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Subject: Re: [PATCH v4 00/23] device-dax: Support sub-dividing soft-reserved
 ranges
To: Dan Williams <dan.j.williams@intel.com>
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CC: Andrew Morton <akpm@linux-foundation.org>, Ard Biesheuvel <ardb@kernel.org>, Mike Rapoport <rppt@linux.ibm.com>, Borislav Petkov <bp@alien8.de>, David Airlie <airlied@linux.ie>, Will Deacon <will@kernel.org>, Catalin Marinas <catalin.marinas@arm.com>, Ard Biesheuvel <ard.biesheuvel@linaro.org>, Joao Martins <joao.m.martins@oracle.com>, Tom Lendacky <thomas.lendacky@amd.com>, "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>, Jonathan Cameron <Jonathan.Cameron@huawei.com>, X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Pavel Tatashin <pasha.tatashin@soleen.com>, Peter Zijlstra <peterz@infradead.org>, Ben Skeggs <bskeggs@redhat.com>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Jason Gunthorpe <jgg@mellanox.com>, Jia He <justin.he@arm.com>, Ingo Molnar <mingo@redhat.com>, Dave Hansen <dave.hansen@linux.intel.com>, Paul Mackerras <paulus@ozlabs.org>, Brice Goglin <Brice.Goglin@inr
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>>
>> 1. On x86-64, e820 indicates "soft-reserved" memory. This memory is not
>> automatically used in the buddy during boot, but remains untouched
>> (similar to pmem). But as it involves ACPI as well, it could also be
>> used on arm64 (-e820), correct?
> 
> Correct, arm64 also gets the EFI support for enumerating memory this
> way. However, I would clarify that whether soft-reserved is given to
> the buddy allocator by default or not is the kernel's policy choice,
> "buddy-by-default" is ok and is what will happen anyways with older
> kernels on platforms that enumerate a memory range this way.

Is "soft-reserved" then the right terminology for that? It sounds very
x86-64/e820 specific. Maybe a compressed for of "performance
differentiated memory" might be a better fit to expose to user space, no?

> 
>> 2. Soft-reserved memory is volatile RAM with differing performance
>> characteristics ("performance differentiated memory"). What would be
>> examples of such memory?
> 
> Likely the most prominent one that drove the creation of the "EFI
> Specific Purpose" attribute bit is high-bandwidth memory. One concrete
> example of that was a platform called Knights Landing [1] that ended
> up shipping firmware that lied to the OS about the latency
> characteristics of the memory to try to reverse engineer OS behavior
> to not allocate from that memory range by default. With the EFI
> attribute firmware performance tables can tell the truth about the
> performance characteristics of the memory range *and* indicate that
> the OS not use it for general purpose allocations by default.

Thanks for clarifying!

> 
> [1]: https://software.intel.com/content/www/us/en/develop/blogs/an-intro-to-mcdram-high-bandwidth-memory-on-knights-landing.html
> 
>> Like, memory that is faster than RAM (scratch
>> pad), or slower (pmem)? Or both? :)
> 
> Both, but note that PMEM is already hard-reserved by default.
> Soft-reserved is about a memory range that, for example, an
> administrator may want to reserve 100% for a weather simulation where
> if even a small amount of memory was stolen for the page cache the
> application may not meet its performance targets. It could also be a
> memory range that is so slow that only applications with higher
> latency tolerances would be prepared to consume it.
> 
> In other words the soft-reserved memory can be used to indicate memory
> that is either too precious, or too slow for general purpose OS
> allocations.

