On 12/16/19 11:14 AM, Martin Lau wrote:
> On Fri, Dec 13, 2019 at 05:59:54PM -0800, Eric Dumazet wrote:
>>
>>
>> On 12/13/19 4:47 PM, Martin KaFai Lau wrote:
>>> This patch adds a helper to handle jiffies. Some of the
>>> tcp_sock's timing is stored in jiffies. Although things
>>> could be deduced by CONFIG_HZ, having an easy way to get
>>> jiffies will make the later bpf-tcp-cc implementation easier.
>>>
>>
>> ...
>>
>>> +
>>> +BPF_CALL_2(bpf_jiffies, u64, in, u64, flags)
>>> +{
>>> + if (!flags)
>>> + return get_jiffies_64();
>>> +
>>> + if (flags & BPF_F_NS_TO_JIFFIES) {
>>> + return nsecs_to_jiffies(in);
>>> + } else if (flags & BPF_F_JIFFIES_TO_NS) {
>>> + if (!in)
>>> + in = get_jiffies_64();
>>> + return jiffies_to_nsecs(in);
>>> + }
>>> +
>>> + return 0;
>>> +}
>>
>> This looks a bit convoluted :)
>>
>> Note that we could possibly change net/ipv4/tcp_cubic.c to no longer use jiffies at all.
>>
>> We have in tp->tcp_mstamp an accurate timestamp (in usec) that can be converted to ms.
> Thanks for the feedbacks!
>
> I have a few questions that need some helps.
>
> Does it mean tp->tcp_mstamp can be used as the "now" in cubic?
TCP makes sure to update tp->tcp_mstamp at least once when handling
a particular packet. We did that to avoid calling possibly expensive
kernel time service (Some platforms do not have fast TSC)
> or tcp_clock_ns() should still be called in cubic, e.g. to replace
> bictcp_clock() and tcp_jiffies32?
Yeah, there is this lsndtime and tcp_jiffies32 thing, but maybe
we can find a way to fetch jiffies32 without having to call a bpf helper ?
>
> BPF currently has a helper calling ktime_get_mono_fast_ns() which looks
> different from tcp_clock_ns().
That's maybe because of NMI requirements.
TCP in the other hand is in process or BH context.
But it should not matter, cubic could should not have to call them.
