Re: [RFC PATCH 3/7] cgroup: rstat: remove cgroup_rstat_flush_irqsafe()

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Mar 23, 2023 at 04:00:33AM +0000, Yosry Ahmed wrote:
> The naming of cgroup_rstat_flush_irqsafe() can be confusing.
> It can read like "irqsave", which means that it disables
> interrupts throughout, but it actually doesn't. It is just "safe" to
> call from contexts with interrupts disabled.
> 
> Furthermore, this is only used today by mem_cgroup_flush_stats(), which
> will be changed by a later patch to optionally allow sleeping. Simplify
> the code and make it easier to reason about by instead passing in an
> explicit @may_sleep argument to cgroup_rstat_flush(), which gets passed
> directly to cgroup_rstat_flush_locked().
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> ---
>  block/blk-cgroup.c     |  2 +-
>  include/linux/cgroup.h |  3 +--
>  kernel/cgroup/cgroup.c |  4 ++--
>  kernel/cgroup/rstat.c  | 24 +++++-------------------
>  mm/memcontrol.c        |  6 +++---
>  5 files changed, 12 insertions(+), 27 deletions(-)
> 
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index bd50b55bdb61..3fe313ce5e6b 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -1043,7 +1043,7 @@ static int blkcg_print_stat(struct seq_file *sf, void *v)
>  	if (!seq_css(sf)->parent)
>  		blkcg_fill_root_iostats();
>  	else
> -		cgroup_rstat_flush(blkcg->css.cgroup);
> +		cgroup_rstat_flush(blkcg->css.cgroup, true);
>  
>  	rcu_read_lock();
>  	hlist_for_each_entry_rcu(blkg, &blkcg->blkg_list, blkcg_node) {
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index 3410aecffdb4..743c345b6a3f 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -691,8 +691,7 @@ static inline void cgroup_path_from_kernfs_id(u64 id, char *buf, size_t buflen)
>   * cgroup scalable recursive statistics.
>   */
>  void cgroup_rstat_updated(struct cgroup *cgrp, int cpu);
> -void cgroup_rstat_flush(struct cgroup *cgrp);
> -void cgroup_rstat_flush_irqsafe(struct cgroup *cgrp);
> +void cgroup_rstat_flush(struct cgroup *cgrp, bool may_sleep);
>  void cgroup_rstat_flush_hold(struct cgroup *cgrp);
>  void cgroup_rstat_flush_release(void);
>  
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index 935e8121b21e..58df0fc379f6 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -5393,7 +5393,7 @@ static void css_release_work_fn(struct work_struct *work)
>  	if (ss) {
>  		/* css release path */
>  		if (!list_empty(&css->rstat_css_node)) {
> -			cgroup_rstat_flush(cgrp);
> +			cgroup_rstat_flush(cgrp, true);
>  			list_del_rcu(&css->rstat_css_node);
>  		}
>  
> @@ -5406,7 +5406,7 @@ static void css_release_work_fn(struct work_struct *work)
>  		/* cgroup release path */
>  		TRACE_CGROUP_PATH(release, cgrp);
>  
> -		cgroup_rstat_flush(cgrp);
> +		cgroup_rstat_flush(cgrp, true);

This is an anti-pattern, please don't do this. Naked bool arguments
are a pain to comprehend at the callsite and an easy vector for bugs.

cgroup_rstat_flush_atomic() would be a better name for, well, atomic
callsites.

> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
> index af11e28bd055..fe8690bb1e1c 100644
> --- a/kernel/cgroup/rstat.c
> +++ b/kernel/cgroup/rstat.c
> @@ -243,30 +243,16 @@ static bool should_skip_flush(void)
>   * This function is safe to call from contexts that disable interrupts, but
>   * @may_sleep must be set to false, otherwise the function may block.
>   */
> -__bpf_kfunc void cgroup_rstat_flush(struct cgroup *cgrp)
> +__bpf_kfunc void cgroup_rstat_flush(struct cgroup *cgrp, bool may_sleep)
>  {
>  	if (should_skip_flush())
>  		return;
>  
> -	might_sleep();
> -	spin_lock(&cgroup_rstat_lock);
> -	cgroup_rstat_flush_locked(cgrp, true);
> -	spin_unlock(&cgroup_rstat_lock);
> -}
> -
> -/**
> - * cgroup_rstat_flush_irqsafe - irqsafe version of cgroup_rstat_flush()
> - * @cgrp: target cgroup
> - *
> - * This function can be called from any context.
> - */
> -void cgroup_rstat_flush_irqsafe(struct cgroup *cgrp)
> -{
> -	if (should_skip_flush())
> -		return;
> +	if (may_sleep)
> +		might_sleep();

might_sleep_if()