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 <[email protected]> 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 <[email protected]> Signed-off-by: Fan Du <[email protected]> Signed-off-by: Jun Miao <[email protected]> --- 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
