Messages in this thread | | | Date | Thu, 13 Feb 2014 21:32:22 +0400 | From | Kirill Tkhai <> | Subject | Re: [PATCH] sched/core: Create new task with twice disabled preemption |
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On 13.02.2014 20:00, Peter Zijlstra wrote: > On Thu, Feb 13, 2014 at 07:51:56PM +0400, Kirill Tkhai wrote: >> For archs without __ARCH_WANT_UNLOCKED_CTXSW set this means >> that all newly created tasks execute finish_arch_post_lock_switch() >> and post_schedule() with preemption enabled. > > That's IA64 and MIPS; do they have a 'good' reason to use this?
It seems my description misleads reader, I'm sorry if so.
I mean all architectures *except* IA64 and MIPS. All, which has no __ARCH_WANT_UNLOCKED_CTXSW defined.
IA64 and MIPS already have preempt_enable() in schedule_tail():
#ifdef __ARCH_WANT_UNLOCKED_CTXSW /* In this case, finish_task_switch does not reenable preemption */ preempt_enable(); #endif
Their initial preemption is not decremented in finish_lock_switch().
So, we speak about x86, ARM64 etc.
Look at ARM64's finish_arch_post_lock_switch(). It looks a task must to not be preempted between switch_mm() and this function. But in case of new task this is possible.
Example: RT thread p0 and RT thread p1 are on shared mm. System has 2 cpu.
p0 is bound to CPU0. p1 is bound to CPU1.
p1 has set timer and it is sleeping.
p0 create fair thread f. Task f wakes on CPU1.
When f is between raw_spin_unlock_irq() and finish_arch_post_lock_switch(), preemption is enabled. In this moment the process p1 is waking on CPU1.
For p1 the check
if (!cpumask_test_and_set_cpu(cpu, mm_cpumask(next)) || prev != next)
in switch_mm() is not passed, because mm is the same. So, later we do not do cpu_switch_mm() in finish_arch_post_lock_switch() and we just go to userspace.
This is the problem I tried to solve. I don't know arm64, and I can't say how it is serious.
But it looks the place is buggy.
Kirill
> That is; the alternative is to fix those two archs and remove the > __ARCH_WANT_UNLOCKED_CTXSW clutter alltogether; which seems like a big > win to me. > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ >
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