Right, so actually performance-differentiated in any way :)

> 
>> Is it a valid use case to use pmem
>> in a hypervisor to back this memory?
> 
> Depends on the pmem. That performance capability is indicated by the
> ACPI HMAT, not the EFI soft-reserved designation.
> 
>> 3. There seem to be use cases where "soft-reserved" memory is used via
>> DAX. What is an example use case? I assume it's *not* to treat it like
>> PMEM but instead e.g., use it as a fast buffer inside applications or
>> similar.
> 
> Right, in that weather-simulation example that application could just
> mmap /dev/daxX.Y and never worry about contending for the "fast
> memory" resource on the platform. Alternatively if that resource needs
> to be shared and/or over-commited then kernel memory-management
> services are needed and that dax-device can be assigned to kmem.
> 
>> 4. There seem to be use cases where some part of "soft-reserved" memory
>> is used via DAX, some other is given to the buddy. What is an example
>> use case? Is this really necessary or only some theoretical use case?
> 
> It's as necessary as pmem namespace partitioning, or the inclusion of
> dax-kmem upstream in the first place. In that kmem case the motivation
> was that some users want a portion of pmem provisioned for storage and
> some for volatile usage. The motivation is similar here, platform
> firmware can only identify memory attributes on coarse boundaries,
> finer grained provisioning decisions are up to the administrator /
> platform-owner and the kernel is a just a facilitator of that policy.
> 
>>
>> 5. The "provisioned along performance relevant address boundaries." part
>> is unclear to me. Can you give an example of how this would look like
>> from user space? Like, split that memory in blocks of size X with
>> alignment Y and give them to separate applications?
> 
> One example of platform address boundaries are the memory address
> ranges that alias in a direct-mapped memory-side-cache. In the
> direct-map-cache aliasing may repeat every N GBs where N is the ratio
> of far-to-near memory. ("Near memory" ==  cache "Far memory" ==
> backing memory). Also refer back to the background in the page
> allocator shuffling patches [2]. With this partitioning mechanism you
> could, for one example use case, assign different VMs to exclusive
> colors in the memory side cache.

Interesting, thanks!

> 
> [2]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e900a918b098
> 
>> 6. If you add such memory to the buddy, is there any way the system can
>> differentiate it from other memory? E.g., via fake/other NUMA nodes?
> 
> Numa node numbers / are how performance differentiated memory ranges
> are enumerated. The expectation is that all distinct performance
> memory targets have unique ACPI proximity domains and Linux numa node
> numbers as a result.

Makes sense to me (although it's somehow weird, because memory of the
same socket/node would be represented via different NUMA nodes), thanks!

> 
>> Also, can you give examples of how kmem-added memory is represented in
>> /proc/iomem for a) pmem and b) soft-resered memory after this series
>> (skimming over the patches, I think there is a change for pmem, right?)?
> 
> I don't expect a change. The only difference is the parent resource
> will be marked "Soft Reserved" instead of "Persistent Memory".

Right, I misread patch #11 while skimming - I thought the device
resource would be dropped.

> 
>> I am really wondering if it's the right approach to squeeze this into
>> our pmem/nvdimm infrastructure just because it's easy to do. E.g., man
>> "ndctl" - "ndctl - Manage "libnvdimm" subsystem devices (Non-volatile
>> Memory)" speaks explicitly about non-volatile memory.
> 
> In fact it's not squeezed into PMEM infrastructure. dax-kmem and
> device-dax are independent of PMEM. PMEM is one source of potential
> device-dax instances, soft-reserved memory is another orthogonal
> source. This is why device-dax needs its own userspace policy directed
> partitioning mechanism because there is no PMEM to store the
> configuration for partitioned higph-bandwidth memory. The userspace
> tooling for this mechanism is targeted for a tool called daxctl that
> has no PMEM dependencies. Look to Joao's use case that is using this
> infrastructure independent of PMEM with manual soft-reservations
> specified on the kernel command-line.

Thanks for clarifying, I was under the impression we would be reusing
libnvdimm to manage that memory.

