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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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