netdev - Re: [PATCH v3 bpf-next 1/4] bpf: enable program stats

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Date:   Mon, 25 Feb 2019 19:10:49 -0800
From:   Stanislav Fomichev <sdf@...ichev.me>
To:     Alexei Starovoitov <ast@...com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v3 bpf-next 1/4] bpf: enable program stats

On 02/25, Alexei Starovoitov wrote:
> On 2/25/19 3:07 PM, Stanislav Fomichev wrote:
> >> +#define BPF_PROG_RUN(prog, ctx)	({				\
> >> +	u32 ret;						\
> >> +	cant_sleep();						\
> >> +	if (static_branch_unlikely(&bpf_stats_enabled_key)) {	\
> >> +		struct bpf_prog_stats *stats;			\
> >> +		u64 start = sched_clock();			\
> > QQ: why sched_clock() and not, for example, ktime_get_ns() which we do
> > in the bpf_test_run()? Or even why not local_clock?
> > I'm just wondering what king of trade off we are doing here
> > regarding precision vs run time cost.
> 
> 
> I'm making this decision based on documentation:
> Documentation/timers/timekeeping.txt
> "Compared to clock sources, sched_clock() has to be very fast: it is 
> called much more often, especially by the scheduler. If you have to do 
> trade-offs between accuracy compared to the clock source, you may 
> sacrifice accuracy for speed in sched_clock()."
So sched_clock is fast, but imprecise; and ktime_get_ns (and
lock_clock?) are slow(er), but more precise?

If that's the case, would it make sense to use a more precise
measurement? I suppose the BPF program execution time is on the order of
nanoseconds and if sched_close has msec or usec resolution, all we get is
essentially noise?

I understand that you want this feature to have almost no overhead, but
since it's gated by the static key, should we aim for a higher precision?