-- 
Thanks,

David / dhildenb
_______________________________________________
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Subject: Re: [PATCH v4 00/23] device-dax: Support sub-dividing soft-reserved
 ranges
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Ira Weiny <ira.weiny@intel.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Borislav Petkov <bp@alien8.de>,
        Vishal Verma <vishal.l.verma@intel.com>,
        David Airlie <airlied@linux.ie>, Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Joao Martins <joao.m.martins@oracle.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Dave Jiang <dave.jiang@intel.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Wei Yang <richardw.yang@linux.intel.com>,
        X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ben Skeggs <bskeggs@redhat.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Jason Gunthorpe <jgg@mellanox.com>, Jia He <justin.he@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Brice Goglin <Brice.Goglin@inria.fr>,
        Jeff Moyer <jmoyer@redhat.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Daniel Vetter <daniel@ffwll.ch>,
        Andy Lutomirski <luto@kernel.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        linux-nvdimm <linux-nvdimm@lists.01.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux ACPI <linux-acpi@vger.kernel.org>,
        Maling list - DRI developers 
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From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
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Date:   Fri, 21 Aug 2020 12:15:03 +0200
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>>
>> 1. On x86-64, e820 indicates "soft-reserved" memory. This memory is not
>> automatically used in the buddy during boot, but remains untouched
>> (similar to pmem). But as it involves ACPI as well, it could also be
>> used on arm64 (-e820), correct?
> 
> Correct, arm64 also gets the EFI support for enumerating memory this
> way. However, I would clarify that whether soft-reserved is given to
> the buddy allocator by default or not is the kernel's policy choice,
> "buddy-by-default" is ok and is what will happen anyways with older
> kernels on platforms that enumerate a memory range this way.

Is "soft-reserved" then the right terminology for that? It sounds very
x86-64/e820 specific. Maybe a compressed for of "performance
differentiated memory" might be a better fit to expose to user space, no?

> 
>> 2. Soft-reserved memory is volatile RAM with differing performance
>> characteristics ("performance differentiated memory"). What would be
>> examples of such memory?
> 
> Likely the most prominent one that drove the creation of the "EFI
> Specific Purpose" attribute bit is high-bandwidth memory. One concrete
> example of that was a platform called Knights Landing [1] that ended
> up shipping firmware that lied to the OS about the latency
> characteristics of the memory to try to reverse engineer OS behavior
> to not allocate from that memory range by default. With the EFI
> attribute firmware performance tables can tell the truth about the
> performance characteristics of the memory range *and* indicate that
> the OS not use it for general purpose allocations by default.

Thanks for clarifying!

> 
> [1]: https://software.intel.com/content/www/us/en/develop/blogs/an-intro-to-mcdram-high-bandwidth-memory-on-knights-landing.html
> 
>> Like, memory that is faster than RAM (scratch
>> pad), or slower (pmem)? Or both? :)
> 
> Both, but note that PMEM is already hard-reserved by default.
> Soft-reserved is about a memory range that, for example, an
> administrator may want to reserve 100% for a weather simulation where
> if even a small amount of memory was stolen for the page cache the
> application may not meet its performance targets. It could also be a
> memory range that is so slow that only applications with higher
> latency tolerances would be prepared to consume it.
> 
> In other words the soft-reserved memory can be used to indicate memory
> that is either too precious, or too slow for general purpose OS
> allocations.

Right, so actually performance-differentiated in any way :)

