Re: [PATCH] Re: softirq buggy

[Manfred Spraul <manfred@colorfullife.com>]

Andrea Arcangeli wrote:
> 
> your cpu_is_idle will return 0 in the need_resched != 0 check even if the cpu
> is idle (because of the -1 trick for avoiding the SMP-IPI to notify the cpu).
>
Fixed.

> The issue you are addressing is quite londstanding and it is not only related
> to the loop with an idle cpu.
> 
> This is the way I prefer to fix it:
> 
>         ftp://ftp.us.kernel.org/pub/linux/kernel/people/andrea/patches/v2.4/2.4.4pre1/ksoftirqd-1
>
The return path to user space checks for pending softirqs. A delay of
1/HZ is only possible if the cpu loops in kernel space without returning
to user space - and the functions that can loop check
'current->need_resched'. That means that either cpu_is_idle() must be
renamed to schedule_required() and all 'need_resched' users should use
that function, or something like your patch.

Is a full thread really necessary? Just setting 'need_resched' should be
enough, schedule() checks for pending softirqs.
And do you have a rough idea how often that new thread is scheduled
under load?

Btw, you don't schedule the ksoftirqd thread if do_softirq() returns
from the 'if(in_interrupt())' check.
I assume that this is the most common case of delayed softirq
processing:

; in process context
spin_lock_bh();
; hw interrupt arrives
; do_softirq returns immediately
spin_unlock_bh();


--
	Manfred