On Mon, 4 May 2020, Emanuele Giuseppe Esposito wrote: > There is currently no common way for Linux kernel subsystems to expose > statistics to userspace shared throughout the Linux kernel; subsystems > have to take care of gathering and displaying statistics by themselves, > for example in the form of files in debugfs. For example KVM has its own > code section that takes care of this in virt/kvm/kvm_main.c, where it sets > up debugfs handlers for displaying values and aggregating them from > various subfolders to obtain information about the system state (i.e. > displaying the total number of exits, calculated by summing all exits of > all cpus of all running virtual machines). > > Allowing each section of the kernel to do so has two disadvantages. First, > it will introduce redundant code. Second, debugfs is anyway not the right > place for statistics (for example it is affected by lockdown) > > In this patch series I introduce statsfs, a synthetic ram-based virtual > filesystem that takes care of gathering and displaying statistics for the > Linux kernel subsystems. > This is exciting, we have been looking in the same area recently. Adding Jonathan Adams <jwadams@xxxxxxxxxx>. In your diffstat, one thing I notice that is omitted: an update to Documentation/* :) Any chance of getting some proposed Documentation/ updates with structure of the fs, the per subsystem breakdown, and best practices for managing the stats from the kernel level? > The file system is mounted on /sys/kernel/stats and would be already used > by kvm. Statsfs was initially introduced by Paolo Bonzini [1]. > > Statsfs offers a generic and stable API, allowing any kind of > directory/file organization and supporting multiple kind of aggregations > (not only sum, but also average, max, min and count_zero) and data types > (all unsigned and signed types plus boolean). The implementation, which is > a generalization of KVM’s debugfs statistics code, takes care of gathering > and displaying information at run time; users only need to specify the > values to be included in each source. > > Statsfs would also be a different mountpoint from debugfs, and would not > suffer from limited access due to the security lock down patches. Its main > function is to display each statistics as a file in the desired folder > hierarchy defined through the API. Statsfs files can be read, and possibly > cleared if their file mode allows it. > > Statsfs has two main components: the public API defined by > include/linux/statsfs.h, and the virtual file system which should end up > in /sys/kernel/stats. > > The API has two main elements, values and sources. Kernel subsystems like > KVM can use the API to create a source, add child > sources/values/aggregates and register it to the root source (that on the > virtual fs would be /sys/kernel/statsfs). > > Sources are created via statsfs_source_create(), and each source becomes a > directory in the file system. Sources form a parent-child relationship; > root sources are added to the file system via statsfs_source_register(). > Every other source is added to or removed from a parent through the > statsfs_source_add_subordinate and statsfs_source_remote_subordinate APIs. > Once a source is created and added to the tree (via add_subordinate), it > will be used to compute aggregate values in the parent source. > > Values represent quantites that are gathered by the statsfs user. Examples > of values include the number of vm exits of a given kind, the amount of > memory used by some data structure, the length of the longest hash table > chain, or anything like that. Values are defined with the > statsfs_source_add_values function. Each value is defined by a struct > statsfs_value; the same statsfs_value can be added to many different > sources. A value can be considered "simple" if it fetches data from a > user-provided location, or "aggregate" if it groups all values in the > subordinates sources that include the same statsfs_value. > This seems like it could have a lot of overhead if we wanted to periodically