> 
>> Is it a valid use case to use pmem
>> in a hypervisor to back this memory?
> 
> Depends on the pmem. That performance capability is indicated by the
> ACPI HMAT, not the EFI soft-reserved designation.
> 
>> 3. There seem to be use cases where "soft-reserved" memory is used via
>> DAX. What is an example use case? I assume it's *not* to treat it like
>> PMEM but instead e.g., use it as a fast buffer inside applications or
>> similar.
> 
> Right, in that weather-simulation example that application could just
> mmap /dev/daxX.Y and never worry about contending for the "fast
> memory" resource on the platform. Alternatively if that resource needs
> to be shared and/or over-commited then kernel memory-management
> services are needed and that dax-device can be assigned to kmem.
> 
>> 4. There seem to be use cases where some part of "soft-reserved" memory
>> is used via DAX, some other is given to the buddy. What is an example
>> use case? Is this really necessary or only some theoretical use case?
> 
> It's as necessary as pmem namespace partitioning, or the inclusion of
> dax-kmem upstream in the first place. In that kmem case the motivation
> was that some users want a portion of pmem provisioned for storage and
> some for volatile usage. The motivation is similar here, platform
> firmware can only identify memory attributes on coarse boundaries,
> finer grained provisioning decisions are up to the administrator /
> platform-owner and the kernel is a just a facilitator of that policy.
> 
>>
>> 5. The "provisioned along performance relevant address boundaries." part
>> is unclear to me. Can you give an example of how this would look like
>> from user space? Like, split that memory in blocks of size X with
>> alignment Y and give them to separate applications?
> 
> One example of platform address boundaries are the memory address
> ranges that alias in a direct-mapped memory-side-cache. In the
> direct-map-cache aliasing may repeat every N GBs where N is the ratio
> of far-to-near memory. ("Near memory" ==  cache "Far memory" ==
> backing memory). Also refer back to the background in the page
> allocator shuffling patches [2]. With this partitioning mechanism you
> could, for one example use case, assign different VMs to exclusive
> colors in the memory side cache.

Interesting, thanks!

> 
> [2]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e900a918b098
> 
>> 6. If you add such memory to the buddy, is there any way the system can
>> differentiate it from other memory? E.g., via fake/other NUMA nodes?
> 
> Numa node numbers / are how performance differentiated memory ranges
> are enumerated. The expectation is that all distinct performance
> memory targets have unique ACPI proximity domains and Linux numa node
> numbers as a result.

Makes sense to me (although it's somehow weird, because memory of the
same socket/node would be represented via different NUMA nodes), thanks!

> 
>> Also, can you give examples of how kmem-added memory is represented in
>> /proc/iomem for a) pmem and b) soft-resered memory after this series
>> (skimming over the patches, I think there is a change for pmem, right?)?
> 
> I don't expect a change. The only difference is the parent resource
> will be marked "Soft Reserved" instead of "Persistent Memory".

Right, I misread patch #11 while skimming - I thought the device
resource would be dropped.

> 
>> I am really wondering if it's the right approach to squeeze this into
>> our pmem/nvdimm infrastructure just because it's easy to do. E.g., man
>> "ndctl" - "ndctl - Manage "libnvdimm" subsystem devices (Non-volatile
>> Memory)" speaks explicitly about non-volatile memory.
> 
> In fact it's not squeezed into PMEM infrastructure. dax-kmem and
> device-dax are independent of PMEM. PMEM is one source of potential
> device-dax instances, soft-reserved memory is another orthogonal
> source. This is why device-dax needs its own userspace policy directed
> partitioning mechanism because there is no PMEM to store the
> configuration for partitioned higph-bandwidth memory. The userspace
> tooling for this mechanism is targeted for a tool called daxctl that
> has no PMEM dependencies. Look to Joao's use case that is using this
> infrastructure independent of PMEM with manual soft-reservations
> specified on the kernel command-line.

Thanks for clarifying, I was under the impression we would be reusing
libnvdimm to manage that memory.

