On Apr 2, 2021, at 10:34 AM, Cong Wang [off-list ref] wrote:

On Thu, Apr 1, 2021 at 1:17 PM Song Liu [off-list ref] wrote:

quoted

quoted

On Apr 1, 2021, at 10:28 AM, Cong Wang [off-list ref] wrote:

On Wed, Mar 31, 2021 at 11:38 PM Song Liu [off-list ref] wrote:

quoted

quoted

On Mar 31, 2021, at 9:26 PM, Cong Wang [off-list ref] wrote:

From: Cong Wang <redacted>

(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