Re: [PATCH v29 10/13] Documentation: Add documents for DAMON

[SeongJae Park]

From: SeongJae Park <sjpark@xxxxxxxxx>

On Fri, 11 Jun 2021 17:44:18 +0000 "Boehme, Markus" <markubo@xxxxxxxxx> wrote:

> On Thu, 2021-05-20 at 07:56 +0000, SeongJae Park wrote:
> > From: SeongJae Park <sjpark@xxxxxxxxx>
> > 
> > This commit adds documents for DAMON under
> > `Documentation/admin-guide/mm/damon/` and `Documentation/vm/damon/`.
> > 
> > Signed-off-by: SeongJae Park <sjpark@xxxxxxxxx>
> > ---
> >  Documentation/admin-guide/mm/damon/guide.rst | 158 +++++++++++++
> >  Documentation/admin-guide/mm/damon/index.rst |  15 ++
> >  Documentation/admin-guide/mm/damon/plans.rst |  29 +++
> >  Documentation/admin-guide/mm/damon/start.rst | 114 +++++++++
> >  Documentation/admin-guide/mm/damon/usage.rst | 112 +++++++++
> >  Documentation/admin-guide/mm/index.rst       |   1 +
> >  Documentation/vm/damon/api.rst               |  20 ++
> >  Documentation/vm/damon/design.rst            | 166 +++++++++++++
> >  Documentation/vm/damon/eval.rst              | 232 +++++++++++++++++++
> >  Documentation/vm/damon/faq.rst               |  58 +++++
> >  Documentation/vm/damon/index.rst             |  31 +++
> >  Documentation/vm/index.rst                   |   1 +
> >  12 files changed, 937 insertions(+)
> >  create mode 100644 Documentation/admin-guide/mm/damon/guide.rst
> >  create mode 100644 Documentation/admin-guide/mm/damon/index.rst
> >  create mode 100644 Documentation/admin-guide/mm/damon/plans.rst
> >  create mode 100644 Documentation/admin-guide/mm/damon/start.rst
> >  create mode 100644 Documentation/admin-guide/mm/damon/usage.rst
> >  create mode 100644 Documentation/vm/damon/api.rst
> >  create mode 100644 Documentation/vm/damon/design.rst
> >  create mode 100644 Documentation/vm/damon/eval.rst
> >  create mode 100644 Documentation/vm/damon/faq.rst
> >  create mode 100644 Documentation/vm/damon/index.rst
> > 
> > [...]
> >
> > diff --git a/Documentation/admin-guide/mm/damon/start.rst b/Documentation/admin-guide/mm/damon/start.rst
> > new file mode 100644
> > index 000000000000..f5bbf1e36836
> > --- /dev/null
> > +++ b/Documentation/admin-guide/mm/damon/start.rst
> > @@ -0,0 +1,114 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +===============
> > +Getting Started
> > +===============
> > +
> > +This document briefly describes how you can use DAMON by demonstrating its
> > +default user space tool.  Please note that this document describes only a part
> > +of its features for brevity.  Please refer to :doc:`usage` for more details.
> > [...]
> > +
> > +
> > +Prerequisites
> > +=============
> > +
> > +Kernel
> > +------
> > +
> > +You should first ensure your system is running on a kernel built with
> > +``CONFIG_DAMON_*=y``.
> > +
> > +
> > +User Space Tool
> > +---------------
> > +
> > +For the demonstration, we will use the default user space tool for DAMON,
> > +called DAMON Operator (DAMO).  It is available at
> > +https://github.com/awslabs/damo.  For brevity, below examples assume you set
> > +``$PATH`` to point it.  It's not mandatory, though.
> 
> "The examples below assume ``damo`` is on your ``$PATH``."?

That's much better for readability.  I will update so in the next spin.

