On 27 Apr 2023 13:35:31 +0000 Bernd Schubert <bschubert@xxxxxxx>
> Btw, a very hackish way to 'solve' the issue is this
>
> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> index cd7aa679c3ee..dd32effb5010 100644
> --- a/fs/fuse/dev.c
> +++ b/fs/fuse/dev.c
> @@ -373,6 +373,26 @@ static void request_wait_answer(struct fuse_req *req)
> int err;
> int prev_cpu = task_cpu(current);
>
> + /* When running over uring and core affined userspace threads, we
> + * do not want to let migrate away the request submitting process.
> + * Issue is that even after waking up on the right core, processes
> + * that have submitted requests might get migrated away, because
> + * the ring thread is still doing a bit of work or is in the process
> + * to go to sleep. Assumption here is that processes are started on
> + * the right core (i.e. idle cores) and can then stay on that core
> + * when they come and do file system requests.
> + * Another alternative way is to set SCHED_IDLE for ring threads,
> + * but that would have an issue if there are other processes keeping
> + * the cpu busy.
> + * SCHED_IDLE or this hack here result in about factor 3.5 for
> + * max meta request performance.
> + *
> + * Ideal would to tell the scheduler that ring threads are not disturbing
> + * that migration away from it should very very rarely happen.
> + */
> + if (fc->ring.ready)
> + migrate_disable();
> +
> if (!fc->no_interrupt) {
> /* Any signal may interrupt this */
> err = wait_event_interruptible(req->waitq,
>
If I understand it correctly, the seesaw workload hint to scheduler looks
like the diff below, leaving scheduler free to pull the two players apart
across CPU and to migrate anyone.
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -421,6 +421,7 @@ static void __fuse_request_send(struct f
/* acquire extra reference, since request is still needed
after fuse_request_end() */
__fuse_get_request(req);
+ current->seesaw = 1;
queue_request_and_unlock(fiq, req);
request_wait_answer(req);
@@ -1229,6 +1230,7 @@ static ssize_t fuse_dev_do_read(struct f
fc->max_write))
return -EINVAL;
+ current->seesaw = 1;
restart:
for (;;) {
spin_lock(&fiq->lock);
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -953,6 +953,7 @@ struct task_struct {
/* delay due to memory thrashing */
unsigned in_thrashing:1;
#endif
+ unsigned seesaw:1;
unsigned long atomic_flags; /* Flags requiring atomic access. */
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7424,6 +7424,8 @@ select_task_rq_fair(struct task_struct *
if (wake_flags & WF_TTWU) {
record_wakee(p);
+ if (p->seesaw && current->seesaw)
+ return cpu;
if (sched_energy_enabled()) {
new_cpu = find_energy_efficient_cpu(p, prev_cpu);
if (new_cpu >= 0)
