linux-kernel - Re: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers

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Message-ID: <CAAPL-u96pwSEA00NM_km6VD8FbxDtZbZpm3or_byZT2FGbvwFg@mail.gmail.com>
Date:   Tue, 7 Jun 2022 13:18:31 -0700
From:   Wei Xu <weixugc@...gle.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Huang Ying <ying.huang@...el.com>,
        Greg Thelen <gthelen@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim C Chen <tim.c.chen@...el.com>,
        Brice Goglin <brice.goglin@...il.com>,
        Michal Hocko <mhocko@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hesham Almatary <hesham.almatary@...wei.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Alistair Popple <apopple@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Feng Tang <feng.tang@...el.com>,
        Jagdish Gediya <jvgediya@...ux.ibm.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers

On Tue, Jun 7, 2022 at 11:43 AM Tim Chen <tim.c.chen@...ux.intel.com> wrote:
>
> On Fri, 2022-06-03 at 19:12 +0530, Aneesh Kumar K.V wrote:
> >
> >
> > The nodes which are part of a specific memory tier can be listed
> > via
> > /sys/devices/system/memtier/memtierN/nodelist
> >
> > "Rank" is an opaque value. Its absolute value doesn't have any
> > special meaning. But the rank values of different memtiers can be
> > compared with each other to determine the memory tier order.
> >
> > For example, if we have 3 memtiers: memtier0, memtier1, memiter2, and
> > their rank values are 300, 200, 100, then the memory tier order is:
> > memtier0 -> memtier2 -> memtier1,
>
> Why is memtier2 (rank 100) higher than memtier1 (rank 200)?  Seems like
> the order should be memtier0 -> memtier1 -> memtier2?
>                     (rank 300)  (rank 200)  (rank 100)

I think this is a copy-and-modify typo from my original memory tiering
kernel interface RFC (v4,
https://lore.kernel.org/linux-mm/CAAPL-u9Wv+nH1VOZTj=9p9S70Y3Qz3+63EkqncRDdHfubsrjfw@mail.gmail.com/T/):
where the rank values are 100, 10, 50 (i.e the rank of memtier2 is
higher than memtier1).

> > where memtier0 is the highest tier
> > and memtier1 is the lowest tier.
>
> I think memtier2 is the lowest as it has the lowest rank value.
> >
> > The rank value of each memtier should be unique.
> >
> >
> > +
> > +static void memory_tier_device_release(struct device *dev)
> > +{
> > +     struct memory_tier *tier = to_memory_tier(dev);
> > +
>
> Do we need some ref counts on memory_tier?
> If there is another device still using the same memtier,
> free below could cause problem.
>
> > +     kfree(tier);
> > +}
> > +
> >
> ...
> > +static struct memory_tier *register_memory_tier(unsigned int tier)
> > +{
> > +     int error;
> > +     struct memory_tier *memtier;
> > +
> > +     if (tier >= MAX_MEMORY_TIERS)
> > +             return NULL;
> > +
> > +     memtier = kzalloc(sizeof(struct memory_tier), GFP_KERNEL);
> > +     if (!memtier)
> > +             return NULL;
> > +
> > +     memtier->dev.id = tier;
> > +     memtier->rank = get_rank_from_tier(tier);
> > +     memtier->dev.bus = &memory_tier_subsys;
> > +     memtier->dev.release = memory_tier_device_release;
> > +     memtier->dev.groups = memory_tier_dev_groups;
> > +
>
> Should you take the mem_tier_lock before you insert to
> memtier-list?
>
> > +     insert_memory_tier(memtier);
> > +
> > +     error = device_register(&memtier->dev);
> > +     if (error) {
> > +             list_del(&memtier->list);
> > +             put_device(&memtier->dev);
> > +             return NULL;
> > +     }
> > +     return memtier;
> > +}
> > +
> > +__maybe_unused // temporay to prevent warnings during bisects
> > +static void unregister_memory_tier(struct memory_tier *memtier)
> > +{
>
> I think we should take mem_tier_lock before modifying memtier->list.
>
> > +     list_del(&memtier->list);
> > +     device_unregister(&memtier->dev);
> > +}
> > +
> >
>
> Thanks.
>
> Tim
>
>