[PATCH v2] x86/sgx: Fix RCU Tasks stalls in EPC sanitization loop

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

Large EPC configurations can spend a significant amount of time
sanitizing EPC pages during SGX initialization.  The sanitization
loop invokes cond_resched() while processing pages, but if the
scheduler does not request rescheduling, the CPU may remain in the
kernel for an extended period without reporting a quiescent state to
RCU Tasks.

cond_resched() only schedules when rescheduling is needed and does
not guarantee a quiescent state for RCU Tasks.  Replace it with
cond_resched_rcu_qs(), which explicitly reports an RCU Tasks
quiescent state even when no context switch occurs.

Kai suggested that, this is a common problem at the scheduler and RCU layer,
but not specific to SGX. More detail please see:
bde6c3aa9930 ("rcu: Provide cond_resched_rcu_qs() to force quiescent states in long loops").

Without this patch, instead, virtual machines (VMs) experience a long OS boot times:

[    4.110549] systemd[1]: Detected architecture x86-64.
[    4.115279] systemd[1]: Hostname set to <i2bp1g0g0m0i8406er0g1zX2>.
[    4.115554] systemd[1]: Installed transient /etc/machine-id file.
[   14.262158] rcu_tasks_wait_gp: rcu_tasks grace period number 1 (since boot) is 10087 jiffies old.
[   14.374158] rcu_tasks_wait_gp: rcu_tasks grace period number 1 (since boot) is 40199 jiffies old.
[  134.806157] rcu_tasks_wait_gp: rcu_tasks grace period number 1 (since boot) is 130631 jiffies old.
[  248.086158] INFO: task systemd:1 blocked for more than 122 seconds.
[  248.086491] Not tainted 6.8.0-90-generic #91-Ubuntu
[  248.086739] 'echo 0 > /proc/sys/kernel/hung_task_timeout_secs' disables this message.
[  248.086993] task:systemd    state:D stack:0    pid:1    tpid:1    ppid:0    flags:0x00000002
[  248.087274] Call Trace:
[  248.087434] <TASK>
[  248.087557] __schedule+0x27c/0x6b0
[  248.087770] schedule+0x33/0x110
[  248.087939] schedule_timeout+0x157/0x170
[  248.088120] wait_for_completion+0x88/0x150
[  248.088304] __wait_rcu_gp+0x17e/0x190
[  248.088481] synchronize_rcu_tasks_generic+0x64/0x60
[  248.088672] ? __pfx_call_rcu_tasks+0x10/0x10
[  248.088858] ? __pfx_wakeme_after_rcu+0x10/0x10
[  248.089047] synchronize_rcu_tasks+0x15/0x20
[  248.089260] register_ftrace_direct+0x31f/0x350
[  248.089445] ? __pfx_bpf_lsm_file_open+0x10/0x10
[  248.089629] bpf_trampoline_update+0x469/0x650
[  248.089814] ? 0xffffffffffffffff
[  248.089988] ? 0xffffffffffffffff
[  248.090153] __bpf_trampoline_link_prog+0x10d/0x330
[  248.090339] bpf_trampoline_link_prog+0x33/0x60
[  248.090518] bpf_tracing_prog_attach+0x3c5/0x5f0
[  248.090699] link_create+0x1a5/0x280
[  248.090886] ? security_bpf+0x3c/0x70
[  248.091101] __sys_bpf+0x4ae/0x10
[  248.091312] __x64_sys_bpf+0x1a/0x30
[  248.091477] x64_sys_call+0x199/0x250
[  248.091647] do_syscall_64+0x7f/0x180
[  248.091818] ? arch_exit_to_user_mode_prepare.isa.0+0x1a/0x60
[  248.092022] ? irqentry_exit_to_user_mode+0x38/0x1e0
[  248.092246] ? irqentry_exit+0x43/0x50
[  248.092401] entry_SYSCALL_64_after_hwframe+0x78/0x80
[  248.092590] RIP: 0033:0x7b53e592728d
[  248.092756] RSP: 002b:00007ffdaa9d696 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
[  248.092856] RAX: ffffffffffffffda RBX: 00007ffdaa9d696 RCX: 00007b53e592728d
[  248.092956] RDX: 0000000000000000 RSI: 00007ffdaa9d696 RDI: 0000000000000001
[  248.093056] RBP: 00007ffdaa9d696 R08: 00007b53e5a03a8 R09: 00007ffdaa9d696
[  248.093156] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
[  248.093256] R13: 0000000000000000 R14: 00005d81ed2cfd0 R15: 00005d81ed2b7ec0
[  248.093406] </TASK>

After this patch test Results:
Before fixed: boot time ~50s (with rcu_tasks grace period stall)
After fixed: boot time ~10.7s (systemd-analyze: 724ms kernel + 1.575s initrd + 8.481s userspace = 10.782s)

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.
 - Change the subject of this patch to fitness function.
---
 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_rcu_qs();
 	}
 
 	list_splice(&dirty, dirty_page_list);
-- 
2.32.0