[PATCH] x86/sgx: Periodically yield in EPC sanitization to unblock rcu_tasks GP

[PATCH] x86/sgx: Periodically yield in EPC sanitization to unblock rcu_tasks GP

[Jun Miao]

During early boot, ksgxd (Intel Software Guard Extensions Kernel Thread)
iterates over all post-kexec dirty EPC pages in a tight loop calling
cond_resched() after each page.  But, on isolated CPUs
(a common configuration in cloud VMs), cond_resched() never triggers a
real context switch because TIF_NEED_RESCHED is not set when no competing
runnable task exists on that CPU.

synchronize_rcu_tasks(), invoked by BPF LSM during initialization, must
wait for every task that was running at the start of the grace period to
pass through a quiescent state (a voluntary sleep or preemption point).
If ksgxd never leaves the CPU, the rcu_tasks grace period stalls, causing
boot delays exceeding 60 seconds on machines with large EPC regions.

Fix this by introducing SGX_SANITIZE_RESCHED_INTERVAL (32768) and forcing
ksgxd to sleep for one jiffy every that many pages, guaranteeing that an
rcu_tasks quiescent state is reached in bounded time regardless of CPU
isolation.  Keep cond_resched() for all other iterations.

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>

Reported-by: challvy <challvy.tee@gmail.com>
Link: https://github.com/systemd/systemd/issues/40423
Fixes: e7e0545299d8 ("x86/sgx: Initialize metadata for Enclave Page Cache (EPC) sections")
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>
---
 arch/x86/kernel/cpu/sgx/main.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
index 4505f808af5e..4642d2d47186 100644
--- a/arch/x86/kernel/cpu/sgx/main.c
+++ b/arch/x86/kernel/cpu/sgx/main.c
@@ -52,6 +52,13 @@ static struct sgx_numa_node *sgx_numa_nodes;
 
 static LIST_HEAD(sgx_dirty_page_list);
 
+/*
+ * Force a voluntary context switch every SGX_SANITIZE_RESCHED_INTERVAL
+ * iterations to let synchronize_rcu_tasks() (e.g. called by BPF LSM at
+ * init) complete its grace period.
+ */
+#define SGX_SANITIZE_RESCHED_INTERVAL	(1 << 15)
+
 /*
  * Reset post-kexec EPC pages to the uninitialized state. The pages are removed
  * from the input list, and made available for the page allocator. SECS pages
@@ -63,6 +70,7 @@ static LIST_HEAD(sgx_dirty_page_list);
 static unsigned long __sgx_sanitize_pages(struct list_head *dirty_page_list)
 {
 	unsigned long left_dirty = 0;
+	unsigned long count = 0;
 	struct sgx_epc_page *page;
 	LIST_HEAD(dirty);
 	int ret;
@@ -72,6 +80,18 @@ static unsigned long __sgx_sanitize_pages(struct list_head *dirty_page_list)
 		if (kthread_should_stop())
 			return 0;
 
+		/*
+		 * On isolated CPUs cond_resched() does not trigger a real
+		 * context switch when no competing runnable task exists.
+		 * Periodically force ksgxd to sleep so that synchronize_rcu_tasks()
+		 * (e.g. BPF LSM) can complete the grace period in bounded time.
+		 * Keep cond_resched() between forced sleeps for higher-priority tasks.
+		 */
+		if (!(++count & (SGX_SANITIZE_RESCHED_INTERVAL - 1)))
+			schedule_timeout_interruptible(1);
+		else
+			cond_resched();
+
 		page = list_first_entry(dirty_page_list, struct sgx_epc_page, list);
 
 		/*
@@ -105,8 +125,6 @@ static unsigned long __sgx_sanitize_pages(struct list_head *dirty_page_list)
 			list_move_tail(&page->list, &dirty);
 			left_dirty++;
 		}
-
-		cond_resched();
 	}
 
 	list_splice(&dirty, dirty_page_list);
-- 
2.43.0

Re: [PATCH] x86/sgx: Periodically yield in EPC sanitization to unblock rcu_tasks GP

[Jarkko Sakkinen]

