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

[Aamir Ahmed]

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@xxxxxxxxxxxxxxx # 6.18+
Assisted-by: LLM
Signed-off-by: Aamir Ahmed <elb12345@xxxxxxxxxxxxx>
---
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