Re: [PATCH RESEND 4/4] dax, kmem: calculate abstract distance with general interface

Alistair Popple
2023-08-22T07:38:46Z

"Huang, Ying" <ying.huang@intel.com> writes:

> Alistair Popple <apopple@nvidia.com> writes:
>
>> "Huang, Ying" <ying.huang@intel.com> writes:
>>
>>> Alistair Popple <apopple@nvidia.com> writes:
>>>
>>>> Huang Ying <ying.huang@intel.com> writes:
>>>>
>>>>> Previously, a fixed abstract distance MEMTIER_DEFAULT_DAX_ADISTANCE is
>>>>> used for slow memory type in kmem driver. This limits the usage of
>>>>> kmem driver, for example, it cannot be used for HBM (high bandwidth
>>>>> memory).
>>>>>
>>>>> So, we use the general abstract distance calculation mechanism in kmem
>>>>> drivers to get more accurate abstract distance on systems with proper
>>>>> support. The original MEMTIER_DEFAULT_DAX_ADISTANCE is used as
>>>>> fallback only.
>>>>>
>>>>> Now, multiple memory types may be managed by kmem. These memory types
>>>>> are put into the "kmem_memory_types" list and protected by
>>>>> kmem_memory_type_lock.
>>>>
>>>> See below but I wonder if kmem_memory_types could be a common helper
>>>> rather than kdax specific?
>>>>
>>>>> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
>>>>> Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>>>>> Cc: Wei Xu <weixugc@google.com>
>>>>> Cc: Alistair Popple <apopple@nvidia.com>
>>>>> Cc: Dan Williams <dan.j.williams@intel.com>
>>>>> Cc: Dave Hansen <dave.hansen@intel.com>
>>>>> Cc: Davidlohr Bueso <dave@stgolabs.net>
>>>>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>>>>> Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
>>>>> Cc: Michal Hocko <mhocko@kernel.org>
>>>>> Cc: Yang Shi <shy828301@gmail.com>
>>>>> Cc: Rafael J Wysocki <rafael.j.wysocki@intel.com>
>>>>> ---
>>>>> drivers/dax/kmem.c | 54 +++++++++++++++++++++++++++---------
>>>>> include/linux/memory-tiers.h | 2 ++
>>>>> mm/memory-tiers.c | 2 +-
>>>>> 3 files changed, 44 insertions(+), 14 deletions(-)
>>>>>
>>>>> diff --git a/drivers/dax/kmem.c b/drivers/dax/kmem.c
>>>>> index 898ca9505754..837165037231 100644
>>>>> --- a/drivers/dax/kmem.c
>>>>> +++ b/drivers/dax/kmem.c
>>>>> @@ -49,14 +49,40 @@ struct dax_kmem_data {
>>>>> struct resource *res[];
>>>>> };
>>>>>
>>>>> -static struct memory_dev_type *dax_slowmem_type;
>>>>> +static DEFINE_MUTEX(kmem_memory_type_lock);
>>>>> +static LIST_HEAD(kmem_memory_types);
>>>>> +
>>>>> +static struct memory_dev_type *kmem_find_alloc_memorty_type(int adist)
>>>>> +{
>>>>> + bool found = false;
>>>>> + struct memory_dev_type *mtype;
>>>>> +
>>>>> + mutex_lock(&kmem_memory_type_lock);
>>>>> + list_for_each_entry(mtype, &kmem_memory_types, list) {
>>>>> + if (mtype->adistance == adist) {
>>>>> + found = true;
>>>>> + break;
>>>>> + }
>>>>> + }
>>>>> + if (!found) {
>>>>> + mtype = alloc_memory_type(adist);
>>>>> + if (!IS_ERR(mtype))
>>>>> + list_add(&mtype->list, &kmem_memory_types);
>>>>> + }
>>>>> + mutex_unlock(&kmem_memory_type_lock);
>>>>> +
>>>>> + return mtype;
>>>>> +}
>>>>> +
>>>>> static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
>>>>> {
>>>>> struct device *dev = &dev_dax->dev;
>>>>> unsigned long total_len = 0;
>>>>> struct dax_kmem_data *data;
>>>>> + struct memory_dev_type *mtype;
>>>>> int i, rc, mapped = 0;
>>>>> int numa_node;
>>>>> + int adist = MEMTIER_DEFAULT_DAX_ADISTANCE;
>>>>>
>>>>> /*
>>>>> * Ensure good NUMA information for the persistent memory.
>>>>> @@ -71,6 +97,11 @@ static int dev_dax_kmem_probe(struct dev_dax *dev_dax)
>>>>> return -EINVAL;
>>>>> }
>>>>>
>>>>> + mt_calc_adistance(numa_node, &adist);
>>>>> + mtype = kmem_find_alloc_memorty_type(adist);
>>>>> + if (IS_ERR(mtype))
>>>>> + return PTR_ERR(mtype);
>>>>> +
>>>>
>>>> I wrote my own quick and dirty module to test this and wrote basically
>>>> the same code sequence.
>>>>
>>>> I notice your using a list of memory types here though. I think it would
>>>> be nice to have a common helper that other users could call to do the
>>>> mt_calc_adistance() / kmem_find_alloc_memory_type() /
>>>> init_node_memory_type() sequence and cleanup as my naive approach would
>>>> result in a new memory_dev_type per device even though adist might be
>>>> the same. A common helper would make it easy to de-dup those.
>>>
>>> If it's useful, we can move kmem_find_alloc_memory_type() to
>>> memory-tier.c after some revision. But I tend to move it after we have
>>> the second user. What do you think about that?
>>
>> Usually I would agree, but this series already introduces a general
>> interface for calculating adist even though there's only one user and
>> implementation. So if we're going to add a general interface I think it
>> would be better to make it more usable now rather than after variations
>> of it have been cut and pasted into other drivers.
>
> In general, I would like to introduce complexity when necessary. So, we
> can discuss the necessity of the general interface firstly. We can do
> that in [1/4] of the series.

Do we need one memory_dev_type per adistance or per adistance+device?

If IUC correctly I think it's the former. Logically that means
memory_dev_types should be managed by the memory-tiering subsystem
because they are system wide rather than driver specific resources. That
we need to add the list field to struct memory_dev_type specifically for
use by dax/kmem supports that idea.

Also I'm not sure why you consider moving the
kmem_memory_types/kmem_find_alloc_memory_type()/etc. functions into
mm/memory-tiers.c to add complexity. Isn't it just moving code around or
am I missing some other subtlety that makes this hard? I really think
logically memory-tiering.c is where management of the various
memory_dev_types belongs.

Thanks.
Alistair