> On Apr 2, 2021, at 10:34 AM, Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote:
>
> On Thu, Apr 1, 2021 at 1:17 PM Song Liu <songliubraving@xxxxxx> wrote:
>>
>>
>>
>>> On Apr 1, 2021, at 10:28 AM, Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote:
>>>
>>> On Wed, Mar 31, 2021 at 11:38 PM Song Liu <songliubraving@xxxxxx> wrote:
>>>>
>>>>
>>>>
>>>>> On Mar 31, 2021, at 9:26 PM, Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote:
>>>>>
>>>>> From: Cong Wang <cong.wang@xxxxxxxxxxxxx>
>>>>>
>>>>> (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
