linux-kernel - Re: [PATCH 3/3] IPI: Avoid to use 2 cache lines for one call_single

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Message-ID: <20170804092754.hyhbhyr2r4gonpu4@hirez.programming.kicks-ass.net>
Date:   Fri, 4 Aug 2017 11:27:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Juergen Gross <jgross@...e.com>, Aaron Lu <aaron.lu@...el.com>
Subject: Re: [PATCH 3/3] IPI: Avoid to use 2 cache lines for one
 call_single_data

On Fri, Aug 04, 2017 at 10:05:55AM +0800, Huang, Ying wrote:
> "Huang, Ying" <ying.huang@...el.com> writes:
> > Peter Zijlstra <peterz@...radead.org> writes:

> >> +struct __call_single_data {
> >>  	struct llist_node llist;
> >>  	smp_call_func_t func;
> >>  	void *info;
> >>  	unsigned int flags;
> >>  };
> >>  
> >> +typedef struct __call_single_data call_single_data_t
> >> +	__aligned(sizeof(struct __call_single_data));
> >> +
> >
> > Another requirement of the alignment is that it should be the power of
> > 2.  Otherwise, for example, if someone adds a field to struct, so that
> > the size becomes 40 on x86_64.  The alignment should be 64 instead of
> > 40.
> 
> Thanks Aaron, he reminded me that there is a roundup_pow_of_two().  So
> the typedef could be,
> 
> typedef struct __call_single_data call_single_data_t
> 	__aligned(roundup_pow_of_two(sizeof(struct __call_single_data));
> 

Yes, that would take away the requirement to play padding games with the
struct. Then again, maybe its a good thing to have to be explicit about
it.

If you see:

struct __call_single_data {
	struct llist_node llist;
	smp_call_func_t func;
	void *info
	int flags;
	void *extra_field;

	unsigned long __padding[3]; /* make align work */
};

that makes it very clear what is going on. In any case, we can delay
this part because the current structure is a power-of-2 for both ILP32
and LP64. So only the person growing this will have to deal with it ;-)