linux-kernel - Re: [PATCH v18 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

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Date:   Wed, 10 Mar 2021 15:28:51 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Muchun Song <songmuchun@...edance.com>, corbet@....net,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        hpa@...or.com, dave.hansen@...ux.intel.com, luto@...nel.org,
        peterz@...radead.org, viro@...iv.linux.org.uk,
        akpm@...ux-foundation.org, mchehab+huawei@...nel.org,
        pawan.kumar.gupta@...ux.intel.com, rdunlap@...radead.org,
        oneukum@...e.com, anshuman.khandual@....com, jroedel@...e.de,
        almasrymina@...gle.com, rientjes@...gle.com, willy@...radead.org,
        osalvador@...e.de, song.bao.hua@...ilicon.com, david@...hat.com,
        naoya.horiguchi@....com, joao.m.martins@...cle.com,
        duanxiongchun@...edance.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, Chen Huang <chenhuang5@...wei.com>,
        Bodeddula Balasubramaniam <bodeddub@...zon.com>
Subject: Re: [PATCH v18 4/9] mm: hugetlb: alloc the vmemmap pages associated
 with each HugeTLB page

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

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
> 
> > 							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
> >