Re: [PATCH v11 2/3] sched: Move task_mm_cid_work to mm work_struct

[Gabriele Monaco]

On Thu, 2025-02-27 at 16:33 +0100, Gabriele Monaco wrote:
> Currently, the task_mm_cid_work function is called in a task work
> triggered by a scheduler tick to frequently compact the mm_cids of
> each
> process. This can delay the execution of the corresponding thread for
> the entire duration of the function, negatively affecting the
> response
> in case of real time tasks. In practice, we observe task_mm_cid_work
> increasing the latency of 30-35us on a 128 cores system, this order
> of
> magnitude is meaningful under PREEMPT_RT.
> 
> Run the task_mm_cid_work in a new work_struct connected to the
> mm_struct rather than in the task context before returning to
> userspace.
> 
> This work_struct is initialised with the mm and disabled before
> freeing
> it. The queuing of the work happens while returning to userspace in
> __rseq_handle_notify_resume, maintaining the checks to avoid running
> more frequently than MM_CID_SCAN_DELAY.
> To make sure this happens predictably also on long running tasks, we
> trigger a call to __rseq_handle_notify_resume also from the scheduler
> tick if the runtime exceeded a 100ms threshold.
> [...]
> 
> Fixes: 223baf9d17f2 ("sched: Fix performance regression introduced by
> mm_cid")
> Signed-off-by: Gabriele Monaco <gmonaco@xxxxxxxxxx>

Is this patch missing anything?

I refactored a bit to have it build in configurations without RSEQ
and/or MM_CID (which was failing v10)

Thanks,
Gabriele