When the kernel boots from kexec, the EPC pages may have a stale state.
The kernel sanitizes all EPC pages to reset them to a clean state before
their first use in any enclave. The EPC size could be several GBs and
resetting them could take a significant amount of time. Because of that,
the kernel performs the reset in a loop through a kernel thread ksgxd() at
early boot, 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 an RCU-Tasks grace period but the cond_resched() in
ksgxd() never triggers rescheduling. Because cond_resched() doesn't report
a quiescent state when it doesn't trigger rescheduling, the thread that is
waiting for an RCU-Tasks grace period will 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 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
reports quiescent state regardless of 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.
Fixes: e7e0545299d8 ("x86/sgx: Initialize metadata for Enclave Page Cache (EPC)
sections")
Suggested-by: Kai Huang <[email protected]>
Co-developed-by: Fan Du <[email protected]>
Signed-off-by: Fan Du <[email protected]>
Signed-off-by: Jun Miao <[email protected]>
Tested-by: Challvy Tee <[email protected]>
Reviewed-by: Kai Huang <[email protected]>
Link: https://github.com/systemd/systemd/issues/40423
---
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.
v4 -> v5:
- Change the title, not state the problem directly
- Corrected spelling and grammatical errors by Kai
- Add "Reviewed-by: Kai Huang"
---
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..7d2f57663177 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
