netdev - Re: [PATCH 01/11] xfrm: Add support for per cpu xfrm state handling.

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Message-ID: <Zzrde4aJcmzjDnqI@gauss3.secunet.de>
Date: Mon, 18 Nov 2024 07:23:55 +0100
From: Steffen Klassert <steffen.klassert@...unet.com>
To: Jakub Kicinski <kuba@...nel.org>
CC: David Miller <davem@...emloft.net>, Herbert Xu
	<herbert@...dor.apana.org.au>, <netdev@...r.kernel.org>
Subject: Re: [PATCH 01/11] xfrm: Add support for per cpu xfrm state handling.

On Fri, Nov 15, 2024 at 06:09:08PM -0800, Jakub Kicinski wrote:
> On Fri, 15 Nov 2024 09:33:33 +0100 Steffen Klassert wrote:
> > +	/* We need the cpu id just as a lookup key,
> > +	 * we don't require it to be stable.
> > +	 */
> > +	pcpu_id = get_cpu();
> > +	put_cpu();
> 
> Why not smp_processor_id() ?

This might be executed in preemptable code sections,
smp_processor_id() will throw a warning if that happens.
Maybe raw_smp_processor_id() can be used instead here,
but was not sure if that's the right thing.

> 
> > +	if (attrs[XFRMA_SA_PCPU]) {
> > +		x->pcpu_num = nla_get_u32(attrs[XFRMA_SA_PCPU]);
> > +		if (x->pcpu_num >= num_possible_cpus())
> > +			goto error;
> > +	}
> 
> cpu ids can be sparse, shouldn't it be checking if the CPU is online ?

I thought about that. But then we must wait for an IKE negotiation
before we can use a fresh booted cpu. If we pre-negotiate a SA,
the cpu can be used right away. I depends a bit on the usecase what's
preferred here.