On Sat, Oct 31, 2026 at 01:33:13AM +0000, Aamir Ahmed wrote:
> Since commit e1fad12dcb66 ("sched/fair: Switch to task based throttle
> model"), a throttled task is only dequeued when TWA_RESUME task_work
> runs on its way back to user mode.
>
> vhost workers never return to user mode, and vhost_task_fn() only runs
> task_work via get_signal() when a signal is pending. A busy vhost worker
> therefore ignores its cgroup's cpu.max, while its runtime is still
> charged to the group, throttling the group's other tasks (typically the
> VM's vCPUs) harder.
>
> Run pending notify-resume work at the top of the worker loop, as io-wq
> does in io_run_task_work(). Commit 12013e3d4695 ("KVM: powerpc: Use
> generic xfer to guest work function") fixed the same problem for KVM.
>
> Fixes: e1fad12dcb66 ("sched/fair: Switch to task based throttle model")
> Cc: [email protected] # 6.18+
> Assisted-by: LLM
> Signed-off-by: Aamir Ahmed <[email protected]>
Could you pls respond to Sashiko's review?
> ---
> Tested on x86_64 with a vhost_task-based reproducer: in a 0.2-CPU cgroup
> a busy worker drops from 100% to 19% of a CPU. Reproducer available on
> request.
>
> kernel/vhost_task.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/kernel/vhost_task.c b/kernel/vhost_task.c
> index 3717885a599..7fa96e1b85b 100644
> --- a/kernel/vhost_task.c
> +++ b/kernel/vhost_task.c
> @@ -6,6 +6,7 @@
> #include <linux/completion.h>
> #include <linux/sched/task.h>
> #include <linux/sched/vhost_task.h>
> +#include <linux/resume_user_mode.h>
> #include <linux/sched/signal.h>
>
> enum vhost_task_flags {
> @@ -31,6 +32,14 @@ static int vhost_task_fn(void *data)
> for (;;) {
> bool did_work;
>
> + /*
> + * We never return to user mode, so TWA_RESUME task_work (e.g.
> CFS
> + * bandwidth throttling) would otherwise never run. Run it here,
> + * where no locks are held, like io-wq workers do.
> + */
> + if (test_thread_flag(TIF_NOTIFY_RESUME))
> + resume_user_mode_work(NULL);
> +
> if (signal_pending(current)) {
> struct ksignal ksig;
>
>
> base-commit: c03c1a14507dfaf8b856c2608f9ac216021fc222
> --
> 2.43.0
