Re: [PATCH] vhost: Run TIF_NOTIFY_RESUME work in the vhost_task loop

["Michael S. Tsirkin" <mst@redhat.com>]

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: stable@vger.kernel.org # 6.18+
> Assisted-by: LLM
> Signed-off-by: Aamir Ahmed <elb12345@hotmail.co.uk>

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