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

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Mar 23, 2023 at 8:43 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> 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.

Thanks for pointing this out. I will rename it to
cgroup_rstat_flush_atomic() in upcoming versions instead. I will also
do the same for mem_cgroup_flush_stats() as I introduce a similar
boolean argument for it in the following patch.

>
> > 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()

Thanks for pointing this out. I don't think it will be needed if we
keep cgroup_rstat_flush_irqsafe() and only rename it to
cgroup_rstat_flush_atomic(), but it is useful to know that it exists
for future reference.