Messages in this thread | | | Subject | Re: [PATCH bpf-next 4/4] bpf: runqslower: use task local storage | From | Yonghong Song <> | Date | Mon, 11 Jan 2021 23:33:42 -0800 |
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On 1/11/21 11:14 PM, Andrii Nakryiko wrote: > 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.
The current approach works fine. What I suggested is to tighten sched_switch only for target_pid. wakeup (doing queuing) will be more relaxed than sched_switch to ensure task local storage creation is always there for target_pid regardless of attachment timing. I think it should work, but we have to experiment to see actual results...
> >> >> 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 >>>
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