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

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Message-ID: <a3ed82c1679e4bb5a8ab4233df38a03c@AcuMS.aculab.com>
Date:   Fri, 16 Nov 2018 14:41:45 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     '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

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.

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

	David

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