Re: [PATCH v11 2/2] memory tier: create CPUless memory tiers after obtaining HMAT info

["Ho-Ren (Jack) Chuang" <horenchuang@bytedance.com>]

On Fri, Apr 5, 2024 at 7:03 AM Jonathan Cameron
<Jonathan.Cameron@huawei.com> wrote:
>
> On Fri,  5 Apr 2024 00:07:06 +0000
> "Ho-Ren (Jack) Chuang" <horenchuang@bytedance.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@bytedance.com>
> > Signed-off-by: Hao Xiang <hao.xiang@bytedance.com>
> > Reviewed-by: "Huang, Ying" <ying.huang@intel.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.

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