Re: [PATCH v2] vhost: Run TIF_NOTIFY_RESUME work in the vhost_task loop

[Bradley Morgan <brads@mainlining.org>]

On 3 November 2026 10:54:40 GMT, Aamir Ahmed <elb12345@hotmail.co.uk>
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.
>
>This also fixes memory.high and block cgroup throttling for vhost
>workers, which are handled in resume_user_mode_work() as well.
>
>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+
>Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
>Tested-by: Shakeel Butt <shakeel.butt@linux.dev>

Reviewed-by: Bradley Morgan <brads@mainlining.org>

>Assisted-by: LLM
>Signed-off-by: Aamir Ahmed <elb12345@hotmail.co.uk>
>---
>v2:
>- Also describe the memory.high and block cgroup throttling fixes
>  (Shakeel)
>- Add Shakeel's Reviewed-by and Tested-by
>- Update the code comment to match
>- v1: https://lore.kernel.org/AS8P251MB00012469E6ECBEA2B7D87BB0C86C2@AS8P251MB0001.EURP251.PROD.OUTLOOK.COM
>
>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..f63ac005214 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 and
>+		 * memory.high/blkcg throttling would otherwise never run. Run
>+		 * them 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
>


--- Thanks!