linux-kernel - Re: [PATCH v3 updated] mm/demotion: Expose memory tier details via sysfs

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Date:   Fri, 2 Sep 2022 00:02:05 -0700
From:   Wei Xu <weixugc@...gle.com>
To:     Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
Cc:     "Huang, Ying" <ying.huang@...el.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yang Shi <shy828301@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim C Chen <tim.c.chen@...el.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>,
        jvgediya.oss@...il.com, Bharata B Rao <bharata@....com>,
        Greg Thelen <gthelen@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH v3 updated] mm/demotion: Expose memory tier details via sysfs

On Thu, Sep 1, 2022 at 11:44 PM Aneesh Kumar K V
<aneesh.kumar@...ux.ibm.com> wrote:
>
> On 9/2/22 12:10 PM, Huang, Ying wrote:
> > Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com> writes:
> >
> >> On 9/2/22 11:42 AM, Huang, Ying wrote:
> >>> Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com> writes:
> >>>
> >>>> On 9/2/22 11:10 AM, Huang, Ying wrote:
> >>>>> Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com> writes:
> >>>>>
> >>>>>> On 9/2/22 10:39 AM, Wei Xu wrote:
> >>>>>>> On Thu, Sep 1, 2022 at 5:33 PM Huang, Ying <ying.huang@...el.com> wrote:
> >>>>>>>>
> >>>>>>>> Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com> writes:
> >>>>>>>>
> >>>>>>>>> On 9/1/22 12:31 PM, Huang, Ying wrote:
> >>>>>>>>>> "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com> writes:
> >>>>>>>>>>
> >>>>>>>>>>> This patch adds /sys/devices/virtual/memory_tiering/ where all memory tier
> >>>>>>>>>>> related details can be found. All allocated memory tiers will be listed
> >>>>>>>>>>> there as /sys/devices/virtual/memory_tiering/memory_tierN/
> >>>>>>>>>>>
> >>>>>>>>>>> The nodes which are part of a specific memory tier can be listed via
> >>>>>>>>>>> /sys/devices/virtual/memory_tiering/memory_tierN/nodes
> >>>>>>>>>>
> >>>>>>>>>> I think "memory_tier" is a better subsystem/bus name than
> >>>>>>>>>> memory_tiering.  Because we have a set of memory_tierN devices inside.
> >>>>>>>>>> "memory_tier" sounds more natural.  I know this is subjective, just my
> >>>>>>>>>> preference.
> >>>>>>>>>>
> >>>>>>
> >>>>>>
> >>>>>> I missed replying to this earlier. I will keep memory_tiering as subsystem name in v4
> >>>>>> because we would want it to a susbsystem where all memory tiering related details can be found
> >>>>>> including memory type in the future. This is as per discussion
> >>>>>>
> >>>>>> https://lore.kernel.org/linux-mm/CAAPL-u9TKbHGztAF=r-io3gkX7gorUunS2UfstudCWuihrA=0g@mail.gmail.com
> >>>>>
> >>>>> I don't think that it's a good idea to mix 2 types of devices in one
> >>>>> subsystem (bus).  If my understanding were correct, that breaks the
> >>>>> driver core convention.
> >>>>>
> >>>>
> >>>> All these are virtual devices .I am not sure i follow what you mean by 2 types of devices.
> >>>> memory_tiering is a subsystem that represents all the details w.r.t memory tiering. It shows
> >>>> details of memory tiers and can possibly contain details of different memory types .
> >>>
> >>> IMHO, memory_tier and memory_type are 2 kind of devices.  They have
> >>> almost totally different attributes (sysfs file).  So, we should create
> >>> 2 buses for them.  Each has its own attribute group.  "virtual" itself
> >>> isn't a subsystem.
> >>
> >> Considering both the details are related to memory tiering, wouldn't it be much simpler we consolidate
> >> them within the same subdirectory? I am still not clear why you are suggesting they need to be in different
> >> sysfs hierarchy.  It doesn't break any driver core convention as you mentioned earlier.
> >>
> >> /sys/devices/virtual/memory_tiering/memory_tierN
> >> /sys/devices/virtual/memory_tiering/memory_typeN
> >
> > I think we should add
> >
> >  /sys/devices/virtual/memory_tier/memory_tierN
> >  /sys/devices/virtual/memory_type/memory_typeN
> >
>
> I am trying to find if there is a technical reason to do the same?
>
> > I don't think this is complex.  Devices of same bus/subsystem should
> > have mostly same attributes.  This is my understanding of driver core
> > convention.
> >
>
> I was not looking at this from code complexity point. Instead of having multiple directories
> with details w.r.t memory tiering, I was looking at consolidating the details
> within the directory /sys/devices/virtual/memory_tiering. (similar to all virtual devices
> are consolidated within /sys/devics/virtual/).
>
> -aneesh

Here is an example of /sys/bus/nd/devices (I know it is not under
/sys/devices/virtual, but it can still serve as a reference):

ls -1 /sys/bus/nd/devices

namespace2.0
namespace3.0
ndbus0
nmem0
nmem1
region0
region1
region2
region3

So I think it is not unreasonable if we want to group memory tiering
related interfaces within a single top directory.

Wei