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    SubjectRe: [PATCH] local_irq_disable removal
    On Sat, 11 Jun 2005, Daniel Walker wrote:

    > On Sat, 11 Jun 2005, Esben Nielsen wrote:
    > > On Fri, 10 Jun 2005, Daniel Walker wrote:
    > >
    > > > On Sat, 2005-06-11 at 01:37 +0200, Esben Nielsen wrote:
    > > > [...]
    > > > > As far as I can see the only solution is to replace them with a per-cpu
    > > > > mutex. Such a mutex can be the rt_mutex for now, but someone may want to
    > > > > make a more optimized per-cpu version where a raw_spin_lock isn't used.
    > > > > That would make it nearly as cheap as cli()/sti() when there is no
    > > > > congestion. One doesn't need PI for this region either as the RT
    > > > > subsystems will not hit it anyway.
    > > >
    > > > I don't like this solution mainly because it's so expensive. cli/sti may
    > > > take a few cycles at most, what your suggesting may take 50 times that,
    > > > which would similar in speed to put linux under adeos..
    > >
    > > We are only talking about the local_irq_disable()/enable() in drivers, not
    > > the core system, right? Therefore making it into a mutex will not be that
    > > expensive overall.
    > No, core system . We're talking about everything, including
    > raw_spinlock_t.
    I think that is a really bad idea then. It only helps on irq-latency. The
    rest of the system will see lower performance. It should certainly not be
    on as a default thing with PREEMPT_RT. Such low latencies are rarely

    I think extremely low irq-latencies can better obtained with other
    solutions closer to the sub-kernel approach, i.e. taking it completely
    away from the scheduler such that the whole kernel - including the
    scheduler, raw_spinlock etc. - runs with irqs enabled.

    Actually, I think there should be 3 "levels" of irq-macroes:
    local_irq_disable() should not compile under PREEMPT_RT (!!)
    raw_local_irq_disable() should be used within the core kernel code.
    hard_local_irq_disable() for the very low latency interrupt systems.
    Under normal circumstances raw_local and hard_local should refer to the
    hardware but raw_local can be made into a soft-interrupt state close to
    the sub-kernel approach such that one or two very special interupts can
    come through.


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