-- 
Thanks,

David / dhildenb


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Subject: Re: [PATCH v4 00/23] device-dax: Support sub-dividing soft-reserved
 ranges
To: Dan Williams <dan.j.williams@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Ira Weiny
 <ira.weiny@intel.com>, Ard Biesheuvel <ardb@kernel.org>,
 Mike Rapoport <rppt@linux.ibm.com>, Borislav Petkov <bp@alien8.de>,
 Vishal Verma <vishal.l.verma@intel.com>, David Airlie <airlied@linux.ie>,
 Will Deacon <will@kernel.org>, Catalin Marinas <catalin.marinas@arm.com>,
 Ard Biesheuvel <ard.biesheuvel@linaro.org>,
 Joao Martins <joao.m.martins@oracle.com>,
 Tom Lendacky <thomas.lendacky@amd.com>, Dave Jiang <dave.jiang@intel.com>,
 "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
 Jonathan Cameron <Jonathan.Cameron@huawei.com>,
 Wei Yang <richardw.yang@linux.intel.com>, X86 ML <x86@kernel.org>,
 "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>,
 Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
 Pavel Tatashin <pasha.tatashin@soleen.com>,
 Peter Zijlstra <peterz@infradead.org>, Ben Skeggs <bskeggs@redhat.com>,
 Benjamin Herrenschmidt <benh@kernel.crashing.org>,
 Jason Gunthorpe <jgg@mellanox.com>, Jia He <justin.he@arm.com>,
 Ingo Molnar <mingo@redhat.com>, Dave Hansen <dave.hansen@linux.intel.com>,
 Paul Mackerras <paulus@ozlabs.org>, Brice Goglin <Brice.Goglin@inria.fr>,
 Jeff Moyer <jmoyer@redhat.com>, Michael Ellerman <mpe@ellerman.id.au>,
 "Rafael J. Wysocki" <rjw@rjwysocki.net>, Daniel Vetter <daniel@ffwll.ch>,
 Andy Lutomirski <luto@kernel.org>, "Rafael J. Wysocki" <rafael@kernel.org>,
 Linux MM <linux-mm@kvack.org>, linux-nvdimm <linux-nvdimm@lists.01.org>,
 Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
 Linux ACPI <linux-acpi@vger.kernel.org>,
 Maling list - DRI developers <dri-devel@lists.freedesktop.org>
References: <159643094279.4062302.17779410714418721328.stgit@dwillia2-desk3.amr.corp.intel.com>
 <c59111f9-7c94-8b9e-2b8c-4cb96b9aa848@redhat.com>
 <CAPcyv4j8-5nWU5GPDBoFicwR84qM=hWRtd78DkcCg4PW-8i6Vg@mail.gmail.com>
From: David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <6af3de0d-ffdc-8942-3922-ebaeef20dd63@redhat.com>
Date: Fri, 21 Aug 2020 12:15:03 +0200
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>>
>> 1. On x86-64, e820 indicates "soft-reserved" memory. This memory is no=
t
>> automatically used in the buddy during boot, but remains untouched
>> (similar to pmem). But as it involves ACPI as well, it could also be
>> used on arm64 (-e820), correct?
>=20
> Correct, arm64 also gets the EFI support for enumerating memory this
> way. However, I would clarify that whether soft-reserved is given to
> the buddy allocator by default or not is the kernel's policy choice,
> "buddy-by-default" is ok and is what will happen anyways with older
> kernels on platforms that enumerate a memory range this way.

Is "soft-reserved" then the right terminology for that? It sounds very
x86-64/e820 specific. Maybe a compressed for of "performance
differentiated memory" might be a better fit to expose to user space, no?

>=20
>> 2. Soft-reserved memory is volatile RAM with differing performance
>> characteristics ("performance differentiated memory"). What would be
>> examples of such memory?
>=20
> Likely the most prominent one that drove the creation of the "EFI
> Specific Purpose" attribute bit is high-bandwidth memory. One concrete
> example of that was a platform called Knights Landing [1] that ended
> up shipping firmware that lied to the OS about the latency
> characteristics of the memory to try to reverse engineer OS behavior
> to not allocate from that memory range by default. With the EFI
> attribute firmware performance tables can tell the truth about the
> performance characteristics of the memory range *and* indicate that
> the OS not use it for general purpose allocations by default.

Thanks for clarifying!

