netdev - Re: [PATCH net-next v2 2/9] net: sched: act_sample: add action cookie to sample

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Date: Tue, 18 Jun 2024 11:42:52 +0200
From: Ilya Maximets <i.maximets@....org>
To: Adrián Moreno <amorenoz@...hat.com>
Cc: i.maximets@....org, netdev@...r.kernel.org, aconole@...hat.com,
 echaudro@...hat.com, horms@...nel.org, dev@...nvswitch.org,
 Jamal Hadi Salim <jhs@...atatu.com>, Cong Wang <xiyou.wangcong@...il.com>,
 Jiri Pirko <jiri@...nulli.us>, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v2 2/9] net: sched: act_sample: add action cookie
 to sample

On 6/18/24 09:38, Adrián Moreno wrote:
> On Mon, Jun 17, 2024 at 12:00:04PM GMT, Ilya Maximets wrote:
>> On 6/3/24 20:56, Adrian Moreno wrote:
>>> If the action has a user_cookie, pass it along to the sample so it can
>>> be easily identified.
>>>
>>> Signed-off-by: Adrian Moreno <amorenoz@...hat.com>
>>> ---
>>>  net/sched/act_sample.c | 12 ++++++++++++
>>>  1 file changed, 12 insertions(+)
>>>
>>> diff --git a/net/sched/act_sample.c b/net/sched/act_sample.c
>>> index a69b53d54039..5c3f86ec964a 100644
>>> --- a/net/sched/act_sample.c
>>> +++ b/net/sched/act_sample.c
>>> @@ -165,9 +165,11 @@ TC_INDIRECT_SCOPE int tcf_sample_act(struct sk_buff *skb,
>>>  				     const struct tc_action *a,
>>>  				     struct tcf_result *res)
>>>  {
>>> +	u8 cookie_data[TC_COOKIE_MAX_SIZE] = {};
>>
>> Is it necessary to initialize these 16 bytes on every call?
>> Might be expensive.  We're passing the data length around,
>> so the uninitialized parts should not be accessed.
>>
> 
> They "should" not, indeed. I was just trying to be extra careful.
> Are you worried TC_COOKIE_MAX_SIZE could grow or the cycles needed to
> clear the current 16 bytes?

I'm assuming that any extra cycles spent per packet are undesirable,
so should be avoided, if possible.  Even if we save 1-2 cycles per
packet, it's a lot when we talk about millions of packets per second.

In this particular case, it seems, we do not sacrifice anything, so
it's just a couple of cycles back for free.

Best regards, Ilya Maximets.