netdev - Re: [PATCH net-next 2/2] net/sched: act_police: don't use spinlock in the data path

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Date:   Fri, 16 Nov 2018 06:55:34 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     David Laight <David.Laight@...LAB.COM>,
        'Eric Dumazet' <eric.dumazet@...il.com>,
        Davide Caratti <dcaratti@...hat.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 2/2] net/sched: act_police: don't use spinlock in
 the data path



On 11/16/2018 06:41 AM, David Laight wrote:
> From: Eric Dumazet
>> Sent: 16 November 2018 14:35
> ...
>> I suggest to use a single cache line with a dedicated spinlock and these three s64
>>
>> 	spinlock_t  tcfp_lock ____cacheline_aligned_in_smp;
>> 	s64 			...
>> 	s64			...
>> 	s64			...
> 
> Doesn't this do something really stupid when cache lines are big.
> If the spinlock is 8 bytes you never want more than 32 byte alignment.
> If cache lines are 256 bytes you don't even need that.

We do want that, even if cache lines are 256 bytes, thank you.

> 
> Also ISTR that the kmalloc() only guarantees 8 byte alignment on x86_64.
> So aligning structure members to larger offsets is rather pointless.


No it is not, we use these hints all the time.

Just double check and report a bug to mm teams if you disagree.

Please do not send feedback if you are not sure.