Hi Michal, On Mon, Oct 19, 2026 at 12:41:46PM +0000, Michal Blaszczyk wrote: > Add a test for a task which leaves SCHED_EXT while it's waiting. Its > scheduler keeps the task waiting on a DSQ after it wakes up. The test > switches the task to SCHED_OTHER while it's waiting, lets it sleep outside > of SCHED_EXT for three times the watchdog timeout, then switches it back > and wakes it up. The scheduler keeps it waiting again, for longer than the > watchdog interval but shorter than the timeout, which the watchdog must not > report as a stall. > > Without the previous patch, the watchdog charges the task for the time it > spent outside of SCHED_EXT and aborts the scheduler. > > Assisted-by: LLM > Signed-off-by: Michal Blaszczyk <[email protected]>
... > diff --git a/tools/testing/selftests/sched_ext/class_switch_stall.c > b/tools/testing/selftests/sched_ext/class_switch_stall.c > new file mode 100644 > index 0000000000000..489d23c777f05 > --- /dev/null > +++ b/tools/testing/selftests/sched_ext/class_switch_stall.c > @@ -0,0 +1,240 @@ > +// SPDX-License-Identifier: GPL-2.0 > +/* > + * Check that a task which leaves SCHED_EXT while it's waiting doesn't carry > + * that wait over to its next stay on SCHED_EXT. If it did, the watchdog > would > + * count the time the task spent outside SCHED_EXT as waiting and abort the > + * scheduler. > + * > + * Copyright (c) 2026 Google LLC. > + */ > +#define _GNU_SOURCE > + > +#include <bpf/bpf.h> > +#include <poll.h> > +#include <sched.h> > +#include <scx/common.h> > +#include <signal.h> > +#include <stdio.h> > +#include <string.h> > +#include <sys/wait.h> > +#include <unistd.h> > +#include "class_switch_stall.bpf.skel.h" > +#include "scx_test.h" > + > +#ifndef SCHED_EXT > +#define SCHED_EXT 7 > +#endif > + > +/* class_switch_stall_ops.timeout_ms, the watchdog checks every half of it */ > +#define TIMEOUT_MS 1000 > +/* longer than the watchdog interval, but shorter than the timeout */ > +#define PARK_MS 750 > +/* time spent outside of SCHED_EXT, a stale wait would exceed the timeout */ > +#define OUTSIDE_MS (3 * TIMEOUT_MS) Could we increase the watchdog timeout to 3s and the park duration to 2s, updating timeout_ms accordingly? With the current values, there is only about 250ms of headroom for timer delivery, dispatch, and the child actually getting CPU time before it becomes eligible for a watchdog stall The first phase also depends on userspace switching the parked child out within 1s. I haven't observed a false failure, so this is a robustness suggestion (I'm thinking at loaded CI VMs) rather than an observed problem. A 2s park would still span the nominal 1.5s watchdog interval while leaving more headroom. Thanks, -Andrea
