the -G0 irqlock patch can be found at:
http://redhat.com/~mingo/remove-irqlock-patches/remove-irqlock-2.5.27-G0
Changes in -G0:
- fix buggy in_softirq(). Fortunately the bug made the test broader,
which didnt result in algorithmical breakage, just suboptimal
performance.
- move do_softirq() processing into irq_exit() => this also fixes the
softirq processing bugs present in apic.c IRQ handlers that did not
test for softirqs after irq_exit().
- simplify local_bh_enable().
Changes in -F9:
- replace all instances of:
local_save_flags(flags);
local_irq_disable();
with the shorter form of:
local_irq_save(flags);
about 30 files are affected by this change.
Changes in -F8:
- preempt/hardirq/softirq count separation, cleanups.
- skbuff.c fix.
- use irq_count() in scheduler_tick()
Changes in -F3:
- the entry.S cleanups/speedups by Oleg Nesterov.
- a rather critical synchronize_irq() bugfix: if a driver frees an
interrupt that is still being probed then synchronize_irq() locks up.
This bug has caused a spurious boot-lockup on one of my testsystems,
ifconfig would lock up trying to close eth0.
- remove duplicate definitions from asm-i386/system.h, this fixes
compiler warnings.
Ingo
Hello.
I can't understand the preempt_count() check in irq_exit()
and local_bh_enable(). Did you meant irq_count() instead?
I beleive preempt_disable() should not disable soft interrupts.
In local_bh_enable() it is probably unneeded at all, because
nested local_bh_disable() is rare (i think), and do_softirq()
checks in_interrupt().
Now suppose preempt_count() & PREEMPT_MASK == 0.
Then local_bh_enable() has a small preemptible window between
__local_bh_enable() and do_softirq()->local_irq_save(flags).
It is only latency problem.
But in irq_exit() case interrupt context may be preempted
while doing wakeup_softirqd(cpu) after __local_bh_enable()
in do_softirq().
So i suggest something like this pseudo code:
__preempt_hack(offset)
{
barrier();
preempt_count() -= offset
#ifdef CONFIG_PREEMPT
- 1
#endif
;
}
irq_exit()
{
__preempt_hack(HARDIRQ_OFFSET);
if (unlikely(!irq_count() &&
softirq_pending(smp_processor_id())))
do_softirq();
preempt_enable_no_resched();
}
local_bh_enable()
{
__preempt_hack(SOFTIRQ_OFFSET);
if (unlikely(!irq_count() &&
softirq_pending(smp_processor_id())))
do_softirq();
preempt_enable();
}
Or just add extra preempt_disable() in both functions to kill
terrible __preempt_hack().
Sorry, i have no remove-irqlock patches applied, so can't
suggest diff.
Oleg.
On Wed, 24 Jul 2002, Oleg Nesterov wrote:
> I can't understand the preempt_count() check in irq_exit() and
> local_bh_enable(). Did you meant irq_count() instead? I beleive
> preempt_disable() should not disable soft interrupts.
yes, you are right, and i fixed them in -G3.
doh, i *thought* i fixed both places in -G3, but i only fixed irq_exit().
the local_bh_enable() fix is in -G4:
http://redhat.com/~mingo/remove-irqlock-patches/remove-irqlock-2.5.27-G4
> In local_bh_enable() it is probably unneeded at all, because nested
> local_bh_disable() is rare (i think), and do_softirq() checks
> in_interrupt().
but still we dont want to call do_softirq() all the time. local_bh_enable
is used in quite performance-sensitive networking code.
> Now suppose preempt_count() & PREEMPT_MASK == 0.
>
> Then local_bh_enable() has a small preemptible window between
> __local_bh_enable() and do_softirq()->local_irq_save(flags).
> It is only latency problem.
i dont think getting a preemption before softirqs are processed is a big
problem. Such type of preemption comes in form of an interrupt, which
handles softirqs in irq_exit() anyway, so there's no delay.
> But in irq_exit() case interrupt context may be preempted
> while doing wakeup_softirqd(cpu) after __local_bh_enable()
> in do_softirq().
okay - i've fixed irq_exit() once more in -G4, could you double-check it?
Ingo
On Wed, 24 Jul 2002, Ingo Molnar wrote:
> > local_bh_disable() is rare (i think), and do_softirq() checks
> > in_interrupt().
>
> but still we dont want to call do_softirq() all the time. local_bh_enable
> is used in quite performance-sensitive networking code.
in fact the in_interrupt() check in do_softirq() should never trigger with
the latest patch applied - i'll put a debugging printk there and we can
remove it after some time. This will speed things up a bit.
Ingo
Hello.
> > Then local_bh_enable() has a small preemptible window between
>
> i dont think getting a preemption before softirqs are processed is a big
> problem. Such type of preemption comes in form of an interrupt, which
Ah, yes...
> > But in irq_exit() case interrupt context may be preempted
>
> okay - i've fixed irq_exit() once more in -G4
found G5, your forgot to add preempt_disable() in irq_exit()
#define irq_exit()
do {
+ preempt_disable();
preempt_count() -= IRQ_EXIT_OFFSET;
if (!in_interrupt() && softirq_pending(smp_processor_id()))
Oleg.
Hello.
> > Then local_bh_enable() has a small preemptible window between
> > __local_bh_enable() and do_softirq()->local_irq_save(flags).
> > It is only latency problem.
>
> i dont think getting a preemption before softirqs are processed is a big
> problem. Such type of preemption comes in form of an interrupt, which
> handles softirqs in irq_exit() anyway, so there's no delay.
Well, no. Not all smp_xxx_interrupt() use irq_enter/exit().
Reschedule interrupt, for example, do not. But indeed, it is not
big problem.
Oleg.
On Wed, 24 Jul 2002, Oleg Nesterov wrote:
> > okay - i've fixed irq_exit() once more in -G4
>
> found G5, your forgot to add preempt_disable() in irq_exit()
>
> #define irq_exit()
> do {
> + preempt_disable();
> preempt_count() -= IRQ_EXIT_OFFSET;
> if (!in_interrupt() && softirq_pending(smp_processor_id()))
>
> Oleg.
nope, it's tricky, check out the define of IRQ_EXIT_OFFSET, it has the
plus 1 count added already.
using preempt_disable() has the problem of putting a barrier() between the
++ and the -IRQ_OFFSET, causing one more instruction to be generated.
but there was another (not too critical) bug here - irq_enter() needs to
have a barrier() after manipulating the preemption count, irq_exit() needs
to have a barrier() before. Fixed in my tree.
Ingo