> 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
