From: Robert Love <rml@tech9.net>
To: torvalds@transmeta.com
Cc: linux-kernel@vger.kernel.org
Subject: [PATCH] 2.5: don't miss a preemption oppurtunity
Date: 06 Apr 2002 14:09:49 -0500 [thread overview]
Message-ID: <1018120190.899.100.camel@phantasy> (raw)
Linus,
The new kernel preemption code in 2.5 is great. The only missing bit is
a check for need to reschedule at the end of the preemption handling
code in preempt_schedule and entry.S. Anytime preemption is disabled,
there is a possibility kernel code could set need_resched and try to
preempt. We need to check need_resched as soon as we reenable
preemption (preempt_count=0) in these code paths.
Right now this window exists anytime after resume_kernel sets
preempt_count to 0 and after return from schedule when preempt_count is
also set to 0. Note they can last as long as the next interrupt - or
longer if a lock was grabbed in the interim. This is a decade in
real-time land.
This patch, against 2.4.8-pre2, adds a simple check for need_resched to
the end of these codepaths and jumps back into them if needed. Please
apply.
Robert Love
diff -urN linux-2.5.8-pre2/kernel/sched.c linux/kernel/sched.c
--- linux-2.5.8-pre2/kernel/sched.c Fri Apr 5 20:18:12 2002
+++ linux/kernel/sched.c Fri Apr 5 21:02:54 2002
@@ -851,10 +851,15 @@
if (unlikely(ti->preempt_count))
return;
+need_resched:
ti->preempt_count = PREEMPT_ACTIVE;
schedule();
ti->preempt_count = 0;
barrier();
+
+ /* we could miss a preemption between schedule() and now */
+ if (unlikely(test_thread_flag(TIF_NEED_RESCHED)))
+ goto need_resched;
}
#endif /* CONFIG_PREEMPT */
diff -urN linux-2.5.8-pre2/arch/i386/kernel/entry.S linux/arch/i386/kernel/entry.S
--- linux-2.5.8-pre2/arch/i386/kernel/entry.S Fri Apr 5 20:18:10 2002
+++ linux/arch/i386/kernel/entry.S Sat Apr 6 01:59:17 2002
@@ -232,6 +232,7 @@
ENTRY(resume_kernel)
cmpl $0,TI_PRE_COUNT(%ebx)
jnz restore_all
+need_resched:
movl TI_FLAGS(%ebx), %ecx
testb $_TIF_NEED_RESCHED, %cl
jz restore_all
@@ -241,8 +242,9 @@
movl $PREEMPT_ACTIVE,TI_PRE_COUNT(%ebx)
sti
call SYMBOL_NAME(schedule)
- movl $0,TI_PRE_COUNT(%ebx)
- jmp restore_all
+ movl $0,TI_PRE_COUNT(%ebx)
+ cli
+ jmp need_resched
#endif
# system call handler stub
reply other threads:[~2002-04-06 19:09 UTC|newest]
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