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MPAM's cache occupancy counters can take a little while to settle once the monitor has been configured. The maximum settling time is described to the driver via a firmware table. The value could be large enough that it makes sense to sleep. To avoid exposing this to resctrl, it should be hidden behind MPAM's resctrl_arch_rmid_read(). resctrl_arch_rmid_read() may be called via IPI meaning it is unable to sleep. In this case, it should return an error if it needs to sleep. This will only affect MPAM platforms where the cache occupancy counter isn't available immediately, nohz_full is in use, and there are no housekeeping CPUs in the necessary domain. There are three callers of resctrl_arch_rmid_read(): __mon_event_count() and __check_limbo() are both called from a non-migrateable context. mon_event_read() invokes __mon_event_count() using smp_call_on_cpu(), which adds work to the target CPUs workqueue. rdtgroup_mutex() is held, meaning this cannot race with the resctrl cpuhp callback. __check_limbo() is invoked via schedule_delayed_work_on() also adds work to a per-cpu workqueue. The remaining call is add_rmid_to_limbo() which is called in response to a user-space syscall that frees an RMID. This opportunistically reads the LLC occupancy counter on the current domain to see if the RMID is over the dirty threshold. This has to disable preemption to avoid reading the wrong domain's value. Disabling preemption here prevents resctrl_arch_rmid_read() from sleeping. add_rmid_to_limbo() walks each domain, but only reads the counter on one domain. If the system has more than one domain, the RMID will always be added to the limbo list. If the RMIDs usage was not over the threshold, it will be removed from the list when __check_limbo() runs. Make this the default behaviour. Free RMIDs are always added to the limbo list for each domain. The user visible effect of this is that a clean RMID is not available for re-allocation immediately after 'rmdir()' completes. This behaviour was never portable as it never happened on a machine with multiple domains. Removing this path allows resctrl_arch_rmid_read() to sleep if its called with interrupts unmasked. Document this is the expected behaviour, and add a might_sleep() annotation to catch changes that won't work on arm64. Signed-off-by: James Morse <james.morse@arm.com> Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de> Reviewed-by: Shaopeng Tan <tan.shaopeng@fujitsu.com> Reviewed-by: Reinette Chatre <reinette.chatre@intel.com> Reviewed-by: Babu Moger <babu.moger@amd.com> Tested-by: Shaopeng Tan <tan.shaopeng@fujitsu.com> Tested-by: Peter Newman <peternewman@google.com> Tested-by: Babu Moger <babu.moger@amd.com> Tested-by: Carl Worth <carl@os.amperecomputing.com> # arm64 Link: https://lore.kernel.org/r/20240213184438.16675-15-james.morse@arm.com Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de> |
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