Date: Tue, 28 Feb 2006 13:31:23 -0500 From: John Baldwin <jhb@freebsd.org> To: freebsd-hackers@freebsd.org Cc: ups@freebsd.org, Kazuaki Oda <kaakun@highway.ne.jp> Subject: Re: question about preemption code Message-ID: <200602281331.26100.jhb@freebsd.org> In-Reply-To: <440039A1.9030500@highway.ne.jp>
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On Saturday 25 February 2006 06:04, Kazuaki Oda wrote:
> Hi,
>
> When reading kern_switch.c, I noticed odd difference between !SMP and
> SMP in maybe_preempt_in_ksegrp().
>
> In !SMP case:
> =================================================================
> #ifdef PREEMPTION
> #ifndef FULL_PREEMPTION
> if (td->td_priority > PRI_MAX_ITHD) {
> running_thread->td_flags |= TDF_NEEDRESCHED;
> return;
> }
> #endif /* FULL_PREEMPTION */
> =================================================================
>
> In SMP case:
> =================================================================
> #ifdef PREEMPTION
>
> #if !defined(FULL_PREEMPTION)
> if (td->td_priority > PRI_MAX_ITHD) {
> running_thread->td_flags |= TDF_NEEDRESCHED;
> }
> #endif /* ! FULL_PREEMPTION */
> =================================================================
>
> Is there any reason not to return after setting TDF_NEEDRESCHED flag
> in SMP case? Because of this, we do context switch even if td's
> priority is lower than PRI_MAX_ITHD. And, I think, it is not
> PREEMPTION, but FULL_PREEMPTION.
Yes, that is a bug, but it only would affect multithreaded applications
I think. Also, the maybe_preempt_in_kseg() stuff doesn't handle idle
priority threads correctly in the !FULL_PREEMPTION case (the maybe_preempt()
function in !FULL_PREEMPTION always preempts _to_ interrupt threads and
always preempts _from_ idle priority threads).
--
John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve" = http://www.FreeBSD.org
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