On Tue, Apr 19, 2022 at 09:33:48AM +0300, Mike Rapoport wrote: > On Mon, Apr 18, 2022 at 10:27:34AM -0700, Roman Gushchin wrote: > > On Mon, Apr 18, 2022 at 12:27:36PM +0300, Mike Rapoport wrote: > > > On Fri, Apr 15, 2022 at 05:27:51PM -0700, Roman Gushchin wrote: > > > > There are 50+ different shrinkers in the kernel, many with their own bells and > > > > whistles. Under the memory pressure the kernel applies some pressure on each of > > > > them in the order of which they were created/registered in the system. Some > > > > of them can contain only few objects, some can be quite large. Some can be > > > > effective at reclaiming memory, some not. > > > > > > > > The only existing debugging mechanism is a couple of tracepoints in > > > > do_shrink_slab(): mm_shrink_slab_start and mm_shrink_slab_end. They aren't > > > > covering everything though: shrinkers which report 0 objects will never show up, > > > > there is no support for memcg-aware shrinkers. Shrinkers are identified by their > > > > scan function, which is not always enough (e.g. hard to guess which super > > > > block's shrinker it is having only "super_cache_scan"). They are a passive > > > > mechanism: there is no way to call into counting and scanning of an individual > > > > shrinker and profile it. > > > > > > > > To provide a better visibility and debug options for memory shrinkers > > > > this patchset introduces a /sys/kernel/shrinker interface, to some extent > > > > similar to /sys/kernel/slab. > > > > > > Wouldn't debugfs better fit the purpose of shrinker debugging? > > > > I think sysfs fits better, but not a very strong opinion. > > > > Even though the interface is likely not very useful for the general > > public, big cloud instances might wanna enable it to gather statistics > > (and it's certainly what we gonna do at Facebook) and to provide > > additional data when something is off. They might not have debugfs > > mounted. And it's really similar to /sys/kernel/slab. > > And there is also similar /proc/vmallocinfo so why not /proc/shrinker? ;-) > > I suspect slab ended up in sysfs because nobody suggested to use debugfs > back then. I've been able to track the transition from /proc/slabinfo to > /proc/slubinfo to /sys/kernel/slab, but could not find why Christoph chose > sysfs in the end. > > > Are there any reasons why debugfs is preferable? > > debugfs is more flexible because it's not stable kernel ABI so if there > will be need/desire to change the layout and content of the files with > debugfs it can be done more easily. > > Is this a real problem for Facebook to mount debugfs? ;-) Fair enough, switching to debugfs in the next version. Thanks!