track the totality of subsystem stats as a form of telemetry gathering from userspace. To collect telemetry for 1,000 different stats, do we need to issue lseek()+read() syscalls for each of them individually (or, worse, open()+read()+close())? Any thoughts on how that can be optimized? A couple of ideas: - an interface that allows gathering of all stats for a particular interface through a single file that would likely be encoded in binary and the responsibility of userspace to disseminate, or - an interface that extends beyond this proposal and allows the reader to specify which stats they are interested in collecting and then the kernel will only provide these stats in a well formed structure and also be binary encoded. We've found that the one-file-per-stat method is pretty much a show stopper from the performance view and we always must execute at least two syscalls to obtain a single stat. Since this is becoming a generic API (good!!), maybe we can discuss possible ways to optimize gathering of stats in mass? > For more information, please consult the kerneldoc documentation in patch > 2 and the sample uses in the kunit tests and in KVM. > > This series of patches is based on my previous series "libfs: group and > simplify linux fs code" and the single patch sent to kvm "kvm_host: unify > VM_STAT and VCPU_STAT definitions in a single place". The former > simplifies code duplicated in debugfs and tracefs (from which statsfs is > based on), the latter groups all macros definition for statistics in kvm > in a single common file shared by all architectures. > > Patch 1 adds a new refcount and kref destructor wrappers that take a > semaphore, as those are used later by statsfs. Patch 2 introduces the > statsfs API, patch 3 provides extensive tests that can also be used as > example on how to use the API and patch 4 adds the file system support. > Finally, patch 5 provides a real-life example of statsfs usage in KVM. > > [1] https://lore.kernel.org/kvm/5d6cdcb1-d8ad-7ae6-7351-3544e2fa366d@xxxxxxxxxx/?fbclid=IwAR18LHJ0PBcXcDaLzILFhHsl3qpT3z2vlG60RnqgbpGYhDv7L43n0ZXJY8M > > Signed-off-by: Emanuele Giuseppe Esposito <eesposit@xxxxxxxxxx> > > v1->v2 remove unnecessary list_foreach_safe loops, fix wrong indentation, > change statsfs in stats_fs > > Emanuele Giuseppe Esposito (5): > refcount, kref: add dec-and-test wrappers for rw_semaphores > stats_fs API: create, add and remove stats_fs sources and values > kunit: tests for stats_fs API > stats_fs fs: virtual fs to show stats to the end-user > kvm_main: replace debugfs with stats_fs > > MAINTAINERS | 7 + > arch/arm64/kvm/Kconfig | 1 + > arch/arm64/kvm/guest.c | 2 +- > arch/mips/kvm/Kconfig | 1 + > arch/mips/kvm/mips.c | 2 +- > arch/powerpc/kvm/Kconfig | 1 + > arch/powerpc/kvm/book3s.c | 6 +- > arch/powerpc/kvm/booke.c | 8 +- > arch/s390/kvm/Kconfig | 1 + > arch/s390/kvm/kvm-s390.c | 16 +- > arch/x86/include/asm/kvm_host.h | 2 +- > arch/x86/kvm/Kconfig | 1 + > arch/x86/kvm/Makefile | 2 +- > arch/x86/kvm/debugfs.c | 64 -- > arch/x86/kvm/stats_fs.c | 56 ++ > arch/x86/kvm/x86.c | 6 +- > fs/Kconfig | 12 + > fs/Makefile | 1 + > fs/stats_fs/Makefile | 6 + > fs/stats_fs/inode.c | 337 ++++++++++ > fs/stats_fs/internal.h | 35 + > fs/stats_fs/stats_fs-tests.c | 1088 +++++++++++++++++++++++++++++++ > fs/stats_fs/stats_fs.c | 773 ++++++++++++++++++++++ > include/linux/kref.h | 11 + > include/linux/kvm_host.h | 39 +- > include/linux/refcount.h | 2 + > include/linux/stats_fs.h | 304 +++++++++ > include/uapi/linux/magic.h | 1 + > lib/refcount.c | 32 + > tools/lib/api/fs/fs.c | 21 + > virt/kvm/arm/arm.c | 2 +- > virt/kvm/kvm_main.c | 314 ++------- > 32 files changed, 2772 insertions(+), 382 deletions(-) > delete mode 100644 arch/x86/kvm/debugfs.c > create mode 100644 arch/x86/kvm/stats_fs.c > create mode 100644 fs/stats_fs/Makefile > create mode 100644 fs/stats_fs/inode.c > create mode 100644 fs/stats_fs/internal.h > create mode 100644 fs/stats_fs/stats_fs-tests.c > create mode 100644 fs/stats_fs/stats_fs.c > create mode 100644 include/linux/stats_fs.h > > -- > 2.25.2 > >
