On Fri, Apr 2, 2021 at 10:57 AM Song Liu <songliubraving@xxxxxx> wrote:
>
>
>
> > 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.
This is really odd, every name here contains a "test" but it is not for testing
purposes. You probably need to rename/alias it. ;)
So, with this we have to get a user-space daemon running just to keep
this "timer" alive. If I want to run it every 1ms, it means I have to issue
a syscall BPF_PROG_TEST_RUN every 1ms. Even with a timer fd, we
still need poll() and timerfd_settime(). This is a considerable overhead
for just a single timer.
With current design, user-space can just exit after installing the timer,
either it can adjust itself or other eBPF code can adjust it, so the per
timer overhead is the same as a kernel timer.
The visibility to other BPF code is important for the conntrack case,
because each time we get an expired item during a lookup, we may
want to schedule the GC timer to run sooner. At least this would give
users more freedom to decide when to reschedule the timer.
Thanks.
