Re: [RFC PATCH v1 4/6] sched: NUMA-aware per-memory-map concurrency IDs

[Yury Norov]

On Fri, Aug 23, 2024 at 02:59:44PM -0400, Mathieu Desnoyers wrote:
> The issue addressed by this change is the non-locality of NUMA accesses
> to data structures indexed by concurrency IDs: for example, in a
> scenario where a process has two threads, and they periodically run one
> after the other on different NUMA nodes, each will be assigned mm_cid=0.
> As a consequence, they will end up accessing the same pages, and thus at
> least one of the threads will need to perform remote NUMA accesses,
> which is inefficient.
> 
> That being said, the same issue theoretically exists due to false
> sharing of cache lines by threads running on after another on different

running one after another you mean?

> cores/CPUs within a single NUMA node, but the extent of the performance
> impact is lesser than remote NUMA accesses.
> 
> Solve this by making the rseq concurrency ID (mm_cid) NUMA-aware. On
> NUMA systems, when a NUMA-aware concurrency ID is observed by user-space
> to be associated with a NUMA node, guarantee that it never changes NUMA
> node unless either a kernel-level NUMA configuration change happens, or
> scheduler migrations end up migrating tasks across NUMA nodes.
> 
> There is a tradeoff between NUMA locality and compactness of the
> concurrency ID allocation. Favor compactness over NUMA locality when
> the scheduler migrates tasks across NUMA nodes, as this does not cause
> the frequent remote NUMA accesses behavior. This is done by limiting the
> concurrency ID range to minimum between the number of threads belonging
> to the process and the number of allowed CPUs.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Cc: Ben Segall <bsegall@xxxxxxxxxx>
> ---
> Changes since v0:
> - Rename "notandnot" to "nor".