On 10.03.23 19:28, Stefan Roesch wrote:
Patch series "mm: process/cgroup ksm support", v3.
So far KSM can only be enabled by calling madvise for memory regions. To
be able to use KSM for more workloads, KSM needs to have the ability to be
enabled / disabled at the process / cgroup level.
Use case 1:
The madvise call is not available in the programming language. An
example for this are programs with forked workloads using a garbage
collected language without pointers. In such a language madvise cannot
be made available.
In addition the addresses of objects get moved around as they are
garbage collected. KSM sharing needs to be enabled "from the outside"
for these type of workloads.
I guess the interpreter could enable it (like a memory allocator could
enable it for the whole heap). But I get that it's much easier to enable
this per-process, and eventually only when a lot of the same processes
are running in that particular environment.
Use case 2:
The same interpreter can also be used for workloads where KSM brings
no benefit or even has overhead. We'd like to be able to enable KSM on
a workload by workload basis.
Agreed. A per-process control is also helpful to identidy workloads
where KSM might be beneficial (and to which degree).
Use case 3:
With the madvise call sharing opportunities are only enabled for the
current process: it is a workload-local decision. A considerable number
of sharing opportuniites may exist across multiple workloads or jobs.
Only a higler level entity like a job scheduler or container can know
for certain if its running one or more instances of a job. That job
scheduler however doesn't have the necessary internal worklaod knowledge
to make targeted madvise calls.
Security concerns:
In previous discussions security concerns have been brought up. The
problem is that an individual workload does not have the knowledge about
what else is running on a machine. Therefore it has to be very
conservative in what memory areas can be shared or not. However, if the
system is dedicated to running multiple jobs within the same security
domain, its the job scheduler that has the knowledge that sharing can be
safely enabled and is even desirable.
Performance:
Experiments with using UKSM have shown a capacity increase of around
20%.
As raised, it would be great to include more details about the workload
where this particulalry helps (e.g., a lot of Django processes operating
in the same domain).
1. New options for prctl system command
This patch series adds two new options to the prctl system call.
The first one allows to enable KSM at the process level and the second
one to query the setting.
The setting will be inherited by child processes.
With the above setting, KSM can be enabled for the seed process of a
cgroup and all processes in the cgroup will inherit the setting.
2. Changes to KSM processing
When KSM is enabled at the process level, the KSM code will iterate
over all the VMA's and enable KSM for the eligible VMA's.
When forking a process that has KSM enabled, the setting will be
inherited by the new child process.
In addition when KSM is disabled for a process, KSM will be disabled
for the VMA's where KSM has been enabled.
Do we want to make MADV_MERGEABLE/MADV_UNMERGEABLE fail while the new
prctl is enabled for a process?
3. Add general_profit metric
The general_profit metric of KSM is specified in the documentation,
but not calculated. This adds the general profit metric to
/sys/kernel/debug/mm/ksm.
4. Add more metrics to ksm_stat
This adds the process profit and ksm type metric to
/proc/<pid>/ksm_stat.
5. Add more tests to ksm_tests
This adds an option to specify the merge type to the ksm_tests.
This allows to test madvise and prctl KSM. It also adds a new option
to query if prctl KSM has been enabled. It adds a fork test to verify
that the KSM process setting is inherited by client processes.
An update to the prctl(2) manpage has been proposed at [1].
This patch (of 3):
This adds a new prctl to API to enable and disable KSM on a per process
basis instead of only at the VMA basis (with madvise).
1) Introduce new MMF_VM_MERGE_ANY flag
This introduces the new flag MMF_VM_MERGE_ANY flag. When this flag
is set, kernel samepage merging (ksm) gets enabled for all vma's of a
process.
2) add flag to __ksm_enter
This change adds the flag parameter to __ksm_enter. This allows to
distinguish if ksm was called by prctl or madvise.
3) add flag to __ksm_exit call
This adds the flag parameter to the __ksm_exit() call. This allows
to distinguish if this call is for an prctl or madvise invocation.
4) invoke madvise for all vmas in scan_get_next_rmap_item
If the new flag MMF_VM_MERGE_ANY has been set for a process, iterate
over all the vmas and enable ksm if possible. For the vmas that can be
ksm enabled this is only done once.
5) support disabling of ksm for a process
This adds the ability to disable ksm for a process if ksm has been
enabled for the process.
6) add new prctl option to get and set ksm for a process
This adds two new options to the prctl system call
- enable ksm for all vmas of a process (if the vmas support it).
- query if ksm has been enabled for a process.
Did you consider, instead of handling MMF_VM_MERGE_ANY in a special way,
to instead make it reuse the existing MMF_VM_MERGEABLE/VM_MERGEABLE
infrastructure. Especially:
1) During prctl(MMF_VM_MERGE_ANY), set VM_MERGABLE on all applicable
compatible. Further, set MMF_VM_MERGEABLE and enter KSM if not
already set.
2) When creating a new, compatible VMA and MMF_VM_MERGE_ANY is set, set
VM_MERGABLE?
The you can avoid all runtime checks for compatible VMAs and only look
at the VM_MERGEABLE flag. In fact, the VM_MERGEABLE will be completely
expressive then for all VMAs. You don't need vma_ksm_mergeable() then.
Another thing to consider is interaction with arch/s390/mm/gmap.c:
s390x/kvm does not support KSM and it has to disable it for all VMAs. We
