Hi, On Fri, Mar 06, 2026 at 10:03:24PM +0800, zhidao su wrote: > From: Su Zhidao <[email protected]> > > Add a test that verifies the sched_ext bypass mechanism does not > prevent tasks from running to completion. > > The test attaches a minimal global FIFO scheduler, spawns worker > processes that complete a fixed computation, detaches the scheduler > (which triggers bypass mode while workers are still active), and > verifies all workers complete successfully under bypass mode. > > This exercises the scheduler attach/detach lifecycle and verifies > that bypass mode (activated during unregistration to guarantee > forward progress) does not stall running tasks.
I'm not sure this selftest adds much value. Implicitly we're already testing the validity of bypass in the other sched_ext kselftests: if a task is missed or gets stuck due to bypass mode, we would trigger a soft lockup, a hung task timeout, or something similar. > > Signed-off-by: Su Zhidao <[email protected]> > --- > tools/testing/selftests/sched_ext/Makefile | 1 + > .../testing/selftests/sched_ext/bypass.bpf.c | 32 ++++++ > tools/testing/selftests/sched_ext/bypass.c | 105 ++++++++++++++++++ > 3 files changed, 138 insertions(+) > create mode 100644 tools/testing/selftests/sched_ext/bypass.bpf.c > create mode 100644 tools/testing/selftests/sched_ext/bypass.c > > diff --git a/tools/testing/selftests/sched_ext/Makefile > b/tools/testing/selftests/sched_ext/Makefile > index a3bbe2c7911b..5fb6278d3f97 100644 > --- a/tools/testing/selftests/sched_ext/Makefile > +++ b/tools/testing/selftests/sched_ext/Makefile > @@ -162,6 +162,7 @@ endef > all_test_bpfprogs := $(foreach prog,$(wildcard > *.bpf.c),$(INCLUDE_DIR)/$(patsubst %.c,%.skel.h,$(prog))) > > auto-test-targets := \ > + bypass \ > create_dsq \ > dequeue \ > enq_last_no_enq_fails \ > diff --git a/tools/testing/selftests/sched_ext/bypass.bpf.c > b/tools/testing/selftests/sched_ext/bypass.bpf.c > new file mode 100644 > index 000000000000..cb37c8df6834 > --- /dev/null > +++ b/tools/testing/selftests/sched_ext/bypass.bpf.c > @@ -0,0 +1,32 @@ > +// SPDX-License-Identifier: GPL-2.0 > +/* > + * BPF scheduler for bypass mode operational test. > + * > + * Implements a minimal global FIFO scheduler. The userspace side > + * attaches this scheduler, runs worker tasks to completion, and > + * verifies that tasks complete successfully. > + * > + * Copyright (c) 2026 Xiaomi Corporation. > + */ > +#include <scx/common.bpf.h> > + > +char _license[] SEC("license") = "GPL"; > + > +UEI_DEFINE(uei); > + > +void BPF_STRUCT_OPS(bypass_enqueue, struct task_struct *p, u64 enq_flags) > +{ > + scx_bpf_dsq_insert(p, SCX_DSQ_GLOBAL, SCX_SLICE_DFL, enq_flags); > +} We could also remove bypass_enqueue() and sched_ext core will do exactly the same (implicitly enqueue to SCX_DSQ_GLOBAL). > + > +void BPF_STRUCT_OPS(bypass_exit, struct scx_exit_info *ei) > +{ > + UEI_RECORD(uei, ei); > +} > + > +SEC(".struct_ops.link") > +struct sched_ext_ops bypass_ops = { > + .enqueue = (void *)bypass_enqueue, > + .exit = (void *)bypass_exit, > + .name = "bypass_test", > +}; > diff --git a/tools/testing/selftests/sched_ext/bypass.c > b/tools/testing/selftests/sched_ext/bypass.c > new file mode 100644 > index 000000000000..952f09d76bdb > --- /dev/null > +++ b/tools/testing/selftests/sched_ext/bypass.c > @@ -0,0 +1,105 @@ > +// SPDX-License-Identifier: GPL-2.0 > +/* > + * Verify the sched_ext bypass mechanism: spawn worker tasks and ensure > + * they run to completion while a BPF scheduler is active. > + * > + * The bypass mechanism (activated on scheduler unregistration) must > + * guarantee forward progress. This test verifies that worker tasks > + * complete successfully when the scheduler is detached. > + * > + * Copyright (c) 2026 Xiaomi Corporation. > + */ > +#define _GNU_SOURCE > +#include <unistd.h> > +#include <sys/wait.h> > +#include <bpf/bpf.h> > +#include <scx/common.h> > +#include "scx_test.h" > +#include "bypass.bpf.skel.h" > + > +#define NUM_BYPASS_WORKERS 4 > + > +static void worker_fn(void) > +{ > + volatile int sum = 0; > + int i; > + > + /* > + * Do enough work to still be running when bpf_link__destroy() > + * is called, ensuring tasks are active during bypass mode. > + */ > + for (i = 0; i < 10000000; i++) > + sum += i; > +} > + > +static enum scx_test_status setup(void **ctx) > +{ > + struct bypass *skel; > + > + skel = bypass__open(); > + SCX_FAIL_IF(!skel, "Failed to open bypass skel"); > + SCX_ENUM_INIT(skel); > + SCX_FAIL_IF(bypass__load(skel), "Failed to load bypass skel"); > + > + *ctx = skel; > + return SCX_TEST_PASS; > +} > + > +static enum scx_test_status run(void *ctx) > +{ > + struct bypass *skel = ctx; > + struct bpf_link *link; > + pid_t pids[NUM_BYPASS_WORKERS]; > + int i, status; > + > + link = bpf_map__attach_struct_ops(skel->maps.bypass_ops); > + SCX_FAIL_IF(!link, "Failed to attach bypass scheduler"); > + > + /* > + * Spawn worker processes. These must complete successfully > + * even as the scheduler is active and then detached (which > + * triggers bypass mode). > + */ > + for (i = 0; i < NUM_BYPASS_WORKERS; i++) { > + pids[i] = fork(); > + SCX_FAIL_IF(pids[i] < 0, "fork() failed for worker %d", i); > + > + if (pids[i] == 0) { > + worker_fn(); > + _exit(0); > + } > + } There's no synchronization with the parent, so on a fast system the workers may even finish the loop before the parent ever detaches the scheduler. > + > + /* > + * Detach the scheduler while workers are still running. This > + * triggers bypass mode, which must guarantee forward progress > + * for all active tasks. > + */ > + bpf_link__destroy(link); > + > + /* Workers must complete successfully under bypass mode */ > + for (i = 0; i < NUM_BYPASS_WORKERS; i++) { > + SCX_FAIL_IF(waitpid(pids[i], &status, 0) != pids[i], > + "waitpid failed for worker %d", i); > + SCX_FAIL_IF(!WIFEXITED(status) || WEXITSTATUS(status) != 0, > + "Worker %d did not exit cleanly", i); > + } > + > + SCX_EQ(skel->data->uei.kind, EXIT_KIND(SCX_EXIT_UNREG)); > + > + return SCX_TEST_PASS; > +} > + > +static void cleanup(void *ctx) > +{ > + bypass__destroy(ctx); > +} > + > +struct scx_test bypass_test = { > + .name = "bypass", > + .description = "Verify tasks complete during bypass mode", > + .setup = setup, > + .run = run, > + .cleanup = cleanup, > +}; > +REGISTER_SCX_TEST(&bypass_test) > -- > 2.43.0 > Thanks, -Andrea
