Hi Dave, On Wed, Feb 19, 2025 at 2:41 PM Dave Martin <Dave.Martin@xxxxxxx> wrote: > > Hi, > > On Wed, Jan 22, 2025 at 02:20:25PM -0600, Babu Moger wrote: > > Assign/unassign counters on resctrl group creation/deletion. Two counters > > are required per group, one for MBM total event and one for MBM local > > event. > > > > There are a limited number of counters available for assignment. If these > > counters are exhausted, the kernel will display the error message: "Out of > > MBM assignable counters". However, it is not necessary to fail the > > creation of a group due to assignment failures. Users have the flexibility > > to modify the assignments at a later time. > > If we are doing this, should turning mbm_cntr_assign mode on also > trigger auto-assingment for all extant monitoring groups? > > Either way though, this auto-assignment feels like a potential nuisance > for userspace. > > If the userspace use-case requires too many monitoring groups for the > available counters, then the kernel will auto-assign counters to a > random subset of groups which may or may not be the ones that userspace > wanted to monitor; then userspace must manually look for the assigned > counters and unassign some of them before they can be assigned where > userspace actually wanted them. > > This is not impossible for userspace to cope with, but it feels > awkward. > > Is there a way to inhibit auto-assignment? > > Or could automatic assignments be considered somehow "weak", so that > new explicit assignments can steal automatically assigned counters > without the need to unassign them explicitly? We had an incomplete discussion about this early on[1]. I guess I didn't revisit it because I found it was trivial to add a flag that inhibits the assignment behavior during mkdir and had moved on to bigger issues. If an agent creating directories isn't coordinated with the agent managing counters, a series of creating and destroying a group could prevent a monitor assignment from ever succeeding because it's not possible to atomically discover the name of the new directory that stole the previously-available counter and reassign it. However, if the counter-manager can get all the counters assigned once and only move them with atomic reassignments, it will become impossible to snatch them with a mkdir. -Peter [1] https://lore.kernel.org/lkml/CALPaoCihfQ9VtLYzyHB9-PsQzXLc06BW8bzhBXwj9-i+Q8RVFQ@xxxxxxxxxxxxxx/