On Wed, Mar 10, 2021 at 02:10:12PM -0800, Mike Kravetz wrote:

On 3/10/21 1:49 PM, Paul E. McKenney wrote:

quoted

On Wed, Mar 10, 2021 at 10:11:22PM +0100, Michal Hocko wrote:

quoted

On Wed 10-03-21 10:56:08, Mike Kravetz wrote:

quoted

On 3/10/21 7:19 AM, Michal Hocko wrote:

quoted

On Mon 08-03-21 18:28:02, Muchun Song wrote:
[...]

quoted

@@ -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?

There are a -lot- of configs out there, so are you sure that HUGETLB is
really enabled in most of them?  ;-)

More seriously, I was going by earlier emails in this and related threads
plus Michal's "PREEMPT_COUNT should be selectable".  But there are other
situations that would like PREEMPT_COUNT.  And to your point, some who
would rather PREEMPT_COUNT not be universally enabled.  I haven't seen
any performance or kernel-size numbers from any of them, however.

							Thanx, Paul

-- 
Mike Kravetz

quoted

							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