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Message-ID: <88de89a2-606f-471c-1556-cebddba231a1@fb.com> Date: Tue, 26 Feb 2019 03:42:55 +0000 From: Alexei Starovoitov <ast@...com> To: Stanislav Fomichev <sdf@...ichev.me> 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 2/25/19 7:10 PM, Stanislav Fomichev wrote: > 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? > Considering everything I believe sched_clock() strikes the best trade off.
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