>=20
> [1]: https://software.intel.com/content/www/us/en/develop/blogs/an-intr=
o-to-mcdram-high-bandwidth-memory-on-knights-landing.html
>=20
>> Like, memory that is faster than RAM (scratch
>> pad), or slower (pmem)? Or both? :)
>=20
> Both, but note that PMEM is already hard-reserved by default.
> Soft-reserved is about a memory range that, for example, an
> administrator may want to reserve 100% for a weather simulation where
> if even a small amount of memory was stolen for the page cache the
> application may not meet its performance targets. It could also be a
> memory range that is so slow that only applications with higher
> latency tolerances would be prepared to consume it.
>=20
> In other words the soft-reserved memory can be used to indicate memory
> that is either too precious, or too slow for general purpose OS
> allocations.

Right, so actually performance-differentiated in any way :)

>=20
>> Is it a valid use case to use pmem
>> in a hypervisor to back this memory?
>=20
> Depends on the pmem. That performance capability is indicated by the
> ACPI HMAT, not the EFI soft-reserved designation.
>=20
>> 3. There seem to be use cases where "soft-reserved" memory is used via
>> DAX. What is an example use case? I assume it's *not* to treat it like
>> PMEM but instead e.g., use it as a fast buffer inside applications or
>> similar.
>=20
> Right, in that weather-simulation example that application could just
> mmap /dev/daxX.Y and never worry about contending for the "fast
> memory" resource on the platform. Alternatively if that resource needs
> to be shared and/or over-commited then kernel memory-management
> services are needed and that dax-device can be assigned to kmem.
>=20
>> 4. There seem to be use cases where some part of "soft-reserved" memor=
y
>> is used via DAX, some other is given to the buddy. What is an example
>> use case? Is this really necessary or only some theoretical use case?
>=20
> It's as necessary as pmem namespace partitioning, or the inclusion of
> dax-kmem upstream in the first place. In that kmem case the motivation
> was that some users want a portion of pmem provisioned for storage and
> some for volatile usage. The motivation is similar here, platform
> firmware can only identify memory attributes on coarse boundaries,
> finer grained provisioning decisions are up to the administrator /
> platform-owner and the kernel is a just a facilitator of that policy.
>=20
>>
>> 5. The "provisioned along performance relevant address boundaries." pa=
rt
>> is unclear to me. Can you give an example of how this would look like
>> from user space? Like, split that memory in blocks of size X with
>> alignment Y and give them to separate applications?
>=20
> One example of platform address boundaries are the memory address
> ranges that alias in a direct-mapped memory-side-cache. In the
> direct-map-cache aliasing may repeat every N GBs where N is the ratio
> of far-to-near memory. ("Near memory" =3D=3D  cache "Far memory" =3D=3D
> backing memory). Also refer back to the background in the page
> allocator shuffling patches [2]. With this partitioning mechanism you
> could, for one example use case, assign different VMs to exclusive
> colors in the memory side cache.

Interesting, thanks!

>=20
> [2]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git=
/commit/?id=3De900a918b098
>=20
>> 6. If you add such memory to the buddy, is there any way the system ca=
n
>> differentiate it from other memory? E.g., via fake/other NUMA nodes?
>=20
> Numa node numbers / are how performance differentiated memory ranges
> are enumerated. The expectation is that all distinct performance
> memory targets have unique ACPI proximity domains and Linux numa node
> numbers as a result.

Makes sense to me (although it's somehow weird, because memory of the
same socket/node would be represented via different NUMA nodes), thanks!

>=20
>> Also, can you give examples of how kmem-added memory is represented in
>> /proc/iomem for a) pmem and b) soft-resered memory after this series
>> (skimming over the patches, I think there is a change for pmem, right?=
)?
>=20
> I don't expect a change. The only difference is the parent resource
> will be marked "Soft Reserved" instead of "Persistent Memory".

Right, I misread patch #11 while skimming - I thought the device
resource would be dropped.

