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

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Message-ID: <20170804092039.ezsxko32a6n5ra4v@hirez.programming.kicks-ass.net>
Date:   Fri, 4 Aug 2017 11:20:39 +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 09:28:17AM +0800, Huang, Ying wrote:
> Peter Zijlstra <peterz@...radead.org> writes:
> [snip]
> > diff --git a/include/linux/smp.h b/include/linux/smp.h
> > index 68123c1fe549..8d817cb80a38 100644
> > --- a/include/linux/smp.h
> > +++ b/include/linux/smp.h
> > @@ -14,13 +14,16 @@
> >  #include <linux/llist.h>
> >  
> >  typedef void (*smp_call_func_t)(void *info);
> > -struct call_single_data {
> > +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.

Yes I know. This generates a compiler error if sizeof() isn't a
power of 2. That's similar to the BUILD_BUG_ON() you added.