Re: [PATCH v16 13/16] x86/sgx: implement direct reclamation for cgroups

[Jarkko Sakkinen]

On Wed Aug 21, 2024 at 4:54 AM EEST, Haitao Huang wrote:
> sgx_reclaim_direct() was introduced to preemptively reclaim some pages
> as the best effort to avoid on-demand reclamation that can stall forward
> progress in some situations, e.g., allocating pages to load previously
> reclaimed page to perform EDMM operations on [1].
>
> Currently when the global usage is close to the capacity,
> sgx_reclaim_direct() makes one invocation to sgx_reclaim_pages_global()
> but does not guarantee there are free pages available for later
> allocations to succeed. In other words, the only goal here is to reduce
> the chance of on-demand reclamation at allocation time. In cases of
> allocation failure, the caller, the EDMM ioctl()'s, would return -EAGAIN
> to user space and let the user space to decide whether to retry or not.
>
> With EPC cgroups enabled, usage of a cgroup can also reach its limit
> (usually much lower than capacity) and trigger per-cgroup reclamation.
> Implement a similar strategy to reduce the chance of on-demand
> per-cgroup reclamation for this use case.
>
> Create a wrapper, sgx_cgroup_reclaim_direct(), to perform a preemptive
> reclamation at cgroup level, and have sgx_reclaim_direct() call it when
> EPC cgroup is enabled.
>
> [1] https://lore.kernel.org/all/a0d8f037c4a075d56bf79f432438412985f7ff7a.1652137848.git.reinette.chatre@xxxxxxxxx/T/#u
>
> Signed-off-by: Haitao Huang <haitao.huang@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kernel/cpu/sgx/epc_cgroup.c | 15 +++++++++++++++
>  arch/x86/kernel/cpu/sgx/epc_cgroup.h |  3 +++
>  arch/x86/kernel/cpu/sgx/main.c       |  4 ++++
>  3 files changed, 22 insertions(+)
>
> diff --git a/arch/x86/kernel/cpu/sgx/epc_cgroup.c b/arch/x86/kernel/cpu/sgx/epc_cgroup.c
> index 23a61689e0d9..b7d60b2d878d 100644
> --- a/arch/x86/kernel/cpu/sgx/epc_cgroup.c
> +++ b/arch/x86/kernel/cpu/sgx/epc_cgroup.c
> @@ -252,6 +252,21 @@ void sgx_cgroup_reclaim_pages_global(struct mm_struct *charge_mm)
>  	sgx_cgroup_reclaim_pages(&sgx_cg_root, charge_mm, SGX_NR_TO_SCAN);
>  }
>  
> +/**
> + * sgx_cgroup_reclaim_direct() - Preemptive reclamation.
> + *
> + * Scan and attempt to reclaim %SGX_NR_TO_SCAN as best effort to allow caller
> + * make quick progress.
> + */
> +void sgx_cgroup_reclaim_direct(void)
> +{
> +	struct sgx_cgroup *sgx_cg = sgx_get_current_cg();
> +
> +	if (sgx_cgroup_should_reclaim(sgx_cg))
> +		sgx_cgroup_reclaim_pages(sgx_cg, current->mm, SGX_NR_TO_SCAN);
> +	sgx_put_cg(sgx_cg);
> +}
> +
>  /*
>   * Asynchronous work flow to reclaim pages from the cgroup when the cgroup is
>   * at/near its maximum capacity.
> diff --git a/arch/x86/kernel/cpu/sgx/epc_cgroup.h b/arch/x86/kernel/cpu/sgx/epc_cgroup.h
> index c0390111e28c..cf2b946d993e 100644
> --- a/arch/x86/kernel/cpu/sgx/epc_cgroup.h
> +++ b/arch/x86/kernel/cpu/sgx/epc_cgroup.h
> @@ -38,6 +38,8 @@ static inline void __init sgx_cgroup_register(void) { }
>  
>  static inline void sgx_cgroup_reclaim_pages_global(struct mm_struct *charge_mm) { }
>  
> +static inline void sgx_cgroup_reclaim_direct(void) { }
> +
>  #else /* CONFIG_CGROUP_MISC */
>  
>  struct sgx_cgroup {
> @@ -90,6 +92,7 @@ static inline void sgx_put_cg(struct sgx_cgroup *sgx_cg)
>  int sgx_cgroup_try_charge(struct sgx_cgroup *sgx_cg, enum sgx_reclaim reclaim);
>  void sgx_cgroup_uncharge(struct sgx_cgroup *sgx_cg);
>  void sgx_cgroup_reclaim_pages_global(struct mm_struct *charge_mm);
> +void sgx_cgroup_reclaim_direct(void);
>  int __init sgx_cgroup_init(void);
>  void __init sgx_cgroup_register(void);
>  void __init sgx_cgroup_deinit(void);
> diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> index d00cb012838b..9a8f91ebd21b 100644
> --- a/arch/x86/kernel/cpu/sgx/main.c
> +++ b/arch/x86/kernel/cpu/sgx/main.c
> @@ -428,6 +428,10 @@ static void sgx_reclaim_pages_global(struct mm_struct *charge_mm)
>   */
>  void sgx_reclaim_direct(void)
>  {
> +	/* Reduce chance of per-cgroup reclamation for later allocation */
> +	sgx_cgroup_reclaim_direct();
> +
> +	/* Reduce chance of the global reclamation for later allocation */
>  	if (sgx_should_reclaim_global(SGX_NR_LOW_PAGES))
>  		sgx_reclaim_pages_global(current->mm);
>  }


Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>

BR, Jarkko