bpf: Mitigate latency spikes associated with freeing non-preallocated htab

Following the recent upgrade of one of our BPF programs, we encountered
significant latency spikes affecting other applications running on the same
host. After thorough investigation, we identified that these spikes were
primarily caused by the prolonged duration required to free a
non-preallocated htab with approximately 2 million keys.

Notably, our kernel configuration lacks the presence of CONFIG_PREEMPT. In
scenarios where kernel execution extends excessively, other threads might
be starved of CPU time, resulting in latency issues across the system. To
mitigate this, we've adopted a proactive approach by incorporating
cond_resched() calls within the kernel code. This ensures that during
lengthy kernel operations, the scheduler is invoked periodically to provide
opportunities for other threads to execute.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240327032022.78391-1-laoar.shao@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/hashtab.c

@@ -1490,6 +1490,7 @@ static void delete_all_elements(struct bpf_htab *htab)
 			hlist_nulls_del_rcu(&l->hash_node);
 			htab_elem_free(htab, l);
 		}
+		cond_resched();
 	}
 	migrate_enable();
 }