On Fri, 2022-06-24 at 13:54 +0100, Mel Gorman wrote: > struct per_cpu_pages is no longer strictly local as PCP lists can be > drained remotely using a lock for protection. While the use of local_lock > works, it goes against the intent of local_lock which is for "pure > CPU local concurrency control mechanisms and not suited for inter-CPU > concurrency control" (Documentation/locking/locktypes.rst) > > local_lock protects against migration between when the percpu pointer is > accessed and the pcp->lock acquired. The lock acquisition is a preemption > point so in the worst case, a task could migrate to another NUMA node > and accidentally allocate remote memory. The main requirement is to pin > the task to a CPU that is suitable for PREEMPT_RT and !PREEMPT_RT. > > Replace local_lock with helpers that pin a task to a CPU, lookup the > per-cpu structure and acquire the embedded lock. It's similar to local_lock > without breaking the intent behind the API. It is not a complete API > as only the parts needed for PCP-alloc are implemented but in theory, > the generic helpers could be promoted to a general API if there was > demand for an embedded lock within a per-cpu struct with a guarantee > that the per-cpu structure locked matches the running CPU and cannot use > get_cpu_var due to RT concerns. PCP requires these semantics to avoid > accidentally allocating remote memory. > > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > --- Reviewed-by: Nicolas Saenz Julienne <nsaenzju@xxxxxxxxxx> Tested-by: Nicolas Saenz Julienne <nsaenzju@xxxxxxxxxx> Thanks! -- Nicolás Sáenz
