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

[Yonghong Song <yhs@fb.com>]

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.

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
>