[PATCH v4] x86/sgx: Fix RCU Tasks stall in EPC sanitization loop

[Jun Miao <jun.miao@intel.com>]

The kernel resets all EPC pages to a clean state in a loop before using them
for enclaves.  The number of EPC pages could be large (e.g., GBs) thus
resetting them could take a fair amount of time.  Because of that, during
early boot, the kernel resets EPC pages through a kernel thread ksgxd() and
there's a cond_resched() after resetting each EPC page.

This is fine in most cases, but becomes a problem when there's other kernel
code waiting for RCU-Tasks grace period but the cond_resched() in ksgxd()
never triggers rescheduling.  Because cond_resched() doesn't report quiescent
state when it doesn't trigger rescheduling, the thread that is waiting for
RCU-Tasks grace period will need to wait until all EPC pages are reset.

For instance, BPF LSM subsystem can invoke synchronize_rcu_tasks() at kernel
boot time.  A VM with a large EPC assigned and have BPF LSM enabled can take
a long time to boot, with a call trace triggered:

    rcu_tasks_wait_gp: rcu_tasks grace period number 1 (since boot) is 130631 jiffies old.
    INFO: task systemd:1 blocked for more than 122 seconds.
    ...
    task:systemd  state:D stack:0  pid:1  tpid:1  ppid:0  flags:0x00000002
    Call Trace:
    ...
    schedule_timeout+0x157/0x170
    wait_for_completion+0x88/0x150
    __wait_rcu_gp+0x17e/0x190
    synchronize_rcu_tasks_generic+0x64/0x60
    ...
    synchronize_rcu_tasks+0x15/0x20
    register_ftrace_direct+0x31f/0x350
    ...
    bpf_trampoline_link_prog+0x33/0x60
    bpf_tracing_prog_attach+0x3c5/0x5f0

Replace cond_resched() with cond_resched_tasks_rcu_qs() which explicitly report quiescent
regardless whether actual rescheduling is triggered.  Resetting all EPC pages in ksgxd()
isn't performance critical so the extra cost of cond_resched_tasks_rcu_qs() isn't a problem.

Tests showed this reduced the VM kernel boot time from ~50s to ~700ms.

Reported-by: Challvy Tee <challvy.tee@gmail.com>
Link: https://github.com/systemd/systemd/issues/40423
Fixes: e7e0545299d8 ("x86/sgx: Initialize metadata for Enclave Page Cache (EPC) sections")
Tested-by: Challvy Tee <challvy.tee@gmail.com>
Suggested-by: Kai Huang <kai.huang@intel.com>
Co-developed-by: Fan Du <fan.du@intel.com>
Signed-off-by: Fan Du <fan.du@intel.com>
Signed-off-by: Jun Miao <jun.miao@intel.com>

---
v1 -> v2:
 - Clarify the RCU Tasks stall root cause.
 - Use cond_resched_rcu_qs() following the Kai`s suggestion.

v2 -> v3:
 - cee439398933 ("rcu: Rename cond_resched_rcu_qs() to cond_resched_tasks_rcu_qs()")

v3 ->v4:
 - Trim down/rewrite changelog following Kai`s suggestion.

---
 arch/x86/kernel/cpu/sgx/main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
index 4505f808af5e..7ba3d0a5a05d 100644
--- a/arch/x86/kernel/cpu/sgx/main.c
+++ b/arch/x86/kernel/cpu/sgx/main.c
@@ -106,7 +106,7 @@ static unsigned long __sgx_sanitize_pages(struct list_head *dirty_page_list)
 			left_dirty++;
 		}
 
-		cond_resched();
+		cond_resched_tasks_rcu_qs();
 	}
 
 	list_splice(&dirty, dirty_page_list);
-- 
2.32.0