On Thu, Jun 18, 2026 at 06:04:32PM +0800, Jun Miao wrote:
> During early boot, ksgxd (Intel Software Guard Extensions Kernel Thread)
> iterates over all post-kexec dirty EPC pages in a tight loop calling
> cond_resched() after each page.  But, on isolated CPUs
> (a common configuration in cloud VMs), cond_resched() never triggers a
> real context switch because TIF_NEED_RESCHED is not set when no competing
> runnable task exists on that CPU.
> 
> synchronize_rcu_tasks(), invoked by BPF LSM during initialization, must
> wait for every task that was running at the start of the grace period to
> pass through a quiescent state (a voluntary sleep or preemption point).
> If ksgxd never leaves the CPU, the rcu_tasks grace period stalls, causing
> boot delays exceeding 60 seconds on machines with large EPC regions.
> 
> Fix this by introducing SGX_SANITIZE_RESCHED_INTERVAL (32768) and forcing
> ksgxd to sleep for one jiffy every that many pages, guaranteeing that an
> rcu_tasks quiescent state is reached in bounded time regardless of CPU
> isolation.  Keep cond_resched() for all other iterations.
> 
> 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>
> 
> Reported-by: challvy <challvy.tee@gmail.com>
> Link: https://github.com/systemd/systemd/issues/40423
> Fixes: e7e0545299d8 ("x86/sgx: Initialize metadata for Enclave Page Cache (EPC) sections")
> 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>
> ---
>  arch/x86/kernel/cpu/sgx/main.c | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index 4505f808af5e..4642d2d47186 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -52,6 +52,13 @@ static struct sgx_numa_node *sgx_numa_nodes;
>  
>  static LIST_HEAD(sgx_dirty_page_list);
>  
> +/*
> + * Force a voluntary context switch every SGX_SANITIZE_RESCHED_INTERVAL
> + * iterations to let synchronize_rcu_tasks() (e.g. called by BPF LSM at
> + * init) complete its grace period.
> + */
> +#define SGX_SANITIZE_RESCHED_INTERVAL	(1 << 15)

nit: maybe SGX_SANITIZE_PAGES_RECHED_INTERVAL

> +
>  /*
>   * Reset post-kexec EPC pages to the uninitialized state. The pages are removed
>   * from the input list, and made available for the page allocator. SECS pages
> @@ -63,6 +70,7 @@ static LIST_HEAD(sgx_dirty_page_list);
>  static unsigned long __sgx_sanitize_pages(struct list_head *dirty_page_list)
>  {
>  	unsigned long left_dirty = 0;
> +	unsigned long count = 0;
>  	struct sgx_epc_page *page;
>  	LIST_HEAD(dirty);
>  	int ret;

nit: reverse xmas tree order

> @@ -72,6 +80,18 @@ static unsigned long __sgx_sanitize_pages(struct list_head *dirty_page_list)
>  		if (kthread_should_stop())
>  			return 0;
>  
> +		/*
> +		 * On isolated CPUs cond_resched() does not trigger a real
> +		 * context switch when no competing runnable task exists.
> +		 * Periodically force ksgxd to sleep so that synchronize_rcu_tasks()
> +		 * (e.g. BPF LSM) can complete the grace period in bounded time.
> +		 * Keep cond_resched() between forced sleeps for higher-priority tasks.
> +		 */
> +		if (!(++count & (SGX_SANITIZE_RESCHED_INTERVAL - 1)))
> +			schedule_timeout_interruptible(1);
> +		else
> +			cond_resched();
> +
>  		page = list_first_entry(dirty_page_list, struct sgx_epc_page, list);
>  
>  		/*
> @@ -105,8 +125,6 @@ static unsigned long __sgx_sanitize_pages(struct list_head *dirty_page_list)
>  			list_move_tail(&page->list, &dirty);
>  			left_dirty++;
>  		}
> -
> -		cond_resched();
>  	}
>  
>  	list_splice(&dirty, dirty_page_list);
> -- 
> 2.43.0
> 
> 

BR, Jarkko

Re: [PATCH] x86/sgx: Periodically yield in EPC sanitization to unblock rcu_tasks GP

[Huang, Kai]

