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

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Message-ID: <2eeb7f8d-d184-07d1-2b7b-76c93b4b1bfe@fb.com>
Date:   Mon, 25 Feb 2019 23:52:45 +0000
From:   Alexei Starovoitov <ast@...com>
To:     Stanislav Fomichev <sdf@...ichev.me>,
        Alexei Starovoitov <ast@...nel.org>
CC:     "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 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()."