Re: [PATCH] mm/vmscan: respect cpuset policy during page demotion

Waiman Long
2022-10-26T14:36:32Z

On 10/26/22 03:43, Feng Tang wrote:
> In page reclaim path, memory could be demoted from faster memory tier
> to slower memory tier. Currently, there is no check about cpuset's
> memory policy, that even if the target demotion node is not allowd
> by cpuset, the demotion will still happen, which breaks the cpuset
> semantics.
>
> So add cpuset policy check in the demotion path and skip demotion
> if the demotion targets are not allowed by cpuset.
>
> Signed-off-by: Feng Tang <feng.tang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> ---
> Hi reviewers,
>
> For easy bisectable, I combined the cpuset change and mm change
> in one patch, if you prefer to separate them, I can turn it into
> 2 patches.
>
> Thanks,
> Feng
>
> include/linux/cpuset.h | 6 ++++++
> kernel/cgroup/cpuset.c | 29 +++++++++++++++++++++++++++++
> mm/vmscan.c | 35 ++++++++++++++++++++++++++++++++---
> 3 files changed, 67 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> index d58e0476ee8e..6fcce2bd2631 100644
> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -178,6 +178,8 @@ static inline void set_mems_allowed(nodemask_t nodemask)
> task_unlock(current);
> }
>
> +extern void cpuset_get_allowed_mem_nodes(struct cgroup *cgroup,
> + nodemask_t *nmask);
> #else /* !CONFIG_CPUSETS */
>
> static inline bool cpusets_enabled(void) { return false; }
> @@ -299,6 +301,10 @@ static inline bool read_mems_allowed_retry(unsigned int seq)
> return false;
> }
>
> +static inline void cpuset_get_allowed_mem_nodes(struct cgroup *cgroup,
> + nodemask_t *nmask)
> +{
> +}
> #endif /* !CONFIG_CPUSETS */
>
> #endif /* _LINUX_CPUSET_H */
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 3ea2e836e93e..cbb118c0502f 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -3750,6 +3750,35 @@ nodemask_t cpuset_mems_allowed(struct task_struct *tsk)
> return mask;
> }
>
> +/*
> + * Retrieve the allowed memory nodemask for a cgroup.
> + *
> + * Set *nmask to cpuset's effective allowed nodemask for cgroup v2,
> + * and NODE_MASK_ALL (means no constraint) for cgroup v1 where there
> + * is no guaranteed association from a cgroup to a cpuset.
> + */
> +void cpuset_get_allowed_mem_nodes(struct cgroup *cgroup, nodemask_t *nmask)
> +{
> + struct cgroup_subsys_state *css;
> + struct cpuset *cs;
> +
> + if (!is_in_v2_mode()) {
> + *nmask = NODE_MASK_ALL;
> + return;
> + }

You are allowing all nodes to be used for cgroup v1. Is there a reason
why you ignore v1?

> +
> + rcu_read_lock();
> + css = cgroup_e_css(cgroup, &cpuset_cgrp_subsys);
> + if (css) {
> + css_get(css);
> + cs = css_cs(css);
> + *nmask = cs->effective_mems;
> + css_put(css);
> + }
Since you are holding an RCU read lock and copying out the whole
nodemask, you probably don't need to do a css_get/css_put pair.
> +
> + rcu_read_unlock();
> +}
> +
Cheers,

Longman