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

[Song Liu]

> On Jan 11, 2021, at 9:49 AM, Yonghong Song <yhs@xxxxxx> 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@xxxxxx>
>> ---
>>  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?

Thanks,
Song