On 3/10/21 1:49 PM, Paul E. McKenney wrote:
> On Wed, Mar 10, 2021 at 10:11:22PM +0100, Michal Hocko wrote:
>> On Wed 10-03-21 10:56:08, Mike Kravetz wrote:
>>> On 3/10/21 7:19 AM, Michal Hocko wrote:
>>>> On Mon 08-03-21 18:28:02, Muchun Song wrote:
>>>> [...]
>>>>> @@ -1447,7 +1486,7 @@ void free_huge_page(struct page *page)
>>>>> /*
>>>>> * Defer freeing if in non-task context to avoid hugetlb_lock deadlock.
>>>>> */
>>>>> - if (!in_task()) {
>>>>> + if (in_atomic()) {
>>>>
>>>> As I've said elsewhere in_atomic doesn't work for CONFIG_PREEMPT_COUNT=n.
>>>> We need this change for other reasons and so it would be better to pull
>>>> it out into a separate patch which also makes HUGETLB depend on
>>>> PREEMPT_COUNT.
>>>
>>> Yes, the issue of calling put_page for hugetlb pages from any context
>>> still needs work. IMO, that is outside the scope of this series. We
>>> already have code in this path which blocks/sleeps.
>>>
>>> Making HUGETLB depend on PREEMPT_COUNT is too restrictive. IIUC,
>>> PREEMPT_COUNT will only be enabled if we enable:
>>> PREEMPT "Preemptible Kernel (Low-Latency Desktop)"
>>> PREEMPT_RT "Fully Preemptible Kernel (Real-Time)"
>>> or, other 'debug' options. These are not enabled in 'more common'
>>> kernels. Of course, we do not want to disable HUGETLB in common
>>> configurations.
>>
>> I haven't tried that but PREEMPT_COUNT should be selectable even without
>> any change to the preemption model (e.g. !PREEMPT).
>
> It works reliably for me, for example as in the diff below. So,
> as Michal says, you should be able to add "select PREEMPT_COUNT" to
> whatever Kconfig option you need to.
>
Thanks Paul.
I may have been misreading Michal's suggestion of "make HUGETLB depend on
PREEMPT_COUNT". We could "select PREEMPT_COUNT" if HUGETLB is enabled.
However, since HUGETLB is enabled in most configs, then this would
result in PREEMPT_COUNT also being enabled in most configs. I honestly
do not know how much this will cost us? I assume that if it was free or
really cheap it would already be always on?
--
Mike Kravetz
> Thanx, Paul
>
> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> index 3128b7c..7d9f989 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -8,6 +8,7 @@ menu "RCU Subsystem"
> config TREE_RCU
> bool
> default y if SMP
> + select PREEMPT_COUNT
> help
> This option selects the RCU implementation that is
> designed for very large SMP system with hundreds or
>