>=20
>> I am really wondering if it's the right approach to squeeze this into
>> our pmem/nvdimm infrastructure just because it's easy to do. E.g., man
>> "ndctl" - "ndctl - Manage "libnvdimm" subsystem devices (Non-volatile
>> Memory)" speaks explicitly about non-volatile memory.
>=20
> In fact it's not squeezed into PMEM infrastructure. dax-kmem and
> device-dax are independent of PMEM. PMEM is one source of potential
> device-dax instances, soft-reserved memory is another orthogonal
> source. This is why device-dax needs its own userspace policy directed
> partitioning mechanism because there is no PMEM to store the
> configuration for partitioned higph-bandwidth memory. The userspace
> tooling for this mechanism is targeted for a tool called daxctl that
> has no PMEM dependencies. Look to Joao's use case that is using this
> infrastructure independent of PMEM with manual soft-reservations
> specified on the kernel command-line.

Thanks for clarifying, I was under the impression we would be reusing
libnvdimm to manage that memory.

--=20
Thanks,

David / dhildenb



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Subject: Re: [PATCH v4 00/23] device-dax: Support sub-dividing soft-reserved
 ranges
To: Dan Williams <dan.j.williams@intel.com>
References: <159643094279.4062302.17779410714418721328.stgit@dwillia2-desk3.amr.corp.intel.com>
 <c59111f9-7c94-8b9e-2b8c-4cb96b9aa848@redhat.com>
 <CAPcyv4j8-5nWU5GPDBoFicwR84qM=hWRtd78DkcCg4PW-8i6Vg@mail.gmail.com>
From: David Hildenbrand <david@redhat.com>
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Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
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>>
>> 1. On x86-64, e820 indicates "soft-reserved" memory. This memory is not
>> automatically used in the buddy during boot, but remains untouched
>> (similar to pmem). But as it involves ACPI as well, it could also be
>> used on arm64 (-e820), correct?
> 
> Correct, arm64 also gets the EFI support for enumerating memory this
> way. However, I would clarify that whether soft-reserved is given to
> the buddy allocator by default or not is the kernel's policy choice,
> "buddy-by-default" is ok and is what will happen anyways with older
> kernels on platforms that enumerate a memory range this way.

Is "soft-reserved" then the right terminology for that? It sounds very
x86-64/e820 specific. Maybe a compressed for of "performance
differentiated memory" might be a better fit to expose to user space, no?

> 
>> 2. Soft-reserved memory is volatile RAM with differing performance
>> characteristics ("performance differentiated memory"). What would be
>> examples of such memory?
> 
> Likely the most prominent one that drove the creation of the "EFI
> Specific Purpose" attribute bit is high-bandwidth memory. One concrete
> example of that was a platform called Knights Landing [1] that ended
> up shipping firmware that lied to the OS about the latency
> characteristics of the memory to try to reverse engineer OS behavior
> to not allocate from that memory range by default. With the EFI
> attribute firmware performance tables can tell the truth about the
> performance characteristics of the memory range *and* indicate that
> the OS not use it for general purpose allocations by default.

Thanks for clarifying!

> 
> [1]: https://software.intel.com/content/www/us/en/develop/blogs/an-intro-to-mcdram-high-bandwidth-memory-on-knights-landing.html
> 
>> Like, memory that is faster than RAM (scratch
>> pad), or slower (pmem)? Or both? :)
> 
> Both, but note that PMEM is already hard-reserved by default.
> Soft-reserved is about a memory range that, for example, an
> administrator may want to reserve 100% for a weather simulation where
> if even a small amount of memory was stolen for the page cache the
> application may not meet its performance targets. It could also be a
> memory range that is so slow that only applications with higher
> latency tolerances would be prepared to consume it.
> 
> In other words the soft-reserved memory can be used to indicate memory
> that is either too precious, or too slow for general purpose OS
> allocations.