On Thu, 2026-06-18 at 18:04 +0800, Jun Miao wrote:
> During early boot, ksgxd (Intel Software Guard Extensions Kernel Thread)
> iterates over all post-kexec dirty EPC pages in a tight loop calling
> cond_resched() after each page.  But, on isolated CPUs
> (a common configuration in cloud VMs), cond_resched() never triggers a
> real context switch because TIF_NEED_RESCHED is not set when no competing
> runnable task exists on that CPU.
> 
> synchronize_rcu_tasks(), invoked by BPF LSM during initialization, must
> wait for every task that was running at the start of the grace period to
> pass through a quiescent state (a voluntary sleep or preemption point).
> If ksgxd never leaves the CPU, the rcu_tasks grace period stalls, causing
> boot delays exceeding 60 seconds on machines with large EPC regions.
> 
> Fix this by introducing SGX_SANITIZE_RESCHED_INTERVAL (32768) and forcing
> ksgxd to sleep for one jiffy every that many pages, guaranteeing that an
> rcu_tasks quiescent state is reached in bounded time regardless of CPU
> isolation.  Keep cond_resched() for all other iterations.
> 
> Without this patch, instead, virtual machines (VMs) experience a long OS boot times:

This seems a common problem at the scheduler and RCU layer, but not specific to
SGX.  I originally thought cond_resched() should be able to handle this, but
after reading some history it is not the case.

IIUC, cond_resched_tasks_rcu_qs() was specifically introduced to to address this
issue.  See bde6c3aa9930 ("rcu: Provide cond_resched_rcu_qs() to force quiescent
states in long loops").

Could you try this solution?

RE: [PATCH] x86/sgx: Periodically yield in EPC sanitization to unblock rcu_tasks GP

[Miao, Jun]

>This seems a common problem at the scheduler and RCU layer, but not specific to
>SGX.  I originally thought cond_resched() should be able to handle this, but after
>reading some history it is not the case.
>
>IIUC, cond_resched_tasks_rcu_qs() was specifically introduced to to address this
>issue.  See bde6c3aa9930 ("rcu: Provide cond_resched_rcu_qs() to force quiescent
>states in long loops").
>
>Could you try this solution?

Hi, Kai
The result is effective, and I will adopt this more common method you suggested by, submit the v2 version.

Thanks Challvy Tee for validate it as follow:
We have tested the suggested fix (replacing cond_resched() with cond_resched_tasks_rcu_qs()) on our production environment and can confirm it resolves the issue.

Test environment:
- Instance type: ecs.r9it.16xlarge (SGX-enabled cloud VM with isolated CPUs) on ECS
- Kernel: 6.6.102-cbp.git.772159d5f.alnx4.x86_64 (PREEMPT_DYNAMIC)

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

```
# uname -a
Linux iZbp1cp63g07kzbp93uzrhZ 6.6.102-cbp.git.772159d5f.alnx4.x86_64 #1 SMP PREEMPT_DYNAMIC Mon Jun 22 03:10:04 UTC 2026 x86_64 x86_64 x86_64 GNU/Linux
# free -h
               total        used        free      shared  buff/cache   available
Mem:           243Gi       4.5Gi       239Gi       2.3Mi       481Mi       239Gi
Swap:             0B          0B          0B
# systemd-analyze 
Startup finished in 724ms (kernel) + 1.575s (initrd) + 8.481s (userspace) = 10.782s 
multi-user.target reached after 3.939s in userspace.
# systemd-analyze blame | head
5.416s kdump.service
2.743s sys-devices-pci0000:00-0000:00:05.0-virtio1-net-eth0.device
2.743s sys-subsystem-net-devices-eth0.device
2.534s sys-devices-pci0000:00-0000:00:03.0-virtio0-virtio\x2dports-vport0p1.device
2.534s dev-virtio\x2dports-org.qemu.guest_agent.0.device
2.534s dev-vport0p1.device
2.512s sys-module-configfs.device
2.510s dev-ttyS0.device
2.510s sys-devices-pnp0-00:00-tty-ttyS0.device
2.509s sys-devices-platform-serial8250-tty-ttyS1.device
# grep -o 'sgx[^ ]*' /proc/cpuinfo | sort -u
sgx
sgx_lc

# dmesg | grep -i sgx
[    0.355547] sgx: EPC section 0x4040000000-0x7e3fffffff

# ls -la /dev/sgx*
crw-rw---- 1 root sgx  10, 125 Jun 22 11:36 /dev/sgx_enclave
crw------- 1 root root 10, 126 Jun 22 11:36 /dev/sgx_provision

Warm regards
Jun Miao