On 2026-07-07 15:33:32 [+0100], Lorenzo Stoakes wrote:

On Tue, Jul 07, 2026 at 08:59:23PM +0800, Wandun Chen wrote:

quoted

From: Wandun Chen <redacted>

In RT kernels, sysctl_compact_unevictable_allowed is false by default,
when the mlock/mlockall system call try to lock all the present page,
the mlock_pte_range function skips non-present entries. If these
non-present entries are migration entries, and the migration is not
guaranteed to have completed before the mlock/mlockall, it may result
in a page fault on subsequent access, which then waits for the
migration to finish, causing spike latency in RT kernels.

Is this really noticable and measurable?

You want to avoid page faults like this which is why you do mlock in the
first place.
If the thread has a RT priority and it blocks here then other RT threads
will be scheduled and your migration thread, which will resolve this, is
SCHED_OTHER and the last one in the line. Assuming that thread has a 1ms
cycle/ deadline then it will likely miss it.

I suppose not too many ranges should be mock

Sebastian