[PATCH 1/1] io_uring/sqpoll: inherit cpumask of creating process

[Felix Moessbauer]

The submit queue polling threads are "kernel" threads that are started
from the userland. In case the userland task is part of a cgroup with
the cpuset controller enabled, the poller should also stay within that
cpuset. This also holds, as the poller belongs to the same cgroup as
the task that started it.

With the current implementation, a process can "break out" of the
defined cpuset by creating sq pollers consuming CPU time on other CPUs,
which is especially problematic for realtime applications.

Part of this problem was fixed in a5fc1441 by dropping the
PF_NO_SETAFFINITY flag, but this only becomes effective after the first
modification of the cpuset (i.e. the pollers cpuset is correct after the
first update of the enclosing cgroups cpuset).

By inheriting the cpuset of the creating tasks, we ensure that the
poller is created with a cpumask that is a subset of the cgroups mask.
Inheriting the creators cpumask is reasonable, as other userland tasks
also inherit the mask.

Fixes: 37d1e2e3642e ("io_uring: move SQPOLL thread io-wq forked worker")
Cc: stable@xxxxxxxxxxxxxxx # 6.1+
Signed-off-by: Felix Moessbauer <felix.moessbauer@xxxxxxxxxxx>
---
 io_uring/sqpoll.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/io_uring/sqpoll.c b/io_uring/sqpoll.c
index 3b50dc9586d1..4681b2c41a96 100644
--- a/io_uring/sqpoll.c
+++ b/io_uring/sqpoll.c
@@ -289,7 +289,7 @@ static int io_sq_thread(void *data)
 	if (sqd->sq_cpu != -1) {
 		set_cpus_allowed_ptr(current, cpumask_of(sqd->sq_cpu));
 	} else {
-		set_cpus_allowed_ptr(current, cpu_online_mask);
+		set_cpus_allowed_ptr(current, sqd->thread->cpus_ptr);
 		sqd->sq_cpu = raw_smp_processor_id();
 	}
 
-- 
2.39.2