Re: [PATCH bpf-next 4/4] bpf: runqslower: use task local storage

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Mon, Jan 11, 2021 at 7:24 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 1/11/21 2:54 PM, Song Liu wrote:
> >
> >
> >> On Jan 11, 2021, at 9:49 AM, Yonghong Song <yhs@fb.com> wrote:
> >>
> >>
> >>
> >> On 1/8/21 3:19 PM, Song Liu wrote:
> >>> Replace hashtab with task local storage in runqslower. This improves the
> >>> performance of these BPF programs. The following table summarizes average
> >>> runtime of these programs, in nanoseconds:
> >>>                            task-local   hash-prealloc   hash-no-prealloc
> >>> handle__sched_wakeup             125             340               3124
> >>> handle__sched_wakeup_new        2812            1510               2998
> >>> handle__sched_switch             151             208                991
> >>> Note that, task local storage gives better performance than hashtab for
> >>> handle__sched_wakeup and handle__sched_switch. On the other hand, for
> >>> handle__sched_wakeup_new, task local storage is slower than hashtab with
> >>> prealloc. This is because handle__sched_wakeup_new accesses the data for
> >>> the first time, so it has to allocate the data for task local storage.
> >>> Once the initial allocation is done, subsequent accesses, as those in
> >>> handle__sched_wakeup, are much faster with task local storage. If we
> >>> disable hashtab prealloc, task local storage is much faster for all 3
> >>> functions.
> >>> Signed-off-by: Song Liu <songliubraving@fb.com>
> >>> ---
> >>>   tools/bpf/runqslower/runqslower.bpf.c | 26 +++++++++++++++-----------
> >>>   1 file changed, 15 insertions(+), 11 deletions(-)
> >>> diff --git a/tools/bpf/runqslower/runqslower.bpf.c b/tools/bpf/runqslower/runqslower.bpf.c
> >>> index 1f18a409f0443..c4de4179a0a17 100644
> >>> --- a/tools/bpf/runqslower/runqslower.bpf.c
> >>> +++ b/tools/bpf/runqslower/runqslower.bpf.c
> >>> @@ -11,9 +11,9 @@ const volatile __u64 min_us = 0;
> >>>   const volatile pid_t targ_pid = 0;
> >>>     struct {
> >>> -   __uint(type, BPF_MAP_TYPE_HASH);
> >>> -   __uint(max_entries, 10240);
> >>> -   __type(key, u32);
> >>> +   __uint(type, BPF_MAP_TYPE_TASK_STORAGE);
> >>> +   __uint(map_flags, BPF_F_NO_PREALLOC);
> >>> +   __type(key, int);
> >>>     __type(value, u64);
> >>>   } start SEC(".maps");
> >>>   @@ -25,15 +25,19 @@ struct {
> >>>     /* record enqueue timestamp */
> >>>   __always_inline
> >>> -static int trace_enqueue(u32 tgid, u32 pid)
> >>> +static int trace_enqueue(struct task_struct *t)
> >>>   {
> >>> -   u64 ts;
> >>> +   u32 pid = t->pid;
> >>> +   u64 ts, *ptr;
> >>>             if (!pid || (targ_pid && targ_pid != pid))
> >>>             return 0;
> >>>             ts = bpf_ktime_get_ns();
> >>> -   bpf_map_update_elem(&start, &pid, &ts, 0);
> >>> +   ptr = bpf_task_storage_get(&start, t, 0,
> >>> +                              BPF_LOCAL_STORAGE_GET_F_CREATE);
> >>> +   if (ptr)
> >>> +           *ptr = ts;
> >>>     return 0;
> >>>   }
> >>>   @@ -43,7 +47,7 @@ int handle__sched_wakeup(u64 *ctx)
> >>>     /* TP_PROTO(struct task_struct *p) */
> >>>     struct task_struct *p = (void *)ctx[0];
> >>>   - return trace_enqueue(p->tgid, p->pid);
> >>> +   return trace_enqueue(p);
> >>>   }
> >>>     SEC("tp_btf/sched_wakeup_new")
> >>> @@ -52,7 +56,7 @@ int handle__sched_wakeup_new(u64 *ctx)
> >>>     /* TP_PROTO(struct task_struct *p) */
> >>>     struct task_struct *p = (void *)ctx[0];
> >>>   - return trace_enqueue(p->tgid, p->pid);
> >>> +   return trace_enqueue(p);
> >>>   }
> >>>     SEC("tp_btf/sched_switch")
> >>> @@ -70,12 +74,12 @@ int handle__sched_switch(u64 *ctx)
> >>>             /* ivcsw: treat like an enqueue event and store timestamp */
> >>>     if (prev->state == TASK_RUNNING)
> >>> -           trace_enqueue(prev->tgid, prev->pid);
> >>> +           trace_enqueue(prev);
> >>>             pid = next->pid;
> >>>             /* fetch timestamp and calculate delta */
> >>> -   tsp = bpf_map_lookup_elem(&start, &pid);
> >>> +   tsp = bpf_task_storage_get(&start, next, 0, 0);
> >>>     if (!tsp)
> >>>             return 0;   /* missed enqueue */
> >>
> >> Previously, hash table may overflow so we may have missed enqueue.
> >> Here with task local storage, is it possible to add additional pid
> >> filtering in the beginning of handle__sched_switch such that
> >> missed enqueue here can be treated as an error?
> >
> > IIUC, hashtab overflow is not the only reason of missed enqueue. If the
> > wakeup (which calls trace_enqueue) happens before runqslower starts, we
> > may still get missed enqueue in sched_switch, no?
>
> the wakeup won't happen before runqslower starts since runqslower needs
> to start to do attachment first and then trace_enqueue() can run.

I think Song is right. Given wakeup and sched_switch need to be
matched, depending at which exact time we attach BPF programs, we can
end up missing wakeup, but not missing sched_switch, no? So it's not
an error.

>
> For the current implementation trace_enqueue() will happen for any non-0
> pid before setting test_progs tgid, and will happen for any non-0 and
> test_progs tgid if it is set, so this should be okay if we do filtering
> in handle__sched_switch. Maybe you can do an experiment to prove whether
> my point is correct or not.
>
> >
> > Thanks,
> > Song
> >