stable at 70 microseconds ... [it was fluctuating
between 200 and 300 usecs before]
Ingo
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