[PATCH 2/2] io_uring: add a schedule point in __io_req_caches_free()

[Yuanhe Shu <xiangzao@linux.alibaba.com>]

The request cache can grow to the workload's in-flight high-water
mark.  __io_req_caches_free() drains it with one
kmem_cache_free() per request under ctx->uring_lock, with no
scheduling point in between.  With debug features like KASAN or
kmemleak enabled, each free gets expensive, and draining a large
cache can hold the CPU well beyond the soft lockup threshold.

Reproduced on mainline v7.3-rc7 on a 192-CPU debug kernel
(preempt=none), with a test that submits 32K requests per
ring and then kills the rings.  Killing 48 rings at once made one
drain take up to 13 seconds with no scheduling point, with 8
kworkers each stuck for over 12 seconds; a separate run with 96
rings produced 97 soft lockup warnings during teardown, of the
form:

  watchdog: BUG: soft lockup - CPU#2 stuck for 12s! [kworker/u769:5]
  Workqueue: iou_exit io_ring_exit_work
  Call Trace:
    stack_trace_save
    kasan_save_stack
    kmem_cache_free+0x3cf/0x6a0
    __io_req_caches_free+0x196/0x4f0
    io_ring_exit_work+0x287/0x880
    process_one_work

Regardless of the per-free cost, the loop has no bound other than
the workload's in-flight high-water mark, so it belongs with the
other potentially long io_uring loops.

Insert a cond_resched() in the drain loop.  The function is __cold;
all callers run in process context and hold only ctx->uring_lock,
a mutex, so scheduling is allowed.

Assisted-by: LLM
Signed-off-by: Yuanhe Shu <xiangzao@linux.alibaba.com>
---
 io_uring/io_uring.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 26cd91a91aa2..dffce8ce55d5 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -2142,6 +2142,7 @@ static __cold void __io_req_caches_free(struct io_ring_ctx *ctx)
 		io_poison_req(req);
 		kmem_cache_free(req_cachep, req);
 		nr++;
+		cond_resched();
 	}
 	if (nr) {
 		ctx->nr_req_allocated -= nr;
-- 
2.43.7