[RFC PATCH 75/86] treewide: virt: remove cond_resched()

[Ankur Arora]

There are broadly three sets of uses of cond_resched():

1.  Calls to cond_resched() out of the goodness of our heart,
    otherwise known as avoiding lockup splats.

2.  Open coded variants of cond_resched_lock() which call
    cond_resched().

3.  Retry or error handling loops, where cond_resched() is used as a
    quick alternative to spinning in a tight-loop.

When running under a full preemption model, the cond_resched() reduces
to a NOP (not even a barrier) so removing it obviously cannot matter.

But considering only voluntary preemption models (for say code that
has been mostly tested under those), for set-1 and set-2 the
scheduler can now preempt kernel tasks running beyond their time
quanta anywhere they are preemptible() [1]. Which removes any need
for these explicitly placed scheduling points.

The cond_resched() calls in set-3 are a little more difficult.
To start with, given it's NOP character under full preemption, it
never actually saved us from a tight loop.
With voluntary preemption, it's not a NOP, but it might as well be --
for most workloads the scheduler does not have an interminable supply
of runnable tasks on the runqueue.

So, cond_resched() is useful to not get softlockup splats, but not
terribly good for error handling. Ideally, these should be replaced
with some kind of timed or event wait.
For now we use cond_resched_stall(), which tries to schedule if
possible, and executes a cpu_relax() if not.

All the cond_resched() calls here are from set-1. Remove them.

[1] https://lore.kernel.org/lkml/20231107215742.363031-1-ankur.a.arora@xxxxxxxxxx/

Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
---
 drivers/xen/balloon.c      | 2 --
 drivers/xen/gntdev.c       | 2 --
 drivers/xen/xen-scsiback.c | 9 +++++----
 virt/kvm/pfncache.c        | 2 --
 4 files changed, 5 insertions(+), 10 deletions(-)

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index 586a1673459e..a57e516b36f5 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -550,8 +550,6 @@ static int balloon_thread(void *unused)
 		update_schedule();
 
 		mutex_unlock(&balloon_mutex);
-
-		cond_resched();
 	}
 }
 
diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 61faea1f0663..cbf74a2b6a06 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -974,8 +974,6 @@ static long gntdev_ioctl_grant_copy(struct gntdev_priv *priv, void __user *u)
 		ret = gntdev_grant_copy_seg(&batch, &seg, &copy.segments[i].status);
 		if (ret < 0)
 			goto out;
-
-		cond_resched();
 	}
 	if (batch.nr_ops)
 		ret = gntdev_copy(&batch);
diff --git a/drivers/xen/xen-scsiback.c b/drivers/xen/xen-scsiback.c
index 8b77e4c06e43..1ab88ba93166 100644
--- a/drivers/xen/xen-scsiback.c
+++ b/drivers/xen/xen-scsiback.c
@@ -814,9 +814,6 @@ static int scsiback_do_cmd_fn(struct vscsibk_info *info,
 			transport_generic_free_cmd(&pending_req->se_cmd, 0);
 			break;
 		}
-
-		/* Yield point for this unbounded loop. */
-		cond_resched();
 	}
 
 	gnttab_page_cache_shrink(&info->free_pages, scsiback_max_buffer_pages);
@@ -831,8 +828,12 @@ static irqreturn_t scsiback_irq_fn(int irq, void *dev_id)
 	int rc;
 	unsigned int eoi_flags = XEN_EOI_FLAG_SPURIOUS;
 
+	/*
+	 * Process cmds in a tight loop.  The scheduler can preempt when
+	 * it needs to.
+	 */
 	while ((rc = scsiback_do_cmd_fn(info, &eoi_flags)) > 0)
-		cond_resched();
+		;
 
 	/* In case of a ring error we keep the event channel masked. */
 	if (!rc)
diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 2d6aba677830..cc757d5b4acc 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -178,8 +178,6 @@ static kvm_pfn_t hva_to_pfn_retry(struct gfn_to_pfn_cache *gpc)
 				gpc_unmap_khva(new_pfn, new_khva);
 
 			kvm_release_pfn_clean(new_pfn);
-
-			cond_resched();
 		}
 
 		/* We always request a writeable mapping */
-- 
2.31.1