Re: [RFC Patch bpf-next] bpf: introduce bpf timer

[Song Liu <songliubraving@fb.com>]

> On Apr 2, 2021, at 10:34 AM, Cong Wang <xiyou.wangcong@gmail.com> wrote:
> 
> On Thu, Apr 1, 2021 at 1:17 PM Song Liu <songliubraving@fb.com> wrote:
>> 
>> 
>> 
>>> On Apr 1, 2021, at 10:28 AM, Cong Wang <xiyou.wangcong@gmail.com> wrote:
>>> 
>>> On Wed, Mar 31, 2021 at 11:38 PM Song Liu <songliubraving@fb.com> wrote:
>>>> 
>>>> 
>>>> 
>>>>> On Mar 31, 2021, at 9:26 PM, Cong Wang <xiyou.wangcong@gmail.com> wrote:
>>>>> 
>>>>> From: Cong Wang <cong.wang@bytedance.com>
>>>>> 
>>>>> (This patch is still in early stage and obviously incomplete. I am sending
>>>>> it out to get some high-level feedbacks. Please kindly ignore any coding
>>>>> details for now and focus on the design.)
>>>> 
>>>> Could you please explain the use case of the timer? Is it the same as
>>>> earlier proposal of BPF_MAP_TYPE_TIMEOUT_HASH?
>>>> 
>>>> Assuming that is the case, I guess the use case is to assign an expire
>>>> time for each element in a hash map; and periodically remove expired
>>>> element from the map.
>>>> 
>>>> If this is still correct, my next question is: how does this compare
>>>> against a user space timer? Will the user space timer be too slow?
>>> 
>>> Yes, as I explained in timeout hashmap patchset, doing it in user-space
>>> would require a lot of syscalls (without batching) or copying (with batching).
>>> I will add the explanation here, in case people miss why we need a timer.
>> 
>> How about we use a user space timer to trigger a BPF program (e.g. use
>> BPF_PROG_TEST_RUN on a raw_tp program); then, in the BPF program, we can
>> use bpf_for_each_map_elem and bpf_map_delete_elem to scan and update the
>> map? With this approach, we only need one syscall per period.
> 
> Interesting, I didn't know we can explicitly trigger a BPF program running
> from user-space. Is it for testing purposes only?

This is not only for testing. We will use this in perf (starting in 5.13).

/* currently in Arnaldo's tree, tools/perf/util/bpf_counter.c: */

/* trigger the leader program on a cpu */
static int bperf_trigger_reading(int prog_fd, int cpu)
{
        DECLARE_LIBBPF_OPTS(bpf_test_run_opts, opts,
                            .ctx_in = NULL,
                            .ctx_size_in = 0,
                            .flags = BPF_F_TEST_RUN_ON_CPU,
                            .cpu = cpu,
                            .retval = 0,
                );

        return bpf_prog_test_run_opts(prog_fd, &opts);
}

test_run also passes return value (retval) back to user space, so we and 
adjust the timer interval based on retval.

Also, test_run can trigger the program on a specific cpu. This might be 
useful with percpu map (BPF_MAP_TYPE_PERCPU_HASH, etc.). 

Thanks,
Song