Message-ID: <CAKPbEqqH0nhVUAcJUxDc_bPewY85+TqhtO94MyypV35GBo33+A@mail.gmail.com>
Date: Tue, 9 Apr 2024 22:55:54 -0700
From: "Ho-Ren (Jack) Chuang" <horenchuang@...edance.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Jonathan Cameron <Jonathan.Cameron@...wei.com>, Gregory Price <gourry.memverge@...il.com>, 
	aneesh.kumar@...ux.ibm.com, mhocko@...e.com, tj@...nel.org, 
	john@...alactic.com, Eishan Mirakhur <emirakhur@...ron.com>, 
	Vinicius Tavares Petrucci <vtavarespetr@...ron.com>, Ravis OpenSrc <Ravis.OpenSrc@...ron.com>, 
	Alistair Popple <apopple@...dia.com>, Srinivasulu Thanneeru <sthanneeru@...ron.com>, 
	SeongJae Park <sj@...nel.org>, Dan Williams <dan.j.williams@...el.com>, 
	Vishal Verma <vishal.l.verma@...el.com>, Dave Jiang <dave.jiang@...el.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, nvdimm@...ts.linux.dev, 
	linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Linux Memory Management List <linux-mm@...ck.org>, "Ho-Ren (Jack) Chuang" <horenc@...edu>, 
	"Ho-Ren (Jack) Chuang" <horenchuang@...il.com>, qemu-devel@...gnu.org, 
	Hao Xiang <hao.xiang@...edance.com>
Subject: Re: [External] Re: [PATCH v11 2/2] memory tier: create CPUless memory
 tiers after obtaining HMAT info

On Tue, Apr 9, 2024 at 7:33 PM Huang, Ying <ying.huang@...el.com> wrote:
>
> "Ho-Ren (Jack) Chuang" <horenchuang@...edance.com> writes:
>
> > On Fri, Apr 5, 2024 at 7:03 AM Jonathan Cameron
> > <Jonathan.Cameron@...wei.com> wrote:
> >>
> >> On Fri,  5 Apr 2024 00:07:06 +0000
> >> "Ho-Ren (Jack) Chuang" <horenchuang@...edance.com> wrote:
> >>
> >> > The current implementation treats emulated memory devices, such as
> >> > CXL1.1 type3 memory, as normal DRAM when they are emulated as normal memory
> >> > (E820_TYPE_RAM). However, these emulated devices have different
> >> > characteristics than traditional DRAM, making it important to
> >> > distinguish them. Thus, we modify the tiered memory initialization process
> >> > to introduce a delay specifically for CPUless NUMA nodes. This delay
> >> > ensures that the memory tier initialization for these nodes is deferred
> >> > until HMAT information is obtained during the boot process. Finally,
> >> > demotion tables are recalculated at the end.
> >> >
> >> > * late_initcall(memory_tier_late_init);
> >> > Some device drivers may have initialized memory tiers between
> >> > `memory_tier_init()` and `memory_tier_late_init()`, potentially bringing
> >> > online memory nodes and configuring memory tiers. They should be excluded
> >> > in the late init.
> >> >
> >> > * Handle cases where there is no HMAT when creating memory tiers
> >> > There is a scenario where a CPUless node does not provide HMAT information.
> >> > If no HMAT is specified, it falls back to using the default DRAM tier.
> >> >
> >> > * Introduce another new lock `default_dram_perf_lock` for adist calculation
> >> > In the current implementation, iterating through CPUlist nodes requires
> >> > holding the `memory_tier_lock`. However, `mt_calc_adistance()` will end up
> >> > trying to acquire the same lock, leading to a potential deadlock.
> >> > Therefore, we propose introducing a standalone `default_dram_perf_lock` to
> >> > protect `default_dram_perf_*`. This approach not only avoids deadlock
> >> > but also prevents holding a large lock simultaneously.
> >> >
> >> > * Upgrade `set_node_memory_tier` to support additional cases, including
> >> >   default DRAM, late CPUless, and hot-plugged initializations.
> >> > To cover hot-plugged memory nodes, `mt_calc_adistance()` and
> >> > `mt_find_alloc_memory_type()` are moved into `set_node_memory_tier()` to
> >> > handle cases where memtype is not initialized and where HMAT information is
> >> > available.
> >> >
> >> > * Introduce `default_memory_types` for those memory types that are not
> >> >   initialized by device drivers.
> >> > Because late initialized memory and default DRAM memory need to be managed,
> >> > a default memory type is created for storing all memory types that are
> >> > not initialized by device drivers and as a fallback.
> >> >
> >> > Signed-off-by: Ho-Ren (Jack) Chuang <horenchuang@...edance.com>
> >> > Signed-off-by: Hao Xiang <hao.xiang@...edance.com>
> >> > Reviewed-by: "Huang, Ying" <ying.huang@...el.com>
> >>
> >> Hi - one remaining question. Why can't we delay init for all nodes
> >> to either drivers or your fallback late_initcall code.
> >> It would be nice to reduce possible code paths.
> >
> > I try not to change too much of the existing code structure in
> > this patchset.
> >
> > To me, postponing/moving all memory tier registrations to
> > late_initcall() is another possible action item for the next patchset.
> >
> > After tier_mem(), hmat_init() is called, which requires registering
> > `default_dram_type` info. This is when `default_dram_type` is needed.
> > However, it is indeed possible to postpone the latter part,
> > set_node_memory_tier(), to `late_init(). So, memory_tier_init() can
> > indeed be split into two parts, and the latter part can be moved to
> > late_initcall() to be processed together.
>
> I don't think that it's good to move all memory_tier initialization in
> drivers to late_initcall().  It's natural to keep them in
> device_initcall() level.
>
> If so, we can allocate default_dram_type in memory_tier_init(), and call
> set_node_memory_tier() only in memory_tier_lateinit().  We can call
> memory_tier_lateinit() in device_initcall() level too.
>