Right, so actually performance-differentiated in any way :)

> 
>> Is it a valid use case to use pmem
>> in a hypervisor to back this memory?
> 
> Depends on the pmem. That performance capability is indicated by the
> ACPI HMAT, not the EFI soft-reserved designation.
> 
>> 3. There seem to be use cases where "soft-reserved" memory is used via
>> DAX. What is an example use case? I assume it's *not* to treat it like
>> PMEM but instead e.g., use it as a fast buffer inside applications or
>> similar.
> 
> Right, in that weather-simulation example that application could just
> mmap /dev/daxX.Y and never worry about contending for the "fast
> memory" resource on the platform. Alternatively if that resource needs
> to be shared and/or over-commited then kernel memory-management
> services are needed and that dax-device can be assigned to kmem.
> 
>> 4. There seem to be use cases where some part of "soft-reserved" memory
>> is used via DAX, some other is given to the buddy. What is an example
>> use case? Is this really necessary or only some theoretical use case?
> 
> It's as necessary as pmem namespace partitioning, or the inclusion of
> dax-kmem upstream in the first place. In that kmem case the motivation
> was that some users want a portion of pmem provisioned for storage and
> some for volatile usage. The motivation is similar here, platform
> firmware can only identify memory attributes on coarse boundaries,
> finer grained provisioning decisions are up to the administrator /
> platform-owner and the kernel is a just a facilitator of that policy.
> 
>>
>> 5. The "provisioned along performance relevant address boundaries." part
>> is unclear to me. Can you give an example of how this would look like
>> from user space? Like, split that memory in blocks of size X with
>> alignment Y and give them to separate applications?
> 
> One example of platform address boundaries are the memory address
> ranges that alias in a direct-mapped memory-side-cache. In the
> direct-map-cache aliasing may repeat every N GBs where N is the ratio
> of far-to-near memory. ("Near memory" ==  cache "Far memory" ==
> backing memory). Also refer back to the background in the page
> allocator shuffling patches [2]. With this partitioning mechanism you
> could, for one example use case, assign different VMs to exclusive
> colors in the memory side cache.

Interesting, thanks!

> 
> [2]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e900a918b098
> 
>> 6. If you add such memory to the buddy, is there any way the system can
>> differentiate it from other memory? E.g., via fake/other NUMA nodes?
> 
> Numa node numbers / are how performance differentiated memory ranges
> are enumerated. The expectation is that all distinct performance
> memory targets have unique ACPI proximity domains and Linux numa node
> numbers as a result.

Makes sense to me (although it's somehow weird, because memory of the
same socket/node would be represented via different NUMA nodes), thanks!

> 
>> Also, can you give examples of how kmem-added memory is represented in
>> /proc/iomem for a) pmem and b) soft-resered memory after this series
>> (skimming over the patches, I think there is a change for pmem, right?)?
> 
> I don't expect a change. The only difference is the parent resource
> will be marked "Soft Reserved" instead of "Persistent Memory".

Right, I misread patch #11 while skimming - I thought the device
resource would be dropped.

> 
>> I am really wondering if it's the right approach to squeeze this into
>> our pmem/nvdimm infrastructure just because it's easy to do. E.g., man
>> "ndctl" - "ndctl - Manage "libnvdimm" subsystem devices (Non-volatile
>> Memory)" speaks explicitly about non-volatile memory.
> 
> In fact it's not squeezed into PMEM infrastructure. dax-kmem and
> device-dax are independent of PMEM. PMEM is one source of potential
> device-dax instances, soft-reserved memory is another orthogonal
> source. This is why device-dax needs its own userspace policy directed
> partitioning mechanism because there is no PMEM to store the
> configuration for partitioned higph-bandwidth memory. The userspace
> tooling for this mechanism is targeted for a tool called daxctl that
> has no PMEM dependencies. Look to Joao's use case that is using this
> infrastructure independent of PMEM with manual soft-reservations
> specified on the kernel command-line.

Thanks for clarifying, I was under the impression we would be reusing
libnvdimm to manage that memory.

-- 
Thanks,

David / dhildenb

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