Re: [PATCH v6] x86/sgx: Report RCU-Tasks quiescent state in EPC sanitization loop

[Jarkko Sakkinen <jarkko@kernel.org>]

On Fri, Jul 24, 2026 at 10:55:14AM +0800, Jun Miao wrote:
> 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 <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>
> Tested-by: Challvy Tee <challvy.tee@gmail.com>
> Reviewed-by: Kai Huang <kai.huang@intel.com>
> 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"
> 
> v5 -> v6:
>  - Add the exactly kind of reminder that why cond_resched() does 
>    not trigger rescheduling.
> 
> ---
>  arch/x86/kernel/cpu/sgx/main.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index 4505f808af5e..207e680732a4 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -106,7 +106,13 @@ static unsigned long __sgx_sanitize_pages(struct list_head *dirty_page_list)
>  			left_dirty++;
>  		}
> 
> -		cond_resched();
> +		/*
> +		 * cond_resched() only schedules when TIF_NEED_RESCHED is set.
> +		 * During this boot-time loop that condition may not happen for a
> +		 * long time, so report an RCU-Tasks quiescent state explicitly.
> +		 * Therefore, change cond_resched() to cond_resched_tasks_rcu_qs().
> +		 */
> +		cond_resched_tasks_rcu_qs();
>  	}
> 
>  	list_splice(&dirty, dirty_page_list);
> -- 
> 2.43.0
> 

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>

BR, Jarkko