It makes sense to me to leave only `default_dram_type ` and
hotplug_init() in memory_tier_init(), postponing all
set_node_memory_tier()s to memory_tier_late_init()

Would it be possible there is no device_initcall() calling
memory_tier_late_init()? If yes, I think putting memory_tier_late_init()
in late_init() is still necessary.

> --
> Best Regards,
> Huang, Ying
>
> > Doing this all memory-type drivers have to call late_initcall() to
> > register a memory tier. I’m not sure how many they are?
> >
> > What do you guys think?
> >
> >>
> >> Jonathan
> >>
> >>
> >> > ---
> >> >  mm/memory-tiers.c | 94 +++++++++++++++++++++++++++++++++++------------
> >> >  1 file changed, 70 insertions(+), 24 deletions(-)
> >> >
> >> > diff --git a/mm/memory-tiers.c b/mm/memory-tiers.c
> >> > index 516b144fd45a..6632102bd5c9 100644
> >> > --- a/mm/memory-tiers.c
> >> > +++ b/mm/memory-tiers.c
> >>
> >>
> >>
> >> > @@ -855,7 +892,8 @@ static int __init memory_tier_init(void)
> >> >        * For now we can have 4 faster memory tiers with smaller adistance
> >> >        * than default DRAM tier.
> >> >        */
> >> > -     default_dram_type = alloc_memory_type(MEMTIER_ADISTANCE_DRAM);
> >> > +     default_dram_type = mt_find_alloc_memory_type(MEMTIER_ADISTANCE_DRAM,
> >> > +                                                   &default_memory_types);
> >> >       if (IS_ERR(default_dram_type))
> >> >               panic("%s() failed to allocate default DRAM tier\n", __func__);
> >> >
> >> > @@ -865,6 +903,14 @@ static int __init memory_tier_init(void)
> >> >        * types assigned.
> >> >        */
> >> >       for_each_node_state(node, N_MEMORY) {
> >> > +             if (!node_state(node, N_CPU))
> >> > +                     /*
> >> > +                      * Defer memory tier initialization on
> >> > +                      * CPUless numa nodes. These will be initialized
> >> > +                      * after firmware and devices are initialized.
> >>
> >> Could the comment also say why we can't defer them all?
> >>
> >> (In an odd coincidence we have a similar issue for some CPU hotplug
> >>  related bring up where review feedback was move all cases later).
> >>
> >> > +                      */
> >> > +                     continue;
> >> > +
> >> >               memtier = set_node_memory_tier(node);
> >> >               if (IS_ERR(memtier))
> >> >                       /*
> >>



-- 
Best regards,
Ho-Ren (Jack) Chuang
莊賀任

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