> 
> > +
> > +Because DAMO is using the debugfs interface (refer to :doc:`usage` for the
> > +detail) of DAMON, you should ensure debugfs is mounted.  Mount it manually as
> > +below::
> > +
> > +    # mount -t debugfs none /sys/kernel/debug/
> > +
> > +or append below line to your ``/etc/fstab`` file so that your system can
> > +automatically mount debugfs from next booting::
> > +
> > +    debugfs /sys/kernel/debug debugfs defaults 0 0
> > +
> > +
> > [...]
> > diff --git a/Documentation/admin-guide/mm/damon/usage.rst b/Documentation/admin-guide/mm/damon/usage.rst
> > new file mode 100644
> > index 000000000000..ea3fa6e55f8b
> > --- /dev/null
> > +++ b/Documentation/admin-guide/mm/damon/usage.rst
> > @@ -0,0 +1,112 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +===============
> > +Detailed Usages
> > +===============
> > [...]
> > +
> > +Tracepoint for Monitoring Results
> > +=================================
> > +
> > +DAMON provides the monitoring results via a tracepoint,
> > +``damon:damon_aggregated``.  While the monitoring is turned on, you could
> > +record the tracepoint events and show results using tracepoint supporting tools
> > +like ``perf``.  For example::
> > +
> > +    # echo on > monitor_on
> > +    # perf record damon:damon_aggregated &
> 
> I think that needs to be "-e damon:damon_aggregated".

Good catch, I will fix in the next spin!

> 
> > +    # sleep 5
> > +    # kill 9 $(pidof perf)
> > +    # echo off > monitor_on
> > +    # perf script
> > 
> > [...]
> >
> > diff --git a/Documentation/vm/damon/design.rst b/Documentation/vm/damon/design.rst
> > new file mode 100644
> > index 000000000000..727d72093f8f
> > --- /dev/null
> > +++ b/Documentation/vm/damon/design.rst
> > @@ -0,0 +1,166 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +======
> > +Design
> > +======
> > +
> > [...]
> > +
> > +Reference Implementations of Address Space Specific Primitives
> > +==============================================================
> > +
> > +The low level primitives for the fundamental access monitoring are defined in
> > +two parts:
> > +
> > +1. Identification of the monitoring target address range for the address space.
> > +2. Access check of specific address range in the target space.
> > +
> > +DAMON currently provides the implementation of the primitives for only the
> > +virtual address spaces. Below two subsections describe how it works.
> > +
> > +
> > +PTE Accessed-bit Based Access Check
> > +-----------------------------------
> > +
> > +The implementation for the virtual address space uses PTE Accessed-bit for
> > +basic access checks.  It finds the relevant PTE Accessed bit from the address
> > +by walking the page table for the target task of the address.  In this way, the
> > +implementation finds and clears the bit for next sampling target address and
> > +checks whether the bit set again after one sampling period.  This could disturb
> > +other kernel subsystems using the Accessed bits, namely Idle page tracking and
> > +the reclaim logic.  To avoid such disturbances, DAMON makes it mutually
> > +exclusive with Idle page tracking and uses ``PG_idle`` and ``PG_young`` page
> > +flags to solve the conflict with the reclaim logic, as Idle page tracking does.
> > +
> > +
> > +VMA-based Target Address Range Construction
> > +-------------------------------------------
> > +
> > +Only small parts in the super-huge virtual address space of the processes are
> > +mapped to the physical memory and accessed.  Thus, tracking the unmapped
> > +address regions is just wasteful.  However, because DAMON can deal with some
> > +level of noise using the adaptive regions adjustment mechanism, tracking every
> > +mapping is not strictly required but could even incur a high overhead in some
> > +cases.  That said, too huge unmapped areas inside the monitoring target should
> > +be removed to not take the time for the adaptive mechanism.
> > +
> > +For the reason, this implementation converts the complex mappings to three
> > +distinct regions that cover every mapped area of the address space.  The two
> > +gaps between the three regions are the two biggest unmapped areas in the given
> > +address space.  The two biggest unmapped areas would be the gap between the
> > +heap and the uppermost mmap()-ed region, and the gap between the lowermost
> > +mmap()-ed region and the stack in most of the cases.  Because these gaps are
> > +exceptionally huge in usual address spaces, excluding these will be sufficient
> > +to make a reasonable trade-off.  Below shows this in detail::
> > +
> > +    <heap>
> > +    <BIG UNMAPPED REGION 1>
> > +    <uppermost mmap()-ed region>
> > +    (small mmap()-ed regions and munmap()-ed regions)
> > +    <lowermost mmap()-ed region>
> > +    <BIG UNMAPPED REGION 2>
> > +    <stack>
> > +
> 
> Nit: I'd swap these sections so they match the ordered list in the
> section overview.

Good point!  I will do so in the next spin!

> 
> > [...]
> 
> I skipped the files you mentioned would be dropped from the next
> revision.

Thanks, hope it didn't distracted you.


Thanks,
SeongJae Park

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