linux-kernel - Re: [RFC PATCH bpf-next v2 2/7] cgroup: bpf: flush bpf stats on rstat flush

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Date:   Tue, 17 May 2022 14:51:49 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Hao Luo <haoluo@...gle.com>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shuah Khan <shuah@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Michal Hocko <mhocko@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        cgroups@...r.kernel.org
Subject: Re: [RFC PATCH bpf-next v2 2/7] cgroup: bpf: flush bpf stats on rstat flush

On Mon, May 16, 2022 at 7:08 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Sun, May 15, 2022 at 02:34:59AM +0000, Yosry Ahmed wrote:
> > +
> > +void bpf_rstat_flush(struct cgroup *cgrp, int cpu)
> > +{
> > +     struct bpf_rstat_flusher *flusher;
> > +     struct bpf_rstat_flush_ctx ctx = {
> > +             .cgrp = cgrp,
> > +             .parent = cgroup_parent(cgrp),
> > +             .cpu = cpu,
> > +     };
> > +
> > +     rcu_read_lock();
> > +     migrate_disable();
> > +     spin_lock(&bpf_rstat_flushers_lock);
> > +
> > +     list_for_each_entry(flusher, &bpf_rstat_flushers, list)
> > +             (void) bpf_prog_run(flusher->prog, &ctx);
> > +
> > +     spin_unlock(&bpf_rstat_flushers_lock);
> > +     migrate_enable();
> > +     rcu_read_unlock();
> > +}
> > diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
> > index 24b5c2ab5598..0285d496e807 100644
> > --- a/kernel/cgroup/rstat.c
> > +++ b/kernel/cgroup/rstat.c
> > @@ -2,6 +2,7 @@
> >  #include "cgroup-internal.h"
> >
> >  #include <linux/sched/cputime.h>
> > +#include <linux/bpf-rstat.h>
> >
> >  static DEFINE_SPINLOCK(cgroup_rstat_lock);
> >  static DEFINE_PER_CPU(raw_spinlock_t, cgroup_rstat_cpu_lock);
> > @@ -168,6 +169,7 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp, bool may_sleep)
> >                       struct cgroup_subsys_state *css;
> >
> >                       cgroup_base_stat_flush(pos, cpu);
> > +                     bpf_rstat_flush(pos, cpu);
>
> Please use the following approach instead:
>
> __weak noinline void bpf_rstat_flush(struct cgroup *cgrp, struct cgroup *parent, int cpu)
> {
> }
>
> and change above line to:
>   bpf_rstat_flush(pos, cgroup_parent(pos), cpu);
>
> Then tracing bpf fentry progs will be able to attach to bpf_rstat_flush.
> Pretty much the patches 1, 2, 3 are not necessary.
> In patch 4 add bpf_cgroup_rstat_updated/flush as two kfuncs instead of stable helpers.
>
> This way patches 1,2,3,4 will become 2 trivial patches and we will be
> able to extend the interface between cgroup rstat and bpf whenever we need
> without worrying about uapi stability.
>
> We had similar discusison with HID subsystem that plans to use bpf in HID
> with the same approach.
> See this patch set:
> https://lore.kernel.org/bpf/20220421140740.459558-2-benjamin.tissoires@redhat.com/
> You'd need patch 1 from it to enable kfuncs for tracing.
>
> Your patch 5 is needed as-is.
> Yonghong,
> please review it.
> Different approach for patch 1-4 won't affect patch 5.
> Patches 6 and 7 look good.
>
> With this approach that patch 7 will mostly stay as-is. Instead of:
> +SEC("rstat/flush")
> +int vmscan_flush(struct bpf_rstat_flush_ctx *ctx)
> +{
> +       struct vmscan_percpu *pcpu_stat;
> +       struct vmscan *total_stat, *parent_stat;
> +       struct cgroup *cgrp = ctx->cgrp, *parent = ctx->parent;
>
> it will become
>
> SEC("fentry/bpf_rstat_flush")
> int BPF_PROG(vmscan_flush, struct cgroup *cgrp, struct cgroup *parent, int cpu)

Thanks so much for taking the time to look into this.

Indeed, this approach looks cleaner and simpler. I will incorporate
that into a V1 and